1. The 29 MHz band stretches from 29.700 MHz to 29.999 MHz.
2. There are 23 discrete fixed frequency channels numbered 1 to 23.
3. The Off-Road Radio Association is licensed to use three frequencies. These frequencies are:
• Ch. 14 = 29.8725 MHz
• Ch. 15 = 29.8850 MHz
• Ch. 16 = 29.8975 MHz
4. We should be aware that these frequencies are not for the exclusive use of the ORRA. We share them with many other users.
5. Other users of the 29 MHz radio band include:
• a) The ski boat fraternity
• b) Civil Protection organisations (who also use commercial radio frequencies)
• c) The National Sea Rescue Institute (who also use commercial radio frequencies)
• d) Farming communities.
• e) The Land Rover Owners Club and the SA. Jeep Club (both part of ORRA)
• f) Commercial businesses such as private ambulance services, security companies, delivery services, etc.

1. A radio signal consist of two parts, a carrier frequency which defines the operating frequency of the system, and a modulating signal which defines the information being carried (voice, digital data, etc.)
2. The information being carried is mixed onto the carrier wave by a technique called modulation.
3. There are many different types of modulation used for different applications. Some common ones are:
• Amplitude modulation (AM) - Used by us on 29 MHz. Cheap to manufacture but very susceptible to electrical interference and noise.
• Frequency modulation (FM) - Used for commercial two-way radio, amateur radio, broadcast. More expensive to manufacture but immune to electrical noise and interference.
• Single Side Band modulation (SSB) - Very efficient mode of modulation use extensively by radio amateurs. Not cheap when manufactured with a high level of stability.
• Pulse Width modulation (PWM) - Specialist modulation type used in digital data transmission. E.g. -personal pagers, satellite telemetry etc.
• Pulse Code Modulation (PCM) - Same application as PWM.
4. This paper will only deal with the two forms of modulation used in 29 MHz equipment.

1. In a basic amplitude modulation system, the signal to be carried is mixed with a carrier signal in a mixer stage of a radio transmitter.
2. This mixed signal is then filtered to derive only the required component, and fed to a power amplifier stage.
3. The amplified result is again filtered and passed through a matching stage and on to the antenna system where it is radiated into space.
4. In the receiver, the incoming RF (radio frequency) signal is amplified and mixed with a local oscillator signal generated internally.
5. Through appropriate filtering and mixing techniques, the modulating signal is extracted from the RF carrier, filtered and amplified to a level sufficient to drive a speaker.
6. Relatively speaking, AM systems are simple to design and cheap to manufacture.
7. These systems are however very susceptible to natural and man made interference.
8. AM is also not a very efficient modulating system resulting in a low level of effective radiated power for a given power consumption.

1. In AM, most of the RF energy is in the carrier, but at the receiver the carrier is filtered out and discarded. This is the reason for the AM system’s inherent inefficiency.
2. In SSB, the carrier is deliberately suppressed at the transmitter, and then the system changes to utilise as much power in the sidebands as is possible.
3. This mode of operation will generally result in a two to four fold increase in effective radiated power.
4. The side effect of this however, is that both the transmitter and the receiver design is substantially more complicated, and thus more expensive.
5. Also, if the design is kept simple to keep the cost down, then stability can become a problem and the advantage of SSB can be lost.

1. A low power radio with a good antenna can often perform better than a high power radio with a bad antenna.
2. 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.
3. For our operation on 29 MHz, the antenna length is vitally important for efficient operation.
4. 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 in transmit.
5. Its appearance is also a function of where on the vehicle and how it is going to be mounted.

1. All antenna theory, design and manufacