Codan Radio Radio Repeater Systems Training Guide - Manual (Page 28)

TRAINING GUIDE | RADIO REPEATER SYSTEM
Chapter 3: Repeater System Equipment Page 20
ANTENNAS
Antennas are manufactured to operate on a specifi c frequency and should not be altered in any way.
There are fi ve factors of practical importance in the design of an antenna. These are antenna gain,
bandwidth, impedance, physical size / construction, and radiation pattern. Any antenna that you
purchase for use on a repeater system will have a nominal impedance of 50 ohms.
The gain, shown in dB on the manufacturer’s data sheet, is determined by the construction. However,
a gain in one direction is accompanied by a loss in another direction.
When surveying proposed paths, think about the antennas that you will be using and watch for surfaces
on either side of the path that may refl ect minor antenna lobes (see patterns below). Watch for near-
fi eld obstructions when determining antenna placement. Although you will be limited if using a fi berglass
enclosure as to antenna location, try to get the lowest antenna as high as possible to reduce signal
absorption by the ground.
Where climatic extremes are anticipated, heavy duty models may be necessary. In addition, antennas
are manufactured for a rated wind velocity and also for a rated wind velocity with one-half inch of radial
ice load. Ensure that your site conditions are within these limitations.
In the manufacturer’s data, the bandwidth will usually be noted as a percentage of the frequency range.
When pairs of frequencies are used for receiving and transmitting on the same antenna, as is nearly
always the case in a repeater system, the bandwidth of the common antenna must be wide enough to
include these frequencies. For example, if your operating frequencies on VHF are Tx 160 MHz, Rx 165
MHz and the antenna frequency range is 157-174 MHz, the bandwidth would have to be 165-160/174-
157 x 100 or 5/17 x 100 = 29.4%. Bandwidth may also be shown as the voltage standing wave ratio
(VSWR) over the frequency range. A VSWR of 1.5:1.0 or less over the frequency range is adequate
for our purposes. Do not confuse bandwidth with beam width which is noted in degrees and is shown
as the 0.707 voltage or half power point on the antenna pattern. This fi gure will probably be required
when licensing your system.