Antenna System Design

The design of the transmit and receive antenna system is important because it determines how well energy is
transferred from one antenna to the other. Some of these factors are gain, directivity, polarization and height
above the ground.

Antenna height is simply a matter of the higher the better. Increasing the height extends the line-of-sight distance and reduces the effects of objects on the ground. The distance to the horizon is dependent on the height above the surface of the earth. It can be seen that because of the curvature of the earth, the distance to the horizon is longer when viewed from an elevated position.

Radio waves are somewhat like light waves in that they tend to travel in a straight line. However, radio waves also tend to refract or bend as they follow the curvature of the earth. This extends the radio horizon beyond the optical horizon. This bending is caused by the tendency of a radio wave to travel slower as the density of the air increases. Since part of the radio wave travels near the earth where the air is denser, this bending occurs.

When studying the behavior of radio waves in space, it is more convenient to use a path that is a straight line instead of a curve. This requires that the radius of earth curvature be simultaneously readjusted to preserve the correct relationship. For the standard atmosphere, the equivalent radius is 4/3 or 1.3 times the actual radius of the earth as shown by experience. As previously stated the optical and radio wave paths differ. The distance in miles from antenna to the optical and radio wave horizon is determined as follows:

Optical Horizon Distance = Square Root of 2h

Radio Horizon Distance = 1.33 x Square Root of 2h

Where, "h" is height in feet. The maximum possible distance at which direct-wave transmission is possible between transmitting and receiving antennas at given heights (the line-of-sight distance) is equal to the sum of the horizon distances calculated separately for the individual antenna heights. When the distance involved is less than line-of-sight, the path is sometimes referred to as the optical path. The nomogram below shows this relationship.

	As the distance between the transmitting and receiving antennas increases, the energy concentration for a given area decreases. Therefore, the distance from the transmitting antenna also determines how much energy an antenna intercepts. This loss of signal strength due to increased distance is known as path attenuation and is expressed in decibels or dB.
Radio and optical horizon nomogram

The amount of power available at the receiving antenna is dependent on the amount of energy it intercepts. An electrically large antenna will intercept more energy than an electrically small antenna. The actual dimensions of an antenna are related to wavelength. The higher the frequency, the smaller the antenna for a given wavelength. Because a smaller antenna intercepts less energy, there is a decrease in usable range as frequency increases. It is possible to increase the size (in terms of wavelength) of higher frequency antennas so that they intercept more power. These antennas are referred to as "gain" antennas.

Directional "Gain" Antennas

Communication range is calculated by determining the path attenuation and relating it to the power output of the transmitting antenna. Path attenuation places a practical limit on maximum usable range because a point is reached where it is impractical to radiate sufficient power to overcome path loss. While antenna height establishes the maximum possible range, the radiated power determines the practical limit since that determines the signal level at the receiving antenna. Even though base station power could be increased to several thousand watts (regulations permitting); the system "talk back" range would still be limited by the power output capability of the remote units.

If the radio link consists of two fixed stations communicating only with each other, the use of directional (gain) antennas can offer an advantage. A directional antenna normally provides several dB of gain by concentrating the RF energy in only one direction. This minimizes potential interference with other stations on other azimuths. If communication is required with stations on different azimuths, an omnidirectional antenna is probably required. Some gain can also be built into that type of antenna.

There are several ways of adding gain to antenna such as using 5/8 wavelength, yagi, corner reflector, or colinear designs. At VHF frequencies, gain antennas are rather large and cumbersome. However, at the higher frequencies, they become practical and may make up for some of the higher path loss at those frequencies.

Gain antennas usually perform very well in line-of-sight applications. However, when the radiation pattern of a gain antenna is compared to that of a standard quarter-wave unity gain antenna, the gain antenna radiates at a smaller vertical angle. It is possible to be under the radiation pattern and operate at a signal loss with this type of antenna. Therefore, if the received signal is the result of a reflected wave, a gain antenna may not enhance performance.

Directional (gain) antenna

The transmission line that connects the antenna to the receiver or transmitter is also a source of power loss. Typically, this loss is specified in dB per 100 feet of transmission line. For example, 100 feet of RG-8U coaxial cable has a 5.0 dB loss at 400 MHz. However, RG-19U coaxial cable has only a 1.85 dB loss at the same frequency, and 7/8" heliax cable has only a 0.9 dB loss at 450 MHz. Although transmission lines with a lower loss cost more and are larger in diameter, they offer an advantage it the antenna is a considerable distance from the receiver or transmitter.

Antennas generally are horizontally or vertically polarized. If radio link performance is to be maximized, it is important that both the transmitter and receiver antennas be of the same polarization. Opposite polarization results in additional path loss.

Polarization cross coupling can occur when a transmitted signal is reflected off an object. There is less chance of cross coupling a vertical wave to a horizontal wave than vise versa. This is because most buildings, electrical poles, and similar reflectors are parallel to the vertical polarized wave. Therefore, there is less chance of depolarizing a vertical wave by such a reflection. At higher frequencies that are more subject to reflection, vertical antennas may be preferred because less ground reflections will be produced.