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In 1901, the Federation of Australia was the process by which the six separate British self-governing colonies of New South Wales, Queensland, South Australia, Tasmania, Victoria and Western Australia formed one nation. They kept the systems of government that they had developed as separate colonies but also would have a federal government that was responsible for matters concerning the whole nation. When the Constitution of Australia came into force, the colonies collectively became states of the Commonwealth of Australia.
The last days of the Qing Dynasty were marked by civil unrest and foreign invasions. Responding to these civil failures and discontent, the Qing Imperial Court did attempt to reform the government in various ways, such as the decision to draft a constitution in 1906, the establishment of provincial legislatures in 1909, and the preparation for a national parliament in 1910. However, many of these measures were opposed by the conservatives of the Qing Court, and many reformers were either imprisoned or executed outright. The failures of the Imperial Court to enact such reforming measures of political liberalization and modernization caused the reformists to steer toward the road of revolution.
In 1912, the Republic of China was established and Sun Yat-sen was inaugurated in Nanjing as the first Provisional President. But power in Beijing already had passed to Yuan Shikai, who had effective control of the Beiyang Army, the most powerful military force in China at the time. To prevent civil war and possible foreign intervention from undermining the infant republic, leaders agreed to Army's demand that China be united under a Beijing government. On March 10, in Beijing, Shikai was sworn in as the second Provisional President of the Republic of China.
After the early 20th century revolutions, shifting alliances of China's regional warlords waged war for control of the Beijing government. Despite the fact that various warlords gained control of the government in Beijing during the warlord era, this did not constitute a new era of control or governance, because other warlords did not acknowledge the transitory governments in this period and were a law unto themselves. These military-dominated governments were collectively known as the Beiyang government. The warlord era ended around 1927.
Four years into the 20th century saw the Russo-Japanese War with the Battle of Port Arthur establishing the Empire of Japan as a world power. The Russians were in constant pursuit of a warm water port on the Pacific Ocean, for their navy as well as for maritime trade. The Manchurian Campaign of the Russian Empire was fought against the Japanese over Manchuria and Korea. The major theatres of operations were Southern Manchuria, specifically the area around the Liaodong Peninsula and Mukden, and the seas around Korea, Japan, and the Yellow Sea. The resulting campaigns, in which the fledgling Japanese military consistently attained victory over the Russian forces arrayed against them, were unexpected by world observers. These victories, as time transpired, would dramatically transform the distribution of power in East Asia, resulting in a reassessment of Japan's recent entry onto the world stage. The embarrassing string of defeats increased Russian popular dissatisfaction with the inefficient and corrupt Tsarist government.
The Edwardian era in the United Kingdom is the period spanning the reign of King Edward VII up to the end of the First World War, including the years surrounding the sinking of the RMS Titanic. In the early years of the period, the Second Boer War in South Africa split the country into anti- and pro-war factions. The imperial policies of the Conservatives eventually proved unpopular and in the general election of 1906 the Liberals won a huge landslide. The Liberal government was unable to proceed with all of its radical programme without the support of the House of Lords, which was largely Conservative. Conflict between the two Houses of Parliament over the People's Budget led to a reduction in the power of the peers in 1910. The general election in January that year returned a hung parliament with the balance of power held by Labour and Irish Nationalist members.
The causes of World War I included many factors, including the conflicts and antagonisms of the four decades leading up to the war. The Triple Entente was the name given to the loose alignment between the United Kingdom, France, and Russia after the signing of the Anglo-Russian Entente in 1907. The alignment of the three powers, supplemented by various agreements with Japan, the United States, and Spain, constituted a powerful counterweight to the Triple Alliance of Germany, Austria-Hungary, and Italy, the third having concluded an additional secret agreement with France effectively nullifying her Alliance commitments. Militarism, alliances, imperialism, and nationalism played major roles in the conflict. The immediate origins of the war lay in the decisions taken by statesmen and generals during the July Crisis of 1914, the spark (or casus belli) for which was the assassination of Archduke Franz Ferdinand of Austria.
However, the crisis did not exist in a void; it came after a long series of diplomatic clashes between the Great Powers over European and colonial issues in the decade prior to 1914 which had left tensions high. The diplomatic clashes can be traced to changes in the balance of power in Europe since 1870. An example is the Baghdad Railway which was planned to connect the Ottoman Empire cities of Konya and Baghdad with a line through modern-day Turkey, Syria and Iraq. The railway became a source of international disputes during the years immediately preceding World War I. Although it has been argued that they were resolved in 1914 before the war began, it has also been argued that the railroad was a cause of the First World War. Fundamentally the war was sparked by tensions over territory in the Balkans. Austria-Hungary competed with Serbia and Russia for territory and influence in the region and they pulled the rest of the great powers into the conflict through their various alliances and treaties. The Balkan Wars were two wars in South-eastern Europe in 1912–1913 in the course of which the Balkan League (Bulgaria, Montenegro, Greece, and Serbia) first captured Ottoman-held remaining part of Thessaly, Macedonia, Epirus, Albania and most of Thrace and then fell out over the division of the spoils, with incorporation of Romania this time.
The First World War began in 1914 and lasted to the final Armistice in 1918. The Allied Powers, led by the British Empire, France, Russia until March 1918, Japan and the United States after 1917, defeated the Central Powers, led by the German Empire, Austro-Hungarian Empire and the Ottoman Empire. The war caused the disintegration of four empires—the Austro-Hungarian, German, Ottoman, and Russian ones—as well as radical change in the European and West Asian maps. The Allied powers before 1917 are referred to as the Triple Entente, and the Central Powers are referred to as the Triple Alliance.
Much of the fighting in World War I took place along the Western Front, within a system of opposing manned trenches and fortifications (separated by a "No man's land") running from the North Sea to the border of Switzerland. On the Eastern Front, the vast eastern plains and limited rail network prevented a trench warfare stalemate from developing, although the scale of the conflict was just as large. Hostilities also occurred on and under the sea and—for the first time—from the air. More than 9 million soldiers died on the various battlefields, and nearly that many more in the participating countries' home fronts on account of food shortages and genocide committed under the cover of various civil wars and internal conflicts. Notably, more people died of the worldwide influenza outbreak at the end of the war and shortly after than died in the hostilities. The unsanitary conditions engendered by the war, severe overcrowding in barracks, wartime propaganda interfering with public health warnings, and migration of so many soldiers around the world helped the outbreak become a pandemic.
Ultimately, World War I created a decisive break with the old world order that had emerged after the Napoleonic Wars, which was modified by the mid-19th century's nationalistic revolutions. The results of World War I would be important factors in the development of World War II approximately 20 years later. More immediate to the time, the partitioning of the Ottoman Empire was a political event that redrew the political boundaries of West Asia. The huge conglomeration of territories and peoples formerly ruled by the Sultan of the Ottoman Empire was divided into several new nations. The partitioning brought the creation of the modern Arab world and the Republic of Turkey. The League of Nations granted France mandates over Syria and Lebanon and granted the United Kingdom mandates over Mesopotamia and Palestine (which was later divided into two regions: Palestine and Transjordan). Parts of the Ottoman Empire on the Arabian Peninsula became parts of what are today Saudi Arabia and Yemen.
The Russian Revolution is the series of revolutions in Russia in 1917, which destroyed the Tsarist autocracy and led to the creation of the Soviet Union. Following the abdication of Nicholas II of Russia, the Russian Provisional Government was established. In October 1917, a red faction revolution occurred in which the Red Guard, armed groups of workers and deserting soldiers directed by the Bolshevik Party, seized control of Saint Petersburg (then known as Petrograd) and began an immediate armed takeover of cities and villages throughout the former Russian Empire.
