ORRA - Off Road Radio Association

ANTENNA

GENERAL

A low power radio with a good antenna can often perform better than a high power radio with a bad antenna.
Antenna length is critical to its operating frequency, under no circumstances can one say “... the longer the wire the better...”. There are only a few applications requiring special equipment where very long antennas are used.
For our operation on 29MHz and VHF, the antenna length is vitally important for both safety and efficient operation.
Apart from the antenna length, the physical attributes of an antenna will be dictated by the amount of RF power it is expected to handle when the radio is transmitting as well as where on the vehicle, and how, it is going to be mounted.

THE 1/4 WAVE

All antenna theory, design and manufacture is based on the starting point of the ¼ wave radiator.
From this theory, the simplest antenna that can be manufactured is the ¼ wave dipole or the ¼ wave whip antenna.
Because of the relationship between frequency and wavelength, on the 29MHz band a ¼ wavelength is approx. 2750mm. On the VHF band at 150MHz a ¼ wavelength is approx. 430mm.
A ¼ wavelength antenna requires no special treatment and can be mounted on a mobile vehicle very simply.

THE LOADED ANTENNA (LONGER AND SHORTER THAN 1/4 WAVE)

As mentioned before a ¼ wavelength antenna requires no special treatment and can be mounted on a mobile vehicle very simply. However not all people are prepared to fit a 2,75 meter long antenna (for 29 MHz) onto their vehicle. Using loading techniques it is possible to make a smaller antenna that still works well.
Using an antenna length that is “shorter than this 1/4 wavelength standard” requires a matching circuit that will ensure proper radiation of RF energy, as well as correct matching of the antenna to the radio.
This is also true for antennas on VHF which can be longer than a ¼ wave length. We use a matching circuit again to match this “longer than standard” antenna to the transmitter.
And when we start using antennas that are longer than a ¼ wave length and correctly matched to the transmitter, we get an added bonus as we can achieve some level of gain in the system over the standard ¼ wavelength antenna.
Just as antennas longer than a ¼ wave introduce gain, shorter “loaded” antennas as used on 29MHz suffer slightly on efficiency compared to the full size ¼ wave antenna, but this is not enough to outweigh the physical advantages of the installation in most instances.
The simplest and most common matching technique is the introduction of a coil in the antenna’s length somewhere.
Most of the short antennas used by 29MHz radio owners are the top loaded type, meaning the coil is situated in the top section of the whip, but centre and base loaded antennas are also available.
Top loaded antennas work very well, but are susceptible to damage in the field.
Base loaded antennas, not very popular on 29MHz, are common and popular for VHF.
The choice of antenna type is generally personal and vehicle mounting dependant.

INSTALLATION

Because of the low transmitter powers used on 29MHz, correct installation and tuning of the
On VHF systems good installation practices are still very important because now we are dealing with high power levels and a bad installation can very quickly damage the transmitter or cause personal injury to users.
At all times ensure that the connectors being used are of the right type and that they have been connected to the cables in the correct manner.
The base mounting point of the antenna must be earthed to the bodywork and / or the chassis of the vehicle.
As much of the antenna length as is possible must be clear of the vehicle bodywork.
The antenna must be adjusted for optimum SWR (see below) with the applicable test instrumentation, by someone skilled in its use.

STANDING WAVE RATIO (SWR)

The SWR measurement, when done correctly, is a quick and simple way of checking to see that the antenna is correctly matched to the radio.
Although it is a very theoretical and technical subject, the SWR instrument is basically indicating how much of the generated RF power is being effectively radiated out into space by the antenna system.
The function of the instrument is to indicate the ratio of RF power being fed into the antenna versus the amount of that RF power that is not being radiated successfully and is thus being reflected back into the transmitter.
This reflected power can do damage to the transmitter electronics and should be kept to a minimum at all times.
When the SWR is at its lowest reading, it means the antenna is successfully radiating all or most of the generated RF power, and little or none is being reflected back into the radio transmitter.

RADIATION PATTERNS

Various mounting positions on a vehicle have an effect on the radiation pattern of the transmission.
A similar receive sensitivity “pattern” is applicable to an instillation as well.
It must be emphasised at this point however that other physical issues will generally compromise the communications before the effect of these patterns will be noticed.
It is true that under certain circumstances, vehicle orientation w.r.t. the other station can have an effect on the transmitted and the received signals. This is apparent when operating at the limits of range of communications and is not applicable during short range operation.
On VHF systems, the effects of radiation patters from a given antenna location on a vehicle can be far more pronounced, even though the transmitted powers are much higher. This is because the physical vehicle has a bigger influence on the signal patterns at these high frequencies.