Antennas enjoy a very large range of applications, both in the military and commercial world. The most well-known applications of antennas to the average person are those associated with radio, TV, and communication systems. Today, antennas find extensive use in biomedicine, radar, remote sensing, astronomy, navigation, radio frequency identification, controlling space vehicles, collision avoidance, air traffic control, GPS, pagers, wireless telephone, wireless local area networks (LANs) etc. These applications cover a very wide range of frequencies as shown in Table 1 (2,3,40):
Antennas in Communication Systems
Antennas are one of the most critical components in a communication system since they are responsible for the proper transmission and reception of electromagnetic waves. The antenna is the first part of the system that will receive or transmit a signal. A good design can relax some of the complex system requirements involved in a communication link and increase the overall system performance.
The choice of an antenna for a specific application (cellular, satellite based, ground based, etc.), depends on the platform to be used (car, ship, building, spacecraft, etc.), the environment (sea, space, land), the frequency of operation, and the nature of the application (video, audio data, etc.). Communication systems can be broken into several different categories:
Direct (Line-of-Site) Links. A transmission link established between two highly directional antennas. The link can be between two land-based antennas (radio relays); between a tower and a mobile antenna (cellular communication); between a land-based antenna and a satellite antenna (earth- space communication); between two satellite antennas (space communication). Usually these links operate at frequencies between 1 to 25 GHz. A typical distance between two points in a high capacity, digital microwave radio relay system is about 30 miles.
Satellites and Wireless Communications. Antennas on orbiting satellites are used to provide communications between various locations around the earth. In general, most telecommunication satellites are placed in a geostationary orbit
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Table 1. Frequency Bands and General Usage
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satellites operate at the L, S, or Ku band, but increasing demand for mobile telephony and high speed interactive data exchange is pushing the antenna and satellite technology into higher operational frequencies (47). Future satellites will be equipped with antennas at both the Ku and the Ka bands. This will lead to greater bandwidth availability. For example, the ETS-VI (A Japanese satellite comparable to NASA’s TDRS), carries five antennas: an S-band phased array, a 0.4 m reflector for 43/38 GHz, for up and down links, an 0.8 m reflector for 26/33 GHz, a 3.5 m reflector for 20 GHz, and a 2.5 m reflector for 30 GHz and 6/4 GHz. Figure 10 shows a few typical antennas used on satellites. It is expected that millions of households, worldwide, will have access to dual Ku/Ka band dishes in the twenty-first century. These households will be able to enjoy hundreds of TV channels from around the world. Moreover, low cost access to high speed, voice, data, and video communications will be available to more customers (48).
Personal/Mobile Communication Systems. The vehicular antennas used with mobile satellite communications constitute the weak link of the system. If the antenna has high gain, then tracking of the satellite becomes necessary. If the vehicle antenna has low gain, the capacity of the communication system link is diminished. Moreover, hand-held telephone units require ingenious design due to lack of ‘‘real estate’’ on the portable device.
There is more emphasis now in enhancing antenna technologies for wireless communications, especially in cellular communications, which will enhance the link performance and reduce the undesirable visual impact of antenna towers. Techniques that utilize ‘‘smart’’ antennas, fixed multiple beams, and neural networks are now being utilized to increase the capacity of mobile communication systems, whether it is land-based or satellite-based (49). It is anticipated that in the twenty-first century the ‘‘wire’’ will no longer dictate where we must go to use the telephone, fax, email, or run a computer. This will lead to the design of more compact and more sophisticated antennas.
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Satellite JK |
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Satellite dish |
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Satellite dish |
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(GEO), about 22,235 miles above the earth as shown in Fig.
1. There are also some satellites at lower earth orbits (LEOs) that are used for wireless communications. Modern satellites have several receiving and transmitting antennas which can offer services such as video, audio, data transmission, and telephone in areas that are not hard-wired. Moreover, direct — TV is now possible through the use of a small 18-inch reflector antenna with 30 million users in the U. S. today (41,42).
Satellite antennas for telecommunications are used either to form a large area-of-coverage beam for broadcasting or spot beams (small area-of-coverage) for point-to-point communications. Also, multibeam antennas are used to link mobile and fixed users that cannot be linked economically via radio, land — based relays (43-46).