Another action in 1917 that is of note was the armistice signed between Russia and the Central Powers at Brest-Litovsk. As a condition for peace, the treaty by the Central Powers conceded huge portions of the former Russian Empire to Imperial Germany and the Ottoman Empire, greatly upsetting nationalists and conservatives. The Bolsheviks made peace with the German Empire and the Central Powers, as they had promised the Russian people prior to the Revolution. Vladimir Lenin's decision has been attributed to his sponsorship by the foreign office of Wilhelm II, German Emperor, offered by the latter in hopes that with a revolution, Russia would withdraw from World War I. This suspicion was bolstered by the German Foreign Ministry's sponsorship of Lenin's return to Petrograd. The Western Allies expressed their dismay at the Bolsheviks, upset at:
The Russian Civil War was a multi-party war that occurred within the former Russian Empire after the Russian provisional government collapsed and the Soviets under the domination of the Bolshevik party assumed power, first in Petrograd (St. Petersburg) and then in other places. In the wake of the October Revolution, the old Russian Imperial Army had been demobilized; the volunteer-based Red Guard was the Bolsheviks' main military force, augmented by an armed military component of the Cheka, the Bolshevik state security apparatus. There was an instituted mandatory conscription of the rural peasantry into the Red Army. Opposition of rural Russians to Red Army conscription units was overcome by taking hostages and shooting them when necessary in order to force compliance. Former Tsarist officers were utilized as "military specialists" (voenspetsy), taking their families hostage in order to ensure loyalty. At the start of the war, three-fourths of the Red Army officer corps was composed of former Tsarist officers. By its end, 83% of all Red Army divisional and corps commanders were ex-Tsarist soldiers.
The principal fighting occurred between the Bolshevik Red Army and the forces of the White Army. Many foreign armies warred against the Red Army, notably the Allied Forces, yet many volunteer foreigners fought in both sides of the Russian Civil War. Other nationalist and regional political groups also participated in the war, including the Ukrainian nationalist Green Army, the Ukrainian anarchist Black Army and Black Guards, and warlords such as Ungern von Sternberg. The most intense fighting took place from 1918 to 1920. Major military operations ended on 25 October 1922 when the Red Army occupied Vladivostok, previously held by the Provisional Priamur Government. The last enclave of the White Forces was the Ayano-Maysky District on the Pacific coast. The majority of the fighting ended in 1920 with the defeat of General Pyotr Wrangel in the Crimea, but a notable resistance in certain areas continued until 1923 (e.g., Kronstadt Uprising, Tambov Rebellion, Basmachi Revolt, and the final resistance of the White movement in the Far East).
The May Fourth Movement helped to rekindle the then-fading cause of republican revolution. In 1917 Sun Yat-sen had become commander-in-chief of a rival military government in Guangzhou in collaboration with southern warlords. Sun's efforts to obtain aid from the Western democracies were ignored, however, and in 1920 he turned to the Soviet Union, which had recently achieved its own revolution. The Soviets sought to befriend the Chinese revolutionists by offering scathing attacks on Western imperialism. But for political expediency, the Soviet leadership initiated a dual policy of support for both Sun and the newly established Chinese Communist Party (CCP).
In North America, especially the first half of this period, people experienced considerable prosperity in the Roaring Twenties. The social and societal upheaval known as the Roaring Twenties began in North America and spread to Europe in the aftermath of World War I. The Roaring Twenties, often called "The Jazz Age", saw an exposition of social, artistic, and cultural dynamism. 'Normalcy' returned to politics, jazz music blossomed, the flapper redefined modern womanhood, Art Deco peaked. The spirit of the Roaring Twenties was marked by a general feeling of discontinuity associated with modernity, a break with traditions. Everything seemed to be feasible through modern technology. New technologies, especially automobiles, movies and radio proliferated 'modernity' to a large part of the population. The 1920s saw the general favor of practicality, in architecture as well as in daily life. The 1920s was further distinguished by several inventions and discoveries, extensive industrial growth and the rise in consumer demand and aspirations, and significant changes in lifestyle.
Europe spent these years rebuilding and coming to terms with the vast human cost of the conflict. The economy of the United States became increasingly intertwined with that of Europe. In Germany, the Weimar Republic gave way to episodes of political and economic turmoil, which culminated with the German hyperinflation of 1923 and the failed Beer Hall Putsch of that same year. When Germany could no longer afford war payments, Wall Street invested heavily in European debts to keep the European economy afloat as a large consumer market for American mass-produced goods. By the middle of the decade, economic development soared in Europe, and the Roaring Twenties broke out in Germany, Britain and France, the second half of the decade becoming known as the "Golden Twenties". In France and francophone Canada, they were also called the "années folles" ("Crazy Years").
Worldwide prosperity changed dramatically with the onset of the Great Depression in 1929. The Wall Street Crash of 1929 served to punctuate the end of the previous era, as The Great Depression set in. The Great Depression was a worldwide economic downturn starting in most places in 1929 and ending at different times in the 1930s or early 1940s for different countries. It was the largest and most important economic depression in the 20th century, and is used in the 21st century as an example of how far the world's economy can fall.
The depression had devastating effects in virtually every country, rich or poor. International trade plunged by half to two-thirds, as did personal income, tax revenue, prices and profits. Cities all around the world were hit hard, especially those dependent on heavy industry. Construction was virtually halted in many countries. Farming and rural areas suffered as crop prices fell by roughly 60 percent. Facing plummeting demand with few alternate sources of jobs, areas dependent on primary sector industries suffered the most.
The Great Depression ended at different times in different countries with the effect lasting into the next era. America's Great Depression ended in 1941 with America's entry into World War II. The majority of countries set up relief programs, and most underwent some sort of political upheaval, pushing them to the left or right. In some world states, the desperate citizens turned toward nationalist demagogues—the most infamous being Adolf Hitler—setting the stage for the next era of war. The convulsion brought on by the worldwide depression resulted in the rise of Nazism. In Asia, Japan became an ever more assertive power, especially with regards to China.
The interwar period was also marked by a radical change in the international order, away from the balance of power that had dominated pre–World War I Europe. One main institution that was meant to bring stability was the League of Nations, which was created after the First World War with the intention of maintaining world security and peace and encouraging economic growth between member countries. The League was undermined by the bellicosity of Nazi Germany, Imperial Japan, the Soviet Union, and Mussolini's Italy, and by the non-participation of the United States, leading many to question its effectiveness and legitimacy.
A series of international crises strained the League to its limits, the earliest being the invasion of Manchuria by Japan and the Abyssinian crisis of 1935/36 in which Italy invaded Abyssinia, one of the only free African nations at that time. The League tried to enforce economic sanctions upon Italy, but to no avail. The incident highlighted French and British weakness, exemplified by their reluctance to alienate Italy and lose her as their ally. The limited actions taken by the Western powers pushed Mussolini's Italy towards alliance with Hitler's Germany anyway. The Abyssinian war showed Hitler how weak the League was and encouraged the remilitarization of the Rhineland in flagrant disregard of the Treaty of Versailles. This was the first in a series of provocative acts culminating in the invasion of Poland in September 1939 and the beginning of the Second World War.
