Eying Antennas For Microwave Systems

Antennas for RF and microwave systems come in many physical shapes and sizes, with a wide range of performance parameters available to suit many different applications.

Antennas are usually the most visible part of any communications system. While the sizes and shapes are many, the functions of these high-frequency components are similar and vital to wireless communications systems around the world, as well as in deep space. Understanding how antennas work and some of the key parameters for characterizing their performance can help when comparing different models for different applications.

At one time, the rooftops of buildings in almost every major city were dotted with Yagi-Uda antennas, used to receive analog television broadband signals at VHF and UHF bands. Today, for television subscribers not serviced by cable connections, antennas are usually come in the form of small parabolic dishes designed to receive data-compressed microwave video signals from an orbiting satellite. In these and many other communications applications, the antenna is the vital component that receives and/or transmits signals from one location to another. Antennas encompass an extremely wide range of design types, from the simplicity of a pair of wires to the complexity of multiple antenna elements precisely controlled in phase.

An antenna (see figure) is essentially a set of conductors or elements fed by a transmission line. The feed to an antenna may be as simple as a microstrip transmission line connected to the antenna's elements, or it may include numerous components and even a full impedance-matching network. In higher-frequency antenna designs, such as horns, the antenna may be fed by means of a waveguide rather than a microstrip transmission line.

The first antennas were built by German physicist Heinrich Hertz in 1888 as part of his efforts to prove the existence of electromagnetic (EM) waves (as predicted by James Clerk Maxwell and his famous equations). Upon transmission, an antenna converts current into radio waves, producing time-varying EM radiation; upon reception, it captures this radiation at its resonant band of frequencies, converting radio waves into current. Antennas can be designed to produce radiation in all directions (omnidirectional) and they can also be made directional, designed with reflective or directive elements such as parabolic reflectors or horns to direct or focus energy in a certain way.

The types of antennas are many, including wire, microstrip, reflector, traveling-wave, and aperture varieties. Wire antennas are the simplest to fabricate, such as dipoles, monopoles, folded dipoles, and loop antennas. Microstrip antennas are among the smallest, such as patch antennas and planar inverted-F antennas (PIFAs) popular in cellular telephone handsets. Reflector antennas include the large parabolic ("dish") reflector antennas associated with satellite communications (satcom) systems. Traveling-wave antennas include helical antennas, the aforementioned Yagi-Uda antennas, and spirals. Aperture antennas include slotted waveguide antennas, horns, and slot antennas. Because of their directivity, horn antennas, for example, have been used in secure communications systems and in point-to-point radio links. The size of the horn aperture is measured in wavelengths, and horns have been used in practical applications well into the millimeter-wave frequency range.

Phased-array antennas consist of a number of simple antennas or antenna elements connected together by means of an electrical network. Depending upon the phase controlled by the network, usually through some means of phase shifter, various combinations of the antenna elements can be made active, with constructive and destructive interference used to effectively steer the beam as if the antenna array was physically moving. The main beam will point in the direction of increasing phase shift. Typically, a phased-array antenna is formed from identical elements, such as dipoles, loops, or slot antennas.

Antennas can be specified and compared by means of a long list of operating parameters. The list includes frequency range, gain, radiation pattern, polarization, directivity, efficiency, beamwidth, sidelobes, effective aperture, and impedance, among other characteristics. Each antenna is designed to operate at a specific set of resonant frequencies, much like a bandpass filter that processes some signals and rejects all others. An antenna's gain is typically specified with reference to a known, standard type of antenna, even when that antenna is purely theoretical (such as an isotropic radiator). This type of antenna is essentially a dimensionless point in space that radiates equally in all directions. Appropriately, gain referenced to an isotropic radiator is given in units of dBi.

Another type of gain reference antenna is the dipole, which is basically two wires running in opposite directions, with one end of each wire connected to the transmitter/receiver and the other end radiating in free space. It generally produces an omnidirectional radiation pattern in the plane perpendicular to the axis of the antenna. Gain referenced to a dipole is given in units of dBd. The physical size of a dipole is typically about the half-wavelength of the resonant frequency of interest. Other designs may use physical dimensions based on the quarter-wavelength of the resonant frequency of interest.

For efficient radiation of power, an antenna must not only be tuned to a resonant frequency but also impedance matched to the source. Proper impedance matching will also maximize the amount of power received by the antenna. An antenna is usually designed to appear as a purely resistive load, with reactance minimized as much as possible. Reactance can be eliminated by operating an antenna at its resonant frequency, when its capacitive and inductive reactances are equal and opposite, resulting in a net zero reactive current. Once the reactance has been eliminated, what remains is pure resistance, which is the sum of the ohmic resistance of the conductors and the radiation resistance of the antenna. The power absorbed by the ohmic resistance is lost as heat while that absorbed by the radiation resistance becomes radiated as electromagnetic energy. The greater the ratio of radiation resistance to ohmic resistance, the more efficient the antenna.

According to the EM reciprocity theorem, an antenna's transmitting radiation pattern will be the same as its receiving radiation pattern. Also, its other performance parameterssuch as gain, bandwidth, impedance, resonant frequency, and polarizationwill be the same whether the antenna is used for transmission or reception. These conditions assume that the antenna has been designed with materials that exhibit linear behavior. Some materials (such as ferrites) can yield nonreciprocal behavior which, in some applications, may be beneficial. In a radar system, for example, it is preferable to exhibit an enhanced radiation pattern on transmission rather than on reception.

The efficiency of a transmit antenna is simply the amount of power radiated as a percentage of the amount of power presented to the antenna's terminals. When fed with 100 W power, an antenna with 70% efficiency would radiate 70 W power, with the remaining 30 W power absorbed by the antenna circuitry and dissipated as heat. This assumes that the antenna is properly impedance matched to the source, since a poor match can also result in a loss of power.

Smaller antennas (physically a fraction of the resonant frequency wavelength) tend to have small radiation resistance and to be less efficient than physically larger antennas. Polarization essentially refers to the orientation of the antenna's electric field with respect to the Earth's surface. It is determined by the antenna's physical configuration and its orientation. For example, an antenna may have one polarization when mounted horizontally and another when mounted vertically. Polarization is most reliable in line-of-sight communications rather than in communications systems in which the ionosphere can impact the polarization as a form of reflector. Having receivers and transmitters with the same polarization can make a significant impact on the effective gain of the system.

Antenna design in recent years has benefit from rapidly improving commercial EM software simulation tools. Both planar and three-dimensional (3D) EM simulators from many software suppliers can be applied to the design of antennas as simple as microstrip patches to as complex as large phased arrays. Most modern software tools include applications that are useful for optimizing key antenna parameters, including directivity, near- and far-field gain, and efficiency.

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