Antenna Terms and Definitions
This compendium is intended to explain and simplify many antenna terms.
GAIN is a measure of the power amplification factor of the antenna. In proper Yagi beam designs, gain is primarily a function of boom length. At VHF and UHF frequencies, very high gain can be achieved because booms can be physically long with more elements. This has the effect of increasing the transmitting and receiving power of the antenna.
dB is an abbreviation for decibel. It is a mathematical expression that shows the relationship between two values. An increase of 3dB is a doubling of power. Gain figures have to be related to a reference antenna. For horizontally polarized antennas the symbol dBd indicates that a half wave dipole was used as the reference. For vertical antennas, a quarter wavelength vertical over a perfect ground is the standard. When the term dB is used without a reference, then it is assumed that the reference is a theoretical point source called an isotropic radiator, referred to as dBi.
FRONT TO BACK RATIO is a measure of the signal (power) radiated or received from the front of the antenna to signals from the back of the antenna.
RADIATION PATTERN is the result of the designer's skill in achieving good forward gain and front to back ratio. Think of the radiation pattern as similar to a flashlight beam. A narrow beam focuses for greater distance, while a wide beam will not reach as far. Long boom antennas will have narrower patterns which may be focused more accurately to receive and transmit distant signals.
VSWR is voltage standing wave ratio. The 2:1 SWR bandwidth is the range over which the antenna will operate without exceeding 2:1 SWR, allowing your transceiver to operate at its full power output.
Fifty Ohms is the value RF engineers have agreed to design for in applications requiring RF connection between equipment. In most radio communications, antenna feed point impedances between 25 and 100 Ohms are acceptable.
BANDWIDTH is the frequency range over which the antenna provides optimum performance. Specific SWR bandwidths are shown for each model.
POLARIZATION refers to the plane of the antenna elements in relation to the earth, either horizontal or vertical. Cross polarization such as trying to contact a vertically polarized mobile station antenna with a horizontally polarized beam will reduce the efficiency of the link and shorten the range of communications. This theory also applies to fixed or base station antennas. Most antennas used for FM communications are mounted with their elements in the vertical plane. For OSCAR satellite communications, antennas are specially designed to give circular polarization to compensate for satellite rotation. At HF frequencies below 30 MHz polarization is less of a factor because radio signals are subject to change as they reflect from the ionosphere back to earth.
RADIATION ANGLE is used to describe the angle which the signal radiates to or from the antenna in relation to the earth's horizon. It is applied primarily to vertical antennas. Angles of radiation as close to the horizon as possible are preferred for good communications. At HF frequencies, this is accomplished by using half wave antennas. At VHF and UHF frequencies, multiple vertical elements, one above the other, are used to lower the radiation angle and provide gain over a quarter wave vertical.
All antennas have elements. The basic element is a half wave dipole. Yagi beam antennas use more than one element to achieve a desired gain and pattern. Vertical antennas are also a configuration of the half wave dipole. Many verticals are one quarter wavelength long, which requires some type of ground radial system for proper impedance matching.
BEAMS can be described as high performance antennas. They have a center support boom and multiple elements. Beams incorporate many of the techniques described above to improve both the transmission and reception of ham radio signals. Beams are often called Yagi's after their inventors, H. Yagi and S. Uda. At VHF and UHF frequencies beams are usually designed to give maximum performance on one band. They are usually called monoband beams. Space and size may be a problem at HF frequencies so engineers have developed multiband trapped beams.
HF VERTICALS are usually designed to cover many ham bands. They radiate and receive in all directions, eliminating the need for rotators. Verticals are relatively small in size, making them a convenient and cost effective way to operate your station. HF verticals are most often ground or roof mounted. Most verticals need extensive ground radial systems. While they are excellent starter or novice antennas, HF verticals are also used successfully by many experienced ham operators.
TRAPS are L/C circuits used in multiband vertical or beam antennas to divide the antenna into sections for operation on separate bands. Multiband trapped antennas electronically select the proper antenna sections as you change band on your transceiver.
STACKING of antennas is used to increase performance. Two or more antennas may be mounted on a support mast or tower at designed distances. Special coaxial cables, sometimes called harnesses, and power dividers are used. They provide the correct impedance matching and allow the use of a single 50 Ohm coaxial cable from the transceiver. Stacking is used to increase forward gain by compressing the pattern. Theoretically, two stacked antennas have twice the radiated power output (a 3 dB increase) of a single antenna. To increase the gain another 3 dB, two more antennas must be added making a total of four and sometimes called an array.
LOCATION is very important. Put your antenna where it will be safe. Antennas are electrical conductors. When they are used with transceivers and amplifiers, high voltages can be developed on many parts of the antenna. Pick a spot where people and pets will not come in contact with your antenna when it is operating. Ground mounted verticals and their radials should be guarded. Place your antennas in locations where there is no chance of coming in contact with power lines during installation or removal.
WIND SURVIVAL is a measure of how well the antenna will survive in a wind storm. Actual survival will depend upon the quality of your installation. Wind surface area will help you to determine if the tower or support structure which you have selected is adequate to handle your antennas.
The buildup of static electricity and lightning charges can be very harmful to amateur radio equipment. The degree of exposure to this kind of damage can be reduced by several methods. One is the addition of a lightning arrester in the coaxial feed cable. These arresters may be placed just outside the building at the point where the cable enters the building. The air gap type reduces static and is most economical.
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