Few Chinese had any illusions about Japanese designs on China. Hungry for raw materials and pressed by a growing population, Japan initiated the seizure of Manchuria in September 1931 and established ex-Qing emperor Puyi as head of the puppet state of Manchukuo in 1932. During the Sino-Japanese War (1937–1945), the loss of Manchuria, and its vast potential for industrial development and war industries, was a blow to the Kuomintang economy. The League of Nations, established at the end of World War I, was unable to act in the face of the Japanese defiance. After 1940, conflicts between the Kuomintang and Communists became more frequent in the areas not under Japanese control. The Communists expanded their influence wherever opportunities presented themselves through mass organizations, administrative reforms, and the land- and tax-reform measures favoring the peasants—while the Kuomintang attempted to neutralize the spread of Communist influence.
The Second Sino-Japanese War had seen tensions rise between Imperial Japan and the United States; events such as the Panay incident and the Nanking Massacre turned American public opinion against Japan. With the occupation of French Indochina in the years of 1940–41, and with the continuing war in China, the United States placed embargoes on Japan of strategic materials such as scrap metal and oil, which were vitally needed for the war effort. The Japanese were faced with the option of either withdrawing from China and losing face or seizing and securing new sources of raw materials in the resource-rich, European-controlled colonies of South East Asia—specifically British Malaya and the Dutch East Indies (modern-day Indonesia). In 1940, Imperial Japan signed the Tripartite Pact with Nazi Germany and Fascist Italy.
On December 7, 1941, Japan attacked the United States at Pearl Harbor, bringing it too into the war on the Allied side. China also joined the Allies, as eventually did most of the rest of the world. China was in turmoil at the time, and attacked Japanese armies through guerilla-type warfare. By the beginning of 1942, the major combatants were aligned as follows: the British Commonwealth, the United States, and the Soviet Union were fighting Germany and Italy; and the British Commonwealth, China, and the United States were fighting Japan. The United Kingdom, the United States, the Soviet Union and China were referred as a "trusteeship of the powerful" during the World War II and were recognized as the Allied "Big Four" in Declaration by United Nations These four countries were considered as the "Four Policemen" or "Four Sheriffs" of the Allies power and primary victors of World War II. From then through August 1945, battles raged across all of Europe, in the North Atlantic Ocean, across North Africa, throughout Southeast Asia, throughout China, across the Pacific Ocean and in the air over Japan.
It is possible that around 62 million people died in the war; estimates vary greatly. About 60% of all casualties were civilians, who died as a result of disease, starvation, genocide (in particular, the Holocaust), and aerial bombing. The former Soviet Union and China suffered the most casualties. Estimates place deaths in the Soviet Union at around 23 million, while China suffered about 10 million. No country lost a greater portion of its population than Poland: approximately 5.6 million, or 16%, of its pre-war population of 34.8 million died.
The Holocaust (which roughly means "burnt whole") was the deliberate and systematic murder of millions of Jews and other "unwanted" during World War II by the Nazi regime in Germany. Several differing views exist regarding whether it was intended to occur from the war's beginning, or if the plans for it came about later. Regardless, persecution of Jews extended well before the war even started, such as in the Kristallnacht (Night of Broken Glass). The Nazis used propaganda to great effect to stir up anti-Semitic feelings within ordinary Germans.
After World War II, Europe was informally split into Western and Soviet spheres of influence. Western Europe later aligned as the North Atlantic Treaty Organization (NATO) and Eastern Europe as the Warsaw Pact. There was a shift in power from Western Europe and the British Empire to the two new superpowers, the United States and the Soviet Union. These two rivals would later face off in the Cold War. In Asia, the defeat of Japan led to its democratization. China's civil war continued through and after the war, resulting eventually in the establishment of the People's Republic of China. The former colonies of the European powers began their road to independence.
Over the course of the 20th century, the world's per-capita gross domestic product grew by a factor of five, much more than all earlier centuries combined (including the 19th with its Industrial Revolution). Many economists make the case that this understates the magnitude of growth, as many of the goods and services consumed at the end of the 20th century, such as improved medicine (causing world life expectancy to increase by more than two decades) and communications technologies, were not available at any price at its beginning. However, the gulf between the world's rich and poor grew wider, and the majority of the global population remained in the poor side of the divide.
Still, advancing technology and medicine has had a great impact even in the Global South. Large-scale industry and more centralized media made brutal dictatorships possible on an unprecedented scale in the middle of the century, leading to wars that were also unprecedented. However, the increased communications contributed to democratization. Technological developments included the development of airplanes and space exploration, nuclear technology, advancement in genetics, and the dawning of the Information Age.
The Soviet Union created the Eastern Bloc of countries that it occupied, annexing some as Soviet Socialist Republics and maintaining others as satellite states that would later form the Warsaw Pact. The United States and various western European countries began a policy of "containment" of communism and forged myriad alliances to this end, including NATO. Several of these western countries also coordinated efforts regarding the rebuilding of western Europe, including western Germany, which the Soviets opposed. In other regions of the world, such as Latin America and Southeast Asia, the Soviet Union fostered communist revolutionary movements, which the United States and many of its allies opposed and, in some cases, attempted to "roll back". Many countries were prompted to align themselves with the nations that would later form either NATO or the Warsaw Pact, though other movements would also emerge.
The Cold War saw periods of both heightened tension and relative calm. International crises arose, such as the Berlin Blockade (1948–1949), the Korean War (1950–1953), the Berlin Crisis of 1961, the Vietnam War (1959–1975), the Cuban Missile Crisis (1962), the Soviet war in Afghanistan (1979–1989) and NATO exercises in November 1983. There were also periods of reduced tension as both sides sought détente. Direct military attacks on adversaries were deterred by the potential for mutual assured destruction using deliverable nuclear weapons. In the Cold War era, the Generation of Love and the rise of computers changed society in very different, complex ways, including higher social and local mobility.
The Cold War drew to a close in the late 1980s and the early 1990s. The United States under President Ronald Reagan increased diplomatic, military, and economic pressure on the Soviet Union, which was already suffering from severe economic stagnation. In the second half of the 1980s, newly appointed Soviet leader Mikhail Gorbachev introduced the perestroika and glasnost reforms. The Soviet Union collapsed in 1991, leaving the United States as the dominant military power, though Russia retained much of the massive Soviet nuclear arsenal.
In Latin America in the 1970s, leftists acquired a significant political influence which prompted the right-wing, ecclesiastical authorities and a large portion of the individual country's upper class to support coup d'états to avoid what they perceived as a communist threat. This was further fueled by Cuban and United States intervention which led to a political polarization. Most South American countries were in some periods ruled by military dictatorships that were supported by the United States of America. In the 1970s, the regimes of the Southern Cone collaborated in Operation Condor killing many leftist dissidents, including some urban guerrillas. However, by the early 1990s all countries had restored their democracies.
The Space Age is a period encompassing the activities related to the Space Race, space exploration, space technology, and the cultural developments influenced by these events. The Space Age began with the development of several technologies that culminated with the launch of Sputnik 1 by the Soviet Union. This was the world's first artificial satellite, orbiting the Earth in 98.1 minutes and weighing in at 83 kg. The launch of Sputnik 1 ushered a new era of political, scientific and technological achievements that became known as the Space Age. The Space Age was characterized by rapid development of new technology in a close race mostly between the United States and the Soviet Union. The Space Age brought the first human spaceflight during the Vostok programme and reached its peak with the Apollo program which captured the imagination of much of the world's population. The landing of Apollo 11 was an event watched by over 500 million people around the world and is widely recognized as one of the defining moments of the 20th century. Since then and with the end of the space race due to the dissolution of the Soviet Union, public attention has largely moved to other areas.
