Antenna Array Developments: A Perspective on the Past, Present and Future Randy L. Haupt1 and Yahya Rahmat-Samii2 1Department of Electrical Engineering and Computer Science, Colorado School of Mines, Golden, CO 80401 USA E-mail: [email protected] 2Department of Electrical Engineering, University of California, Los Angeles, Los Angeles, CA 90095 USA E-mail: [email protected] Abstract This paper presents a historical development of phased-array antennas as viewed by the authors. Arrays are another approach to high-gain antennas as contrasted with reflector antennas. They originated a little over 100 years ago and received little attention at first. WWII elevated their importance through use in air defense. Since then, the development of computers and solid-state devices has made arrays a very valuable tool in radio-frequency systems. Radio astronomy and defense applications will continue to push the state of the art for many years. Keywords: Antenna; arrays; beamforming; history; phased arrays; radar 1. Introduction Moreover, mechanical steering might be too slow to meet some of the demands on fast-moving platforms such as airplanes. arge antennas collect relatively large amounts of elec- The array, particularly the phased array, makes many per- tromagnetic energy much as large buckets collect large L formance promises but for a price. Some of the unique features amounts of rain. In our companion paper, we described reflec- of a phased-array antenna include: tor antennas (large buckets). Using many small buckets to collect rain corresponds to using small antennas in an array to collect a large amount of electromagnetic energy. As with an- 1. fast wide-angle scanning without moving the antenna; tennas, a large bucket has the advantage of collecting lots of 2. adaptive beamforming; water in one location, whereas using many small buckets has the advantage of being easy to rearrange and move the small 3. graceful degradation in performance over time; buckets in order to better collect the rain. This analogy be- tween buckets and antennas is interesting but limited, because 4. distributed aperture; electromagnetic waves have phase whereas rain does not. 5. multiple beams; Large antennas create the high gain needed to boost the received/transmitted signal for a communications or radar sys- 6. potential for low radar cross section. tem. Today, reflectors and arrays compete for large aperture jobs in many types of systems. In general, the reflector is rela- Comparatively, reflectors are blessed with these advantages: tively inexpensive, that is why it is the antenna of choice for commercial activities, such as satellite TV. If the reflector must 1. high G/T; be moved in order to locate or track a signal, then the gimbals, servomotors, and other mechanical parts become a reliability 2. wide bandwidth; and maintenance issue that significantly increases lifecycle cost. 3. relatively low cost. Digital Object Identifier 10.1109/MAP.2015.2397154 The competition between the better antennas for the job will con- Date of publication: 26 February 2015 tinue with the cost/performance issues decided by the mission 86 1045-9243/15/$26.00 © 2015 IEEE IEEE Antennas and Propagation Magazine, Vol. 57, No. 1, February 2015 and budget. Clearly, hybridization of reflectors and arrays pro- of the 0 phase signals at A and B. They discovered that the vides enhanced opportunity for more sophisticated and high- array radiated a cardioid pattern. Braun was the first to use phase performance antenna systems. to collimate (steer) the main beam. Maybe we can say that he is the inventor of the phased array. This paper presents the historical development of array antennas. Reflectors have a rich history in optics that started The early arrays were physically large but electrically thousands of years ago. Arrays, on the other hand, are only a small. Thus, an array of only a few elements required extensive little more than 100 years old, whereas phased arrays are only real estate. In 1917, Frank Adcock designed a direction-finding a little more than 70 years old. The next section starts around array (see Figure 2) that consists of four uniformly weighted el- the turn of the 20th century and continues up to WWII. WWII ements placed at the four corners of a square whose sides are (see Section 3) motivated the development of high-gain anten- much less than half of a wavelength [6]. The antennas on one nas for defensive reasons. Section 3 continues with develop- diagonal are out of phase with the antennas on the other diag- ments after WWII up to the computer age. Section 4 describes onal. Sir Watson-Watt developed the mathematics to find the the impact of computers on array design and control. Semicon- elevation and azimuth of a source incident on an Adcock array ductor technology led to the development of very