A basic understanding of radio propagation, or how radio energy travels in space, is needed to be able to get the best performance from radio equipment carried in off-road vehicles.
When a transceiver is operating as a transmitter, radio energy is radiated from its antenna. This radio energy travels through space in two different ways - by ground waves and by sky waves.
Radio energy that leaves the antenna as ground waves remains close to the surface of the Earth and reduces in energy quickly, particularly when travelling over land. Consequently, communication by ground waves is only suitable for short ranges. Radio energy leaving the antenna as sky waves travels upwards at many different angles until it reaches the ionosphere (the part of the Earth's atmosphere that lies between 80 and 350 kilometres above the Earth's surface). As the radio energy meets the ionosphere, it is reflected back towards the Earth's surface. Sky waves lose their energy slowly and, due to the reflection from the ionosphere, allow communications over very long distances.
There are many variables that affect the sky wave propagation of radio energy, most importantly, the time of day or night. The seasons and the natural phenomena of solar flares, or sunspots, also substantially affect the behaviour of sky waves.
Radio propagation at 27 MHz citizen band frequencies
Propagation at 27 MHz frequencies is reliable only for short distances. Generally, 27 MHz sky wave radio energy is not reflected by the ionosphere. Communications must be conducted by ground waves and are consequently possible only for short distances. Approximately 20–30 km is the maximum range.
Under certain ionospheric conditions, 27 MHz sky waves are reflected by the ionosphere and return to earth. This allows communications over hundreds or thousands of kilometres. This situation is popularly referred to as 'skip'.
Radio propagation at UHF citizen band frequencies
There is no reflection of ultra high frequency (UHF) radio energy from the ionosphere. Consequently, UHF communications rely primarily on line-of-sight propagation and are effective only for short ranges.
As a general rule, UHF communications between two stations are possible over a maximum range of approximately the combined line-of-sight distance of each station. It follows that the greater the heights of the transmitting and receiving antennas, the greater will be the communication range.
By using UHF repeaters, the range of vehicle-to-vehicle communications can be significantly increased.
Radio propagation at HF frequencies
In high frequency (HF) bands, reliable use can be made of both ground and sky wave energy components, allowing communication over short and long ranges.
HF radio equipment typically provides the operator with a selection of frequencies in different bands. This allows the operator to select a frequency that will be suitable both for the distance over which communication is required, and the time of day and season. The lowest frequency that will provide effective communication should be selected first. This has the effect of minimising the distance that the radio waves will travel and reducing interference to other HF users.
The general rule for frequency selection is to use the lower frequencies when close to the required station and the higher frequencies when further away. During hours of darkness, a frequency lower than that necessary during the day is more likely to be effective.
The lowest available frequency should be selected for communications, using the table as guide. Citizen band (CB) frequencies should be used for inter-vehicle communication when travelling in convoys. It is suggested that only one vehicle maintain communication with the outside world using other frequencies.
Approximate guide to frequency bands and ranges
| Approximate distance | Day | Night |
|---|---|---|
| Less than 300 km | 3–4 MHz | 2–4 MHz |
| Between 300 km and 750 km | 4–6 MHz | 3–7 MHz |
| Between 750 km and 1,500 km | 5–9 MHz | 6–10 MHz |
| Between 1,500 km and 2,500 km | 8–12 MHz | 7–11 MHz |
| Between 2,500 km and 5,000 km | 10–18 MHz | 8–15 MHz |
Radio transceiver controls
To make the most out of radiocommunication, radio operators should be familiar with some of the more important controls that may be found on radio transceivers, as follows.
Squelch or mute control
This control allows an operator to stop constant and annoying background noise from the receiver in the absence of an incoming signal. The correct setting is that which just suppresses the noise.
Selective call
Many transceivers now have a selective call facility, often termed 'Selcall', to alert a particular station that a message is about to be sent to it.
To make a selective call, the station's identification number is entered into a keypad and, after the button to send the selective call is pressed, a burst of tones lasting for approximately 10 seconds is sent on the channel to which the transceiver is tuned. As any conversation in progress will be disrupted by the tone burst, you should listen on the channel first to check whether it is in use before making a selective call.
Clarifier
This control provides a means of fine tuning incoming single sideband (SSB) signals that sound distorted or 'off station', often referred to as 'Donald Duck'. It has no effect on transmitted signals.
Antenna tuning unit
This unit is generally incorporated with an HF transceiver and is necessary to adjust the 'electrical' length of the antenna to ensure maximum transfer of power from the transmitter on different frequency bands.
Noise limiter or noise blanker
This control may be switched on to minimise the effect of loud static or ignition interference on received signals.
AM/SSB emission control
This control will be found on 27 MHz CB transceivers with an SSB option. The SSB mode may improve chances of successful communications under poor conditions or at extreme range.
More information
For more information about radio propagation, contact the ACMA.
The ACMA has fact sheets on a range of topics.
Please note: This document is intended as a guide only and is considered correct at the time of printing. For this reason the information contained herein should not be relied on as legal advice or regarded as a substitute for legal advice in individual cases.