The phrase "51st state" can be used in a positive sense, meaning that a region or territory is so aligned, supportive, and conducive with the United States, that it is like a U.S. state. It can also be used in a pejorative sense, meaning an area or region is perceived to be under excessive American cultural or military influence or control. In various countries around the world, people who believe their local or national culture has become too Americanized sometimes use the term "51st state" in reference to their own countries.
Under Article IV, Section Three of the United States Constitution, which outlines the relationship among the states, Congress has the power to admit new states to the union. The states are required to give "full faith and credit" to the acts of each other's legislatures and courts, which is generally held to include the recognition of legal contracts, marriages, and criminal judgments. The states are guaranteed military and civil defense by the federal government, which is also obliged by Article IV, Section Four, to "guarantee to every state in this union a republican form of government".
Puerto Rico has been discussed as a potential 51st state of the United States. In a 2012 status referendum a majority of voters, 54%, expressed dissatisfaction with the current political relationship. In a separate question, 61% of voters supported statehood (excluding the 26% of voters who left this question blank). On December 11, 2012, Puerto Rico's legislature resolved to request that the President and the U.S. Congress act on the results, end the current form of territorial status and begin the process of admitting Puerto Rico to the Union as a state.
Since 1898, Puerto Rico has had limited representation in the Congress in the form of a Resident Commissioner, a nonvoting delegate. The 110th Congress returned the Commissioner's power to vote in the Committee of the Whole, but not on matters where the vote would represent a decisive participation. Puerto Rico has elections on the United States presidential primary or caucus of the Democratic Party and the Republican Party to select delegates to the respective parties' national conventions although presidential electors are not granted on the Electoral College. As American citizens, Puerto Ricans can vote in U.S. presidential elections, provided they reside in one of the 50 states or the District of Columbia and not in Puerto Rico itself.
Residents of Puerto Rico pay U.S. federal taxes: import/export taxes, federal commodity taxes, social security taxes, therefore contributing to the American Government. Most Puerto Rico residents do not pay federal income tax but do pay federal payroll taxes (Social Security and Medicare). However, federal employees, those who do business with the federal government, Puerto Rico–based corporations that intend to send funds to the U.S. and others do pay federal income taxes. Puerto Ricans may enlist in the U.S. military. Puerto Ricans have participated in all American wars since 1898; 52 Puerto Ricans had been killed in the Iraq War and War in Afghanistan by November 2012.
Puerto Rico has been under U.S. sovereignty for over a century when it was ceded to the U.S. by Spain following the end of the Spanish–American War, and Puerto Ricans have been U.S. citizens since 1917. The island's ultimate status has not been determined as of 2012[update], its residents do not have voting representation in their federal government. Puerto Rico has limited representation in the U.S. Congress in the form of a Resident Commissioner, a delegate with limited no voting rights. Like the states, Puerto Rico has self-rule, a republican form of government organized pursuant to a constitution adopted by its people, and a bill of rights.
This constitution was created when the U.S. Congress directed local government to organize a constitutional convention to write the Puerto Rico Constitution in 1951. The acceptance of that constitution by Puerto Rico's electorate, the U.S. Congress, and the U.S. president occurred in 1952. In addition, the rights, privileges and immunities attendant to United States citizens are "respected in Puerto Rico to the same extent as though Puerto Rico were a state of the union" through the express extension of the Privileges and Immunities Clause of the U.S. Constitution by the U.S. Congress in 1948.
Puerto Rico is designated in its constitution as the "Commonwealth of Puerto Rico". The Constitution of Puerto Rico which became effective in 1952 adopted the name of Estado Libre Asociado (literally translated as "Free Associated State"), officially translated into English as Commonwealth, for its body politic. The island is under the jurisdiction of the Territorial Clause of the U.S. Constitution, which has led to doubts about the finality of the Commonwealth status for Puerto Rico. In addition, all people born in Puerto Rico become citizens of the U.S. at birth (under provisions of the Jones–Shafroth Act in 1917), but citizens residing in Puerto Rico cannot vote for president nor for full members of either house of Congress. Statehood would grant island residents full voting rights at the Federal level. The Puerto Rico Democracy Act (H.R. 2499) was approved on April 29, 2010, by the United States House of Representatives 223–169, but was not approved by the Senate before the end of the 111th Congress. It would have provided for a federally sanctioned self-determination process for the people of Puerto Rico. This act would provide for referendums to be held in Puerto Rico to determine the island's ultimate political status. It had also been introduced in 2007.
In November 2012, a referendum resulted in 54 percent of respondents voting to reject the current status under the territorial clause of the U.S. Constitution, while a second question resulted in 61 percent of voters identifying statehood as the preferred alternative to the current territorial status. The 2012 referendum was by far the most successful referendum for statehood advocates and support for statehood has risen in each successive popular referendum. However, more than one in four voters abstained from answering the question on the preferred alternative status. Statehood opponents have argued that the statehood option garnered only 45 percent of the votes if abstentions are included. If abstentions are considered, the result of the referendum is much closer to 44 percent for statehood, a number that falls under the 50 percent majority mark.
The Washington Post, The New York Times and the Boston Herald have published opinion pieces expressing support for the statehood of Puerto Rico. On November 8, 2012, Washington, D.C. newspaper The Hill published an article saying that Congress will likely ignore the results of the referendum due to the circumstances behind the votes. and U.S. Congressman Luis Gutiérrez U.S. Congresswoman Nydia Velázquez, both of Puerto Rican ancestry, agreed with the The Hill 's statements. Shortly after the results were published Puerto Rico-born U.S. Congressman José Enrique Serrano commented "I was particularly impressed with the outcome of the 'status' referendum in Puerto Rico. A majority of those voting signaled the desire to change the current territorial status. In a second question an even larger majority asked to become a state. This is an earthquake in Puerto Rican politics. It will demand the attention of Congress, and a definitive answer to the Puerto Rican request for change. This is a history-making moment where voters asked to move forward."
Several days after the referendum, the Resident Commissioner Pedro Pierluisi, Governor Luis Fortuño, and Governor-elect Alejandro García Padilla wrote separate letters to the President of the United States Barack Obama addressing the results of the voting. Pierluisi urged Obama to begin legislation in favor of the statehood of Puerto Rico, in light of its win in the referendum. Fortuño urged him to move the process forward. García Padilla asked him to reject the results because of their ambiguity. The White House stance related to the November 2012 plebiscite was that the results were clear, the people of Puerto Rico want the issue of status resolved, and a majority chose statehood in the second question. Former White House director of Hispanic media stated, "Now it is time for Congress to act and the administration will work with them on that effort, so that the people of Puerto Rico can determine their own future."
On May 15, 2013, Resident Commissioner Pierluisi introduced H.R. 2000 to Congress to "set forth the process for Puerto Rico to be admitted as a state of the Union," asking for Congress to vote on ratifying Puerto Rico as the 51st state. On February 12, 2014, Senator Martin Heinrich introduced a bill in the US Senate. The bill would require a binding referendum to be held in Puerto Rico asking whether the territory wants to be admitted as a state. In the event of a yes vote, the president would be asked to submit legislation to Congress to admit Puerto Rico as a state.
Washington, D.C. is often mentioned as a candidate for statehood. In Federalist No. 43 of The Federalist Papers, James Madison considered the implications of the definition of the "seat of government" found in the United States Constitution. Although he noted potential conflicts of interest, and the need for a "municipal legislature for local purposes," Madison did not address the district's role in national voting. Legal scholars disagree on whether a simple act of Congress can admit the District as a state, due to its status as the seat of government of the United States, which Article I, Section 8 of the Constitution requires to be under the exclusive jurisdiction of Congress; depending on the interpretation of this text, admission of the full District as a state may require a Constitutional amendment, which is much more difficult to enact. However, the Constitution does not set a minimum size for the District. Its size has already changed once before, when Virginia reclaimed the portion of the District south of the Potomac. So the constitutional requirement for a federal district can be satisfied by reducing its size to the small central core of government buildings and monuments, giving the rest of the territory to the new state.
Washington, D.C. residents who support the statehood movement sometimes use a shortened version of the Revolutionary War protest motto "No taxation without representation", omitting the initial "No", denoting their lack of Congressional representation; the phrase is now printed on newly issued Washington, D.C. license plates (although a driver may choose to have the Washington, D.C. website address instead). President Bill Clinton's presidential limousine had the "Taxation without representation" license plate late in his term, while President George W. Bush had the vehicle's plates changed shortly after beginning his term in office. President Barack Obama had the license plates changed back to the protest style at the beginning of his second term.
This position was carried by the D.C. Statehood Party, a political party; it has since merged with the local Green Party affiliate to form the D.C. Statehood Green Party. The nearest this movement ever came to success was in 1978, when Congress passed the District of Columbia Voting Rights Amendment. Two years later in 1980, local citizens passed an initiative calling for a constitutional convention for a new state. In 1982, voters ratified the constitution of the state, which was to be called New Columbia. The drive for statehood stalled in 1985, however, when the Washington, D.C. Voting Rights Amendment failed because not enough states ratified the amendment within the seven-year span specified.
Other less likely contenders are Guam and the United States Virgin Islands, both of which are unincorporated organized territories of the United States. Also, the Northern Mariana Islands and American Samoa, an unorganized, unincorporated territory, could both attempt to gain statehood. Some proposals call for the Virgin Islands to be admitted with Puerto Rico as one state (often known as the proposed "Commonwealth of Prusvi", for Puerto Rico/U.S. Virgin Islands, or as "Puerto Virgo"), and for the amalgamation of U.S. territories or former territories in the Pacific Ocean, in the manner of the "Greater Hawaii" concept of the 1960s. Guam and the Northern Mariana Islands would be admitted as one state, along with Palau, the Federated States of Micronesia, and the Marshall Islands (although these latter three entities are now separate sovereign nations, which have Compact of Free Association relationships with the United States). Such a state would have a population of 412,381 (slightly lower than Wyoming's population) and a land area of 911.82 square miles (2,361.6 km2) (slightly smaller than Rhode Island). American Samoa could possibly be part of such a state, increasing the population to 467,900 and the area to 988.65 square miles (2,560.6 km2). Radio Australia, in late May 2008, issued signs of Guam and the Northern Mariana Islands becoming one again and becoming the 51st state.
The Philippines has had small grassroots movements for U.S. statehood. Originally part of the platform of the Progressive Party, then known as the Federalista Party, the party dropped it in 1907, which coincided with the name change. As recently as 2004, the concept of the Philippines becoming a U.S. state has been part of a political platform in the Philippines. Supporters of this movement include Filipinos who believe that the quality of life in the Philippines would be higher and that there would be less poverty there if the Philippines were an American state or territory. Supporters also include Filipinos that had fought as members of the United States Armed Forces in various wars during the Commonwealth period.
In Canada, "the 51st state" is a phrase generally used in such a way as to imply that if a certain political course is taken, Canada's destiny will be as little more than a part of the United States. Examples include the Canada-United States Free Trade Agreement in 1988, the debate over the creation of a common defense perimeter, and as a potential consequence of not adopting proposals intended to resolve the issue of Quebec sovereignty, the Charlottetown Accord in 1992 and the Clarity Act in 1999.
The phrase is usually used in local political debates, in polemic writing or in private conversations. It is rarely used by politicians themselves in a public context, although at certain times in Canadian history political parties have used other similarly loaded imagery. In the 1988 federal election, the Liberals asserted that the proposed Free Trade Agreement amounted to an American takeover of Canada—notably, the party ran an ad in which Progressive Conservative (PC) strategists, upon the adoption of the agreement, slowly erased the Canada-U.S. border from a desktop map of North America. Within days, however, the PCs responded with an ad which featured the border being drawn back on with a permanent marker, as an announcer intoned "Here's where we draw the line."
The implication has historical basis and dates to the breakup of British America during the American Revolution. The colonies that had confederated to form the United States invaded Canada (at the time a term referring specifically to the modern-day provinces of Quebec and Ontario, which had only been in British hands since 1763) at least twice, neither time succeeding in taking control of the territory. The first invasion was during the Revolution, under the assumption that French-speaking Canadians' presumed hostility towards British colonial rule combined with the Franco-American alliance would make them natural allies to the American cause; the Continental Army successfully recruited two Canadian regiments for the invasion. That invasion's failure forced the members of those regiments into exile, and they settled mostly in upstate New York. The Articles of Confederation, written during the Revolution, included a provision for Canada to join the United States, should they ever decide to do so, without needing to seek U.S. permission as other states would. The United States again invaded Canada during the War of 1812, but this effort was made more difficult due to the large number of Loyalist Americans that had fled to what is now Ontario and still resisted joining the republic. The Hunter Patriots in the 1830s and the Fenian raids after the American Civil War were private attacks on Canada from the U.S. Several U.S. politicians in the 19th century also spoke in favour of annexing Canada.
In the late 1940s, during the last days of the Dominion of Newfoundland (at the time a dominion-dependency in the Commonwealth and independent of Canada), there was mainstream support, although not majority, for Newfoundland to form an economic union with the United States, thanks to the efforts of the Economic Union Party and significant U.S. investment in Newfoundland stemming from the U.S.-British alliance in World War II. The movement ultimately failed when, in a 1948 referendum, voters narrowly chose to confederate with Canada (the Economic Union Party supported an independent "responsible government" that they would then push toward their goals).
In the United States, the term "the 51st state" when applied to Canada can serve to highlight the similarities and close relationship between the United States and Canada. Sometimes the term is used disparagingly, intended to deride Canada as an unimportant neighbor. In the Quebec general election, 1989, the political party Parti 51 ran 11 candidates on a platform of Quebec seceding from Canada to join the United States (with its leader, André Perron, claiming Quebec could not survive as an independent nation). The party attracted just 3,846 votes across the province, 0.11% of the total votes cast. In comparison, the other parties in favour of sovereignty of Quebec in that election got 40.16% (PQ) and 1.22% (NPDQ).
Due to geographical proximity of the Central American countries to the U.S. which has powerful military, economic, and political influences, there were several movements and proposals by the United States during the 19th and 20th centuries to annex some or all of the Central American republics (Costa Rica, El Salvador, Guatemala, Honduras with the formerly British-ruled Bay Islands, Nicaragua, Panama which had the U.S.-ruled Canal Zone territory from 1903 to 1979, and formerly British Honduras or Belize since 1981). However, the U.S. never acted on these proposals from some U.S. politicians; some of which were never delivered or considered seriously. In 2001, El Salvador adopted the U.S. dollar as its currency, while Panama has used it for decades due to its ties to the Canal Zone.