The impact of antennas on satellite technology continues to grow. For example, very small aperture terminal dishes (VSAT) at Ku band that can transmit any combination of voice, data, and video using satellite networking, have become valuable tools for several small and large companies. Most
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47.1 ft |
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Ka-band command, ranging, and telemetry antennas |
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7.2-ft, 30-GHz receiving antenna |
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29.9 ft |
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10.8-ft, 20-GHz transmitting antenna |
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3.3-ft steerable antenna |
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Solar array7 |
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^5> |
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Figure 10. Typical antennas on a satellite. (Courtesy, NASA Lewis Center) |
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Dual subreflectors |
Antennas for Biomedical Applications
In many biological applications the antenna operates under very different conditions than the more traditional free-space, far-field counterparts. Near fields and mutual interaction with the body dominate. Also, the antenna radiates in a lossy environment rather than free space. Several antennas, from microstrip antenna to phased arrays, operating at various frequencies, have been developed to couple electromagnetic energy in or out of the body. Most medical applications can be classified into two groups (50): (1) therapeutic and (2) informational. Examples of therapeutic applications are hyperthermia for cancer therapy, enhancement of bone and wound healing, nerve simulation, neural prosthesis, microwave angioplasty, treatment of prostatic hyperlastia, and cardiac ablation. Examples of informational applications are tumor detection using microwave radiometry, imaging using microwave tomography, measurement of lung water content, and dosimetry.
Therapeutic applications are further classified as invasive and noninvasive. Both applications require different types of antennas and different restrictions on their design. In the noninvasive applications (not penetrating the body), antennas are used to generate an electromagnetic field to heat some tissue. Antennas such as helical-coils, ring capacitors, dielectrically loaded waveguides, and microstrip radiators are attractive because of their compactness. Phased arrays are also used to provide focusing and increase the depth of penetration. The designer has to choose the right frequency, size of the antenna, and the spot size that the beam has to cover in the body. The depth of penetration, since the medium of propagation is lossy, is determined by the total power applied or available to the antenna. Invasive applications require some kind of implantation in the tissue. Many single antennas and phased or nonphased arrays have been extensively used for treating certain tumors. A coaxial cable with an extended center conductor is a typical implanted antenna. This type of antenna has also been used in arteries to soften arterial plaque and enlarge the lumen of narrowed arteries.
Antennas have also been used to stimulate certain nerves in the human body. As the technology advances in the areas of materials and in the design of more compact antennas, more antenna applications will be found in the areas of biology and medicine.
Radio Astronomy Applications
Another field where antennas have made a significant impact is the field of astronomy. A radio telescope is an antenna system that astronomers use to detect radio frequency (RF) radiation emitted from extraterrestrial sources. Since radio wavelengths are much longer than those in the visible region, radio telescopes make use of very large antennas to obtain the resolution of optical telescopes. Today, the most powerful radio telescope is located in the Plains of San Augustin, near Sorocco, N. M. It is made of an array of 27 parabolic antennas, each about 25 m in diameter. Its collecting area is equivalent to a 130-m antenna. This antenna is used by over 500 astronomers to study the solar system, the Milky Way Galaxy, and extraterrestrial systems. Puerto Rico is the site of the world’s largest single-antenna radio telescope. It uses a 300-m spherical reflector consisting of perforated aluminum panels. These panels are used to focus the received radio waves on movable antennas placed about 168 meters above the reflector surface. The movable antennas allow the astronomer to track a celestial object in various directions in the sky.
Antennas have also been used in constructing a different type of a radio telescope, called radio interferometer. It consists of two or more separate antennas that are capable of receiving radio waves simultaneously but are connected to one receiver. The radio waves reach the spaced antennas at different times. The idea is to use information from the two antennas (interference) to measure the distance or angular position of an object with a very high degree of accuracy.
Radar Applications
Modern airplanes, both civilian and military, have several antennas on board used for altimetry, speed measurement, collision avoidance, communications, weather detection, naviga-
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Figure 11. A basic radar system. |
tion, and a variety of other functions (40,51-53). Each function requires a certain type of antenna. It is the antenna that makes the operation of a radar system feasible. Figure 11 shows a block diagram of a basic radar system.
Scientists in 1930 observed that electromagnetic waves emitted by a radio source were reflected back by aircrafts (echoes). These echoes could be detected by electronic equipment. In 1937, the first radar system, used in Britain for direction finding of enemy guns, operated around 20 MHz to 30 MHz. Since then, several technological developments have emerged in the area of radar antennas. The desire to operate at various frequencies led to the development of several, very versatile and sophisticated antennas. Radar antennas can be ground-based, mobile, satellite-based, or placed on any aircraft or spacecraft. The space shuttle orbiter, for example, has 23 antennas. Among these, four C-band antennas are used for altimetry, two to receive and two to transmit. There are also six L-band antennas and 3 C-band antennas used for navigation purposes.