sophisticated [7]. Adcock arrays are still popular today. The main purpose of solid-state arrays described in Section 5. Finally, Section 6 looks the first arrays was direction finding. to the future. Our account of history is what we have learned through research and experience. The authors decided to collabo- The magnetron was first developed in 1920 [8]. At first, it rate on two historical papers based on presentations they made only worked at low frequencies. By 1940, however, the British at the IEEE CLASTECH Symposium and Exhibition on Anten- had it working at high power above 1 GHz. This development nas and Microwave Technology, October 2011, Los Angeles, CA. led to higher resolution antennas and radars on aircraft, hence This paper summarizes the presentation made by Randy Haupt: electrically larger antenna arrays. “Phased array antenna design yesterday and today.” A compan- ion paper covers the history of reflector antennas. Due to lim- Harold Friis was the dominant array researcher from the ited space and our limited knowledge, we apologize to those mid-1920s to the mid-1930s. Friis presented the theory behind whose important work has been omitted. the antenna pattern for a two-element array of loop antennas and experimental results that validated his theory [9]. Friis sub- sequently designed a multiple-unit steerable antenna (MUSA) [10], which employed an array of rhombics and was altered for 2. Early Array Developments: 1899–1937 optimum reception of shortwave signals (see Figure 3). In 1927, J. S. Stone received a patent for the binomial amplitude taper, The first antenna array was built over 100 years ago [1]. In which theoretically eliminates array sidelobes [11]. Mutual cou- order to increase the directivity of a single monopole, Brown pling between elements in an array was recognized to be very used two vertical antennas separated by half a wavelength and important in array design at a very early date [12]. fed them out of phase [2]. He found that the directivity was greatest in the plane of the antennas. The first array radiated at endfire. De Forest also noted an increase in gain due to ar- raying two vertical antennas [3]. He and several others used 3. Arrays in WWII: 1937–1945 an array to locate the source of a transmitting station. Shortly after the turn of the century, Marconi performed several ex- WWII motivated countries to tremendously accelerate the periments involving multiple antennas to enhance the gain in development of radars that detect aircraft and ships at a great certain directions [4]. Some even credit him with the invention distance. The radars needed to operate at a high frequency in of the antenna array, even though other lesser known experi- order to resolve targets, but the upper frequency was limited, menters preceded him. Nobel Prize winner Ferdinand Braun because transmitters at that time had insufficient power. The (also given credit as the inventor of the antenna array) placed available frequencies required huge antennas that could not be three monopoles in a triangle, as shown in Figure 1 [5]. The moved. Britain developed the bistatic Chain Home (CH) radar signal at antenna C has a 100 phase and twice the amplitude for air defense in the late 1930s [13]. The 23.1-MHz transmit Figure 1. Braun’s three-element array [5]. IEEE Antennas and Propagation Magazine, Vol. 57, No. 1, February 2015 87 Figure 2. Four-element direction-finding array from Adcock’s patent [6]. Figure 5. SCR-270 antenna array (photo taken by R. Haupt at the National Electronics Museum). As the war progressed in Europe, better radars were needed. A new US-developed long-range radar called the SCR-270 (Figure 5) was available in Hawaii and detected the Japanese for- mation attacking Pearl Harbor. Unlike CH, it could be mechani- cally rotated in azimuth 360 in order to steer the beam and operated at a much higher frequency. The SCR-270 had four rows of eight horizontal dipoles and operates at 110 MHz [15]. This frequency is much higher than CH and allowed for much higher gain and resolution for detecting aircraft as well as the ability to mechanically rotate the much smaller higher frequency antenna. In 1942, Bell Labs built the X-band Mark 8 surface fire control radar that had an array of 14 Â 3 polyrod antennas (see Figure 6) [16]. It used mechanically switched rotary phase shifters attached to the columns for azimuth scanning. Figure 3. MUSA, a six-element rhombic array, with one of This was the first use of the polyrod antenna in an array and the phase-shifting condensers in the lower left [10] Ó1937. the first microwave phased array. array had towers that were about 107 m tall and spaced about Schelkunoff developed a general approach to the analysis 55 m apart (see Figure 4). These towers held eight horizontal and synthesis of linear arrays in 1943 [18].