Cuba, like many Spanish territories, wanted to break free from Spain. A pro-independence movement in Cuba was supported by the U.S., and Cuban guerrilla leaders wanted annexation to the United States, but Cuban revolutionary leader José Martí called for Cuban nationhood. When the U.S. battleship Maine sank in Havana Harbor, the U.S. blamed Spain and the Spanish–American War broke out in 1898. After the U.S. won, Spain relinquished claim of sovereignty over territories, including Cuba. The U.S. administered Cuba as a protectorate until 1902. Several decades later in 1959, the corrupt Cuban government of U.S.-backed Fulgencio Batista was overthrown by Fidel Castro. Castro installed a Marxist–Leninist government allied with the Soviet Union, which has been in power ever since.
Several websites assert that Israel is the 51st state due to the annual funding and defense support it receives from the United States. An example of this concept can be found in 2003 when Martine Rothblatt published a book called Two Stars for Peace that argued for the addition of Israel and the Palestinian territories surrounding it as the 51st state in the Union. The American State of Canaan, is a book published by Prof. Alfred de Grazia, political science and sociologist, in March 2009, proposing the creation of the 51st and 52nd states from Israel and the Palestinian territories.
In Article 3 of the Treaty of San Francisco between the Allied Powers and Japan, which came into force in April 1952, the U.S. put the outlying islands of the Ryukyus, including the island of Okinawa—home to over 1 million Okinawans related to the Japanese—and the Bonin Islands, the Volcano Islands, and Iwo Jima into U.S. trusteeship. All these trusteeships were slowly returned to Japanese rule. Okinawa was returned on May 15, 1972, but the U.S. stations troops in the island's bases as a defense for Japan.
In 2010 there was an attempt to register a 51st State Party with the New Zealand Electoral Commission. The party advocates New Zealand becoming the 51st state of the United States of America. The party's secretary is Paulus Telfer, a former Christchurch mayoral candidate. On February 5, 2010, the party applied to register a logo with the Electoral Commission. The logo – a US flag with 51 stars – was rejected by the Electoral Commission on the grounds that it was likely to cause confusion or mislead electors. As of 2014[update], the party remains unregistered and cannot appear on a ballot.
Albania has often been called the 51st state for its perceived strongly pro-American positions, mainly because of the United States' policies towards it. In reference to President George W. Bush's 2007 European tour, Edi Rama, Tirana's mayor and leader of the opposition Socialists, said: "Albania is for sure the most pro-American country in Europe, maybe even in the world ... Nowhere else can you find such respect and hospitality for the President of the United States. Even in Michigan, he wouldn't be as welcome." At the time of ex-Secretary of State James Baker's visit in 1992, there was even a move to hold a referendum declaring the country as the 51st American state. In addition to Albania, Kosovo which is predominately Albanian is seen as a 51st state due to the heavily presence and influence of the United States. The US has had troops and the largest base outside US territory, Camp Bondsteel in the territory since 1999.
During World War II, when Denmark was occupied by Nazi Germany, the United States briefly controlled Greenland for battlefields and protection. In 1946, the United States offered to buy Greenland from Denmark for $100 million ($1.2 billion today) but Denmark refused to sell it. Several politicians and others have in recent years argued that Greenland could hypothetically be in a better financial situation as a part of the United States; for instance mentioned by professor Gudmundur Alfredsson at University of Akureyri in 2014. One of the actual reasons behind US interest in Greenland could be the vast natural resources of the island. According to Wikileaks, the U.S. appears to be highly interested in investing in the resource base of the island and in tapping the vast expected hydrocarbons off the Greenlandic coast.
Poland has historically been staunchly pro-American, dating back to General Tadeusz Kościuszko and Casimir Pulaski's involvement in the American Revolution. This pro-American stance was reinforced following favorable American intervention in World War I (leading to the creation of an independent Poland) and the Cold War (culminating in a Polish state independent of Soviet influence). Poland contributed a large force to the "Coalition of the Willing" in Iraq. A quote referring to Poland as "the 51st state" has been attributed to James Pavitt, then Central Intelligence Agency Deputy Director for Operations, especially in connection to extraordinary rendition.
The Party of Reconstruction in Sicily, which claimed 40,000 members in 1944, campaigned for Sicily to be admitted as a U.S. state. This party was one of several Sicilian separatist movements active after the downfall of Italian Fascism. Sicilians felt neglected or underrepresented by the Italian government after the annexation of 1861 that ended the rule of the Kingdom of the Two Sicilies based in Naples. The large population of Sicilians in America and the American-led Allied invasion of Sicily in July–August 1943 may have contributed to the sentiment.
There are four categories of terra nullius, land that is unclaimed by any state: the small unclaimed territory of Bir Tawil between Egypt and Sudan, Antarctica, the oceans, and celestial bodies such as the Moon or Mars. In the last three of these, international treaties (the Antarctic Treaty, the United Nations Convention on the Law of the Sea, and the Outer Space Treaty respectively) prevent colonization and potential statehood of any of these uninhabited (and, given current technology, not permanently inhabitable) territories.
An antenna (plural antennae or antennas), or aerial, is an electrical device which converts electric power into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter supplies an electric current oscillating at radio frequency (i.e. a high frequency alternating current (AC)) to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to be amplified.
Typically an antenna consists of an arrangement of metallic conductors (elements), electrically connected (often through a transmission line) to the receiver or transmitter. An oscillating current of electrons forced through the antenna by a transmitter will create an oscillating magnetic field around the antenna elements, while the charge of the electrons also creates an oscillating electric field along the elements. These time-varying fields radiate away from the antenna into space as a moving transverse electromagnetic field wave. Conversely, during reception, the oscillating electric and magnetic fields of an incoming radio wave exert force on the electrons in the antenna elements, causing them to move back and forth, creating oscillating currents in the antenna.
The words antenna (plural: antennas in US English, although both "antennas" and "antennae" are used in International English) and aerial are used interchangeably. Occasionally the term "aerial" is used to mean a wire antenna. However, note the important international technical journal, the IEEE Transactions on Antennas and Propagation. In the United Kingdom and other areas where British English is used, the term aerial is sometimes used although 'antenna' has been universal in professional use for many years.
The origin of the word antenna relative to wireless apparatus is attributed to Italian radio pioneer Guglielmo Marconi. In the summer of 1895, Marconi began testing his wireless system outdoors on his father's estate near Bologna and soon began to experiment with long wire "aerials". Marconi discovered that by raising the "aerial" wire above the ground and connecting the other side of his transmitter to ground, the transmission range was increased. Soon he was able to transmit signals over a hill, a distance of approximately 2.4 kilometres (1.5 mi). In Italian a tent pole is known as l'antenna centrale, and the pole with the wire was simply called l'antenna. Until then wireless radiating transmitting and receiving elements were known simply as aerials or terminals.
Antennas are required by any radio receiver or transmitter to couple its electrical connection to the electromagnetic field. Radio waves are electromagnetic waves which carry signals through the air (or through space) at the speed of light with almost no transmission loss. Radio transmitters and receivers are used to convey signals (information) in systems including broadcast (audio) radio, television, mobile telephones, Wi-Fi (WLAN) data networks, trunk lines and point-to-point communications links (telephone, data networks), satellite links, many remote controlled devices such as garage door openers, and wireless remote sensors, among many others. Radio waves are also used directly for measurements in technologies including radar, GPS, and radio astronomy. In each and every case, the transmitters and receivers involved require antennas, although these are sometimes hidden (such as the antenna inside an AM radio or inside a laptop computer equipped with Wi-Fi).
One example of omnidirectional antennas is the very common vertical antenna or whip antenna consisting of a metal rod (often, but not always, a quarter of a wavelength long). A dipole antenna is similar but consists of two such conductors extending in opposite directions, with a total length that is often, but not always, a half of a wavelength long. Dipoles are typically oriented horizontally in which case they are weakly directional: signals are reasonably well radiated toward or received from all directions with the exception of the direction along the conductor itself; this region is called the antenna blind cone or null.
Both the vertical and dipole antennas are simple in construction and relatively inexpensive. The dipole antenna, which is the basis for most antenna designs, is a balanced component, with equal but opposite voltages and currents applied at its two terminals through a balanced transmission line (or to a coaxial transmission line through a so-called balun). The vertical antenna, on the other hand, is a monopole antenna. It is typically connected to the inner conductor of a coaxial transmission line (or a matching network); the shield of the transmission line is connected to ground. In this way, the ground (or any large conductive surface) plays the role of the second conductor of a dipole, thereby forming a complete circuit. Since monopole antennas rely on a conductive ground, a so-called grounding structure may be employed to provide a better ground contact to the earth or which itself acts as a ground plane to perform that function regardless of (or in absence of) an actual contact with the earth.
Antennas more complex than the dipole or vertical designs are usually intended to increase the directivity and consequently the gain of the antenna. This can be accomplished in many different ways leading to a plethora of antenna designs. The vast majority of designs are fed with a balanced line (unlike a monopole antenna) and are based on the dipole antenna with additional components (or elements) which increase its directionality. Antenna "gain" in this instance describes the concentration of radiated power into a particular solid angle of space, as opposed to the spherically uniform radiation of the ideal radiator. The increased power in the desired direction is at the expense of that in the undesired directions. Power is conserved, and there is no net power increase over that delivered from the power source (the transmitter.)
For instance, a phased array consists of two or more simple antennas which are connected together through an electrical network. This often involves a number of parallel dipole antennas with a certain spacing. Depending on the relative phase introduced by the network, the same combination of dipole antennas can operate as a "broadside array" (directional normal to a line connecting the elements) or as an "end-fire array" (directional along the line connecting the elements). Antenna arrays may employ any basic (omnidirectional or weakly directional) antenna type, such as dipole, loop or slot antennas. These elements are often identical.
However a log-periodic dipole array consists of a number of dipole elements of different lengths in order to obtain a somewhat directional antenna having an extremely wide bandwidth: these are frequently used for television reception in fringe areas. The dipole antennas composing it are all considered "active elements" since they are all electrically connected together (and to the transmission line). On the other hand, a superficially similar dipole array, the Yagi-Uda Antenna (or simply "Yagi"), has only one dipole element with an electrical connection; the other so-called parasitic elements interact with the electromagnetic field in order to realize a fairly directional antenna but one which is limited to a rather narrow bandwidth. The Yagi antenna has similar looking parasitic dipole elements but which act differently due to their somewhat different lengths. There may be a number of so-called "directors" in front of the active element in the direction of propagation, and usually a single (but possibly more) "reflector" on the opposite side of the active element.
At low frequencies (such as AM broadcast), arrays of vertical towers are used to achieve directionality and they will occupy large areas of land. For reception, a long Beverage antenna can have significant directivity. For non directional portable use, a short vertical antenna or small loop antenna works well, with the main design challenge being that of impedance matching. With a vertical antenna a loading coil at the base of the antenna may be employed to cancel the reactive component of impedance; small loop antennas are tuned with parallel capacitors for this purpose.
An antenna lead-in is the transmission line (or feed line) which connects the antenna to a transmitter or receiver. The antenna feed may refer to all components connecting the antenna to the transmitter or receiver, such as an impedance matching network in addition to the transmission line. In a so-called aperture antenna, such as a horn or parabolic dish, the "feed" may also refer to a basic antenna inside the entire system (normally at the focus of the parabolic dish or at the throat of a horn) which could be considered the one active element in that antenna system. A microwave antenna may also be fed directly from a waveguide in place of a (conductive) transmission line.
Monopole antennas consist of a single radiating element such as a metal rod, often mounted over a conducting surface, a ground plane. One side of the feedline from the receiver or transmitter is connected to the rod, and the other side to the ground plane, which may be the Earth. The most common form is the quarter-wave monopole which is one-quarter of a wavelength long and has a gain of 5.12 dBi when mounted over a ground plane. Monopoles have an omnidirectional radiation pattern, so they are used for broad coverage of an area, and have vertical polarization. The ground waves used for broadcasting at low frequencies must be vertically polarized, so large vertical monopole antennas are used for broadcasting in the MF, LF, and VLF bands. Small monopoles are used as nondirectional antennas on portable radios in the HF, VHF, and UHF bands.
The most widely used class of antenna, a dipole antenna consists of two symmetrical radiators such as metal rods or wires, with one side of the balanced feedline from the transmitter or receiver attached to each. A horizontal dipole radiates in two lobes perpendicular to the antenna's axis. A half-wave dipole the most common type, has two collinear elements each a quarter wavelength long and a gain of 2.15 dBi. Used individually as low gain antennas, dipoles are also used as driven elements in many more complicated higher gain types of antennas.
A necessary condition for the aforementioned reciprocity property is that the materials in the antenna and transmission medium are linear and reciprocal. Reciprocal (or bilateral) means that the material has the same response to an electric current or magnetic field in one direction, as it has to the field or current in the opposite direction. Most materials used in antennas meet these conditions, but some microwave antennas use high-tech components such as isolators and circulators, made of nonreciprocal materials such as ferrite. These can be used to give the antenna a different behavior on receiving than it has on transmitting, which can be useful in applications like radar.
Antennas are characterized by a number of performance measures which a user would be concerned with in selecting or designing an antenna for a particular application. Chief among these relate to the directional characteristics (as depicted in the antenna's radiation pattern) and the resulting gain. Even in omnidirectional (or weakly directional) antennas, the gain can often be increased by concentrating more of its power in the horizontal directions, sacrificing power radiated toward the sky and ground. The antenna's power gain (or simply "gain") also takes into account the antenna's efficiency, and is often the primary figure of merit.
Resonant antennas are expected to be used around a particular resonant frequency; an antenna must therefore be built or ordered to match the frequency range of the intended application. A particular antenna design will present a particular feedpoint impedance. While this may affect the choice of an antenna, an antenna's impedance can also be adapted to the desired impedance level of a system using a matching network while maintaining the other characteristics (except for a possible loss of efficiency).
An antenna transmits and receives radio waves with a particular polarization which can be reoriented by tilting the axis of the antenna in many (but not all) cases. The physical size of an antenna is often a practical issue, particularly at lower frequencies (longer wavelengths). Highly directional antennas need to be significantly larger than the wavelength. Resonant antennas usually use a linear conductor (or element), or pair of such elements, each of which is about a quarter of the wavelength in length (an odd multiple of quarter wavelengths will also be resonant). Antennas that are required to be small compared to the wavelength sacrifice efficiency and cannot be very directional. Fortunately at higher frequencies (UHF, microwaves) trading off performance to obtain a smaller physical size is usually not required.
The majority of antenna designs are based on the resonance principle. This relies on the behaviour of moving electrons, which reflect off surfaces where the dielectric constant changes, in a fashion similar to the way light reflects when optical properties change. In these designs, the reflective surface is created by the end of a conductor, normally a thin metal wire or rod, which in the simplest case has a feed point at one end where it is connected to a transmission line. The conductor, or element, is aligned with the electrical field of the desired signal, normally meaning it is perpendicular to the line from the antenna to the source (or receiver in the case of a broadcast antenna).