Today, radar antennas are used for coastal surveillance, air traffic control, weather prediction, surface detection (ground penetrating radar), mine detection, tracking, air-de — fense, speed-detection (traffic radar), burglar alarms, missile guidance, mapping of the surface of the earth, reconnaissance, and other uses.
In general, radar antennas are designed as part of a very complex system that includes high power klystrons, traveling wave tubes, solid state devices, integrated circuits, computers, signal processing, and a myriad of mechanical parts. The requirements on the radar antennas vary depending on the application (continuous wave, pulses radar, Doppler, etc.) and the platform of operation. For example, the 23 antennas on the space shuttle orbiter must have a useful life of 100,000 operational hours over a ten-year period or about 100 orbital missions. These antennas are required to operate at temperatures from -150 °F to 350 °F during re-entry. The antennas also have to withstand a lot of pressure and a direct lightning strike. The antenna designer will have to meet all of these constraints along with the standard antenna problems of polarization, scan rates, frequency agility, etc.
Impact of Antennas in Remote Sensing
Remote sensing is a radar application where antennas such as horns, reflectors, phased arrays, and synthetic apertures are used from an airplane or a satellite to infer the physical properties of planetary atmosphere and surface or take images of objects.
There are two types of remote sensing: active and passive (radiometry) and both are in wide use. In the active case a signal is transmitted and the reflected energy, intercepted by the radar as shown in Figure 12, is used to determine several characteristics of the illuminated object such as temperature, wind, shape, etc. In the passive case the antenna detects the amount of microwave energy radiated by thermal radiation from the objects on the earth. Radiometers are used to measure the thermal radiation of the ground surface and/or atmospheric condition (13,54-56).
Most antennas associated with remote sensing are downward-looking, whose radiation patterns possess small, close — in sidelobes. Remote sensing antennas require a very careful design to achieve high beam efficiency, low antenna losses, low sidelobes, and good polarization properties. Ohmic losses in the antenna is perhaps the most critical parameter since it can modify the apparent temperature observed by the radiometer system.
The degree of resolution of a remote map depends on the ability of the antenna system to separate closely space objects in range and azimuth. To increase the azimuth resolution a technique called ‘‘synthetic aperture’’ is employed. Basically, as an aircraft flies over a target the antenna transmits pulses assuming the value of a single radiating element in a long array. Each time a pulse is transmitted, the antenna, due to the aircraft’s motion, is further along the flight path. By storing and adding up the returned signals from many pulses, the single antenna element acts as the equivalent of a very large antenna, hundreds of feet long. Using this approach, an antenna system can produce maps approaching the quality of good aerial photographs. This synthetic aperture antenna becomes a ‘‘radio camera’’ that can yield excellent remote imagery. Figure 13 depicts a reflectivity map of the earth taken by NASA’s scatterometer.
Today, antennas are used in remote sensing applications for both the military and civilian sectors. For example, in the 1960s the US used remote sensing imaging from satellite and airplanes to track missile activities over Cuba. In the 1970s, remote sensing provided NASA with needed maps of the lunar surface before the Apollo landing. Also in July 1972, NASA launched the first earth resource technology satellite (ERTS-1). This satellite provided data about crops, minerals, soils, urban growth, and other earth features. This program
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Figure 13. A reflectivity map of the earth taken by NASA’s Scatterometer. (Courtesy, NASA/ JPL) |
still continues its original success using the new series of satellites called the Landsats. In 1985, British scientists noted the ozone depletion over Antarctica. In 1986, US and French satellites sensed the Chernobyl nuclear reactor explosion that occurred in Ukraine. Landsat images from 1975 to 1986 proved to be very instrumental in determining the deforestation of the earth, especially in Brazil. In 1992, hurricane Andrew, the most costly natural disaster in the history of the United States, with winds of 160 miles per hour, was detected on time by very high resolution radar on satellites. Because of the ability to detect the hurricane from a distance, on time, through sophisticated antennas and imagery, the casualties from this hurricane were low. In 1993, during the flooding of the Mississippi River, antenna images were used to assist in emergency planning, and locating threatened areas (56). In 1997, NASA, using antennas, managed to receive signals from Mars and have the entire world observe the pathfinder maneuver itself through the rocky Martian terrain.