The first smart antennas were developed for military communications and intelligence gathering. The growth of cellular telephone in the 1980s attracted interest in commercial applications. The upgrade to digital radio technology in the mobile phone, indoor wireless network, and satellite broadcasting industries created new opportunities for smart antennas in the 1990s, culminating in the development of the MIMO (multiple-input multiple-output) technology used in 4G wireless networks.
Directional antennas The earliest success at tracking and controlling wireless signals relied on the antennas’ physical configuration and motion. The German inventor and physicist Karl F. Braun demonstrated beamforming for the first time in 1905. Braun created a phased array by positioning three antennas to reinforce radiation in one direction and diminish radiation in other directions. Guglielmo Marconi experimented with directional antennas in 1906. Directional antennas were rotated to detect and track enemy forces during World War I. The British admiralty used goniometers (radio compasses) to track the German fleet. Edwin H. Armstrong invented the superheterodyne receiver to detect the high frequency noise generated by German warplanes’ ignition systems. The war ended before Armstrong's creation was ready to help direct antiaircraft fire. Multiple elements (a fed dipole, a director, and reflectors) were assembled in the 1920s to create narrow transmit and receive antenna patterns. The Yagi-Uda array, better known as the Yagi antenna, is still widely used. Edmond Bruce and Harald T. Friis developed directional antennas for shortwave and microwave frequencies during the 1930s. AT&T's decision to use microwave to carry inter-city telephone traffic led to the first large-scale commercial deployment of directional antennas (based on Friis’ horn reflector design) in 1947. Directional antennas with alternating polarization enabled a single pair of frequencies to be reused over many consecutive hops. Microwave links are less expensive to deploy and maintain than coaxial cable links.
Phased array radar The first mechanically scanned phased array radar (using a rotating Yagi antenna) was demonstrated in the 1930s. The first electronically scanned radars used electromechanical devices (such as mechanical tuners or switches) to steer the antenna's beam. Germany built the Wullenweber circular array for direction finding during the early years of World War II. The Wullenweber could electronically scan the horizon 360° and determine the direction of any signal with reasonably good accuracy. Circular arrays were enhanced during the Cold War for eavesdropping purposes. The American Physicist Luis Walter Alvarez developed the first ground-controlled approach (GCA) system for landing aircraft in bad weather based on an electronically steered microwave phased array antenna. Alvarez tested and deployed the system in England in 1943. Near the end of the war, Germany's GEMA built an early warning phased array radar system (the PESA Mammut 1) to detect targets up to 300 km away. The polyrod fire control antenna was developed by Bell Laboratories in 1947 using cascaded phase shifters controlled by a rotary switch (spinning at ten revolutions per second) to create a continuous scanning beam. A major push to meet national security response time and coverage requirements called for the development of an all-electronic steerable planar phased array radar. The USSR's launch of Sputnik in 1957 suggested the need for ground-based satellite surveillance systems. Bendix Corporation responded by building its Electronically Steerable Array Radar (ESAR) in 1960. Enhanced beamforming techniques, such as multiple-beam Butler matrices, were developed for detecting and tracking objects in space. The launch of Explorer 1 by the United States in 1958 suggested another application: space-based radar systems for detecting and tracking aircraft, ships, armored vehicles, ballistic missiles, and cruise missiles. These systems required the development of special techniques for canceling the radar clutter seen from space, nulling ground-based jammers, and compensating for Doppler shifts experienced by fast-moving satellites. Space-based radar systems spurred the development of smaller, lighter weight, and less costly components: monolithic microwave integrated circuits (MMICs) for operation at frequencies in the 1 GHz to 30 GHz (microwave) and 30 GHz to 300 GHz (millimeter wave) ranges. The high power levels needed for detection are easier to achieve at microwave frequencies. The narrow beams required for high resolution target tracking are best achieved at millimeter wave frequencies. Companies such as Texas Instruments, Raytheon, RCA, Westinghouse, General Electric, and Hughes Electronics participated in the early development of MMICs. The first all-solid state radar was built for the United States Marines in 1972 by General Electric. It was a mobile 3-D radar system with its array mounted on a rotating platform for scanning the horizon. The first all-solid state phased array radar was the PAVE PAWS (precision acquisition vehicle entry - phased array warning system) UHF radar built in 1978 for the United States Air Force. Phased array antennas are also used in radio astronomy. Karl Jansky, discoverer of the radio waves emanating from the Milky Way galaxy, used a Bruce array for experiments he conducted in 1931. Modern phased array radio telescopes typically consist of a number of small, interconnected antennas such as the Murchison Widefield Array in Australia, constructed in 2012.
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