The radio signal's electrical component induces a voltage in the conductor. This causes an electrical current to begin flowing in the direction of the signal's instantaneous field. When the resulting current reaches the end of the conductor, it reflects, which is equivalent to a 180 degree change in phase. If the conductor is 1⁄4 of a wavelength long, current from the feed point will undergo 90 degree phase change by the time it reaches the end of the conductor, reflect through 180 degrees, and then another 90 degrees as it travels back. That means it has undergone a total 360 degree phase change, returning it to the original signal. The current in the element thus adds to the current being created from the source at that instant. This process creates a standing wave in the conductor, with the maximum current at the feed.
The ordinary half-wave dipole is probably the most widely used antenna design. This consists of two 1⁄4-wavelength elements arranged end-to-end, and lying along essentially the same axis (or collinear), each feeding one side of a two-conductor transmission wire. The physical arrangement of the two elements places them 180 degrees out of phase, which means that at any given instant one of the elements is driving current into the transmission line while the other is pulling it out. The monopole antenna is essentially one half of the half-wave dipole, a single 1⁄4-wavelength element with the other side connected to ground or an equivalent ground plane (or counterpoise). Monopoles, which are one-half the size of a dipole, are common for long-wavelength radio signals where a dipole would be impractically large. Another common design is the folded dipole, which is essentially two dipoles placed side-by-side and connected at their ends to make a single one-wavelength antenna.
The standing wave forms with this desired pattern at the design frequency, f0, and antennas are normally designed to be this size. However, feeding that element with 3f0 (whose wavelength is 1⁄3 that of f0) will also lead to a standing wave pattern. Thus, an antenna element is also resonant when its length is 3⁄4 of a wavelength. This is true for all odd multiples of 1⁄4 wavelength. This allows some flexibility of design in terms of antenna lengths and feed points. Antennas used in such a fashion are known to be harmonically operated.
The quarter-wave elements imitate a series-resonant electrical element due to the standing wave present along the conductor. At the resonant frequency, the standing wave has a current peak and voltage node (minimum) at the feed. In electrical terms, this means the element has minimum reactance, generating the maximum current for minimum voltage. This is the ideal situation, because it produces the maximum output for the minimum input, producing the highest possible efficiency. Contrary to an ideal (lossless) series-resonant circuit, a finite resistance remains (corresponding to the relatively small voltage at the feed-point) due to the antenna's radiation resistance as well as any actual electrical losses.
It is possible to use the impedance matching concepts to construct vertical antennas substantially shorter than the 1⁄4 wavelength at which the antenna is resonant. By adding an inductance in series with the antenna, a so-called loading coil, the capacitive reactance of this antenna can be cancelled leaving a pure resistance which can then be matched to the transmission line. Sometimes the resulting resonant frequency of such a system (antenna plus matching network) is described using the construct of electrical length and the use of a shorter antenna at a lower frequency than its resonant frequency is termed electrical lengthening.
The end result is that the resonant antenna will efficiently feed a signal into the transmission line only when the source signal's frequency is close to that of the design frequency of the antenna, or one of the resonant multiples. This makes resonant antenna designs inherently narrowband, and they are most commonly used with a single target signal. They are particularly common on radar systems, where the same antenna is used for both broadcast and reception, or for radio and television broadcasts, where the antenna is working with a single frequency. They are less commonly used for reception where multiple channels are present, in which case additional modifications are used to increase the bandwidth, or entirely different antenna designs are used.
The amount of signal received from a distant transmission source is essentially geometric in nature due to the inverse square law, and this leads to the concept of effective area. This measures the performance of an antenna by comparing the amount of power it generates to the amount of power in the original signal, measured in terms of the signal's power density in Watts per square metre. A half-wave dipole has an effective area of 0.13 2. If more performance is needed, one cannot simply make the antenna larger. Although this would intercept more energy from the signal, due to the considerations above, it would decrease the output significantly due to it moving away from the resonant length. In roles where higher performance is needed, designers often use multiple elements combined together.
Returning to the basic concept of current flows in a conductor, consider what happens if a half-wave dipole is not connected to a feed point, but instead shorted out. Electrically this forms a single 1⁄2-wavelength element. But the overall current pattern is the same; the current will be zero at the two ends, and reach a maximum in the center. Thus signals near the design frequency will continue to create a standing wave pattern. Any varying electrical current, like the standing wave in the element, will radiate a signal. In this case, aside from resistive losses in the element, the rebroadcast signal will be significantly similar to the original signal in both magnitude and shape. If this element is placed so its signal reaches the main dipole in-phase, it will reinforce the original signal, and increase the current in the dipole. Elements used in this way are known as passive elements.
A Yagi-Uda array uses passive elements to greatly increase gain. It is built along a support boom that is pointed toward the signal, and thus sees no induced signal and does not contribute to the antenna's operation. The end closer to the source is referred to as the front. Near the rear is a single active element, typically a half-wave dipole or folded dipole. Passive elements are arranged in front (directors) and behind (reflectors) the active element along the boom. The Yagi has the inherent quality that it becomes increasingly directional, and thus has higher gain, as the number of elements increases. However, this also makes it increasingly sensitive to changes in frequency; if the signal frequency changes, not only does the active element receive less energy directly, but all of the passive elements adding to that signal also decrease their output as well and their signals no longer reach the active element in-phase.
It is also possible to use multiple active elements and combine them together with transmission lines to produce a similar system where the phases add up to reinforce the output. The antenna array and very similar reflective array antenna consist of multiple elements, often half-wave dipoles, spaced out on a plane and wired together with transmission lines with specific phase lengths to produce a single in-phase signal at the output. The log-periodic antenna is a more complex design that uses multiple in-line elements similar in appearance to the Yagi-Uda but using transmission lines between the elements to produce the output.
Reflection of the original signal also occurs when it hits an extended conductive surface, in a fashion similar to a mirror. This effect can also be used to increase signal through the use of a reflector, normally placed behind the active element and spaced so the reflected signal reaches the element in-phase. Generally the reflector will remain highly reflective even if it is not solid; gaps less than 1⁄10 generally have little effect on the outcome. For this reason, reflectors often take the form of wire meshes or rows of passive elements, which makes them lighter and less subject to wind. The parabolic reflector is perhaps the best known example of a reflector-based antenna, which has an effective area far greater than the active element alone.
Another extreme case of impedance matching occurs when using a small loop antenna (usually, but not always, for receiving) at a relatively low frequency where it appears almost as a pure inductor. Resonating such an inductor with a capacitor at the frequency of operation not only cancels the reactance but greatly magnifies the very small radiation resistance of such a loop.[citation needed] This is implemented in most AM broadcast receivers, with a small ferrite loop antenna resonated by a capacitor which is varied along with the receiver tuning in order to maintain resonance over the AM broadcast band
Antenna tuning generally refers to cancellation of any reactance seen at the antenna terminals, leaving only a resistive impedance which might or might not be exactly the desired impedance (that of the transmission line). Although an antenna may be designed to have a purely resistive feedpoint impedance (such as a dipole 97% of a half wavelength long) this might not be exactly true at the frequency that it is eventually used at. In some cases the physical length of the antenna can be "trimmed" to obtain a pure resistance. On the other hand, the addition of a series inductance or parallel capacitance can be used to cancel a residual capacitative or inductive reactance, respectively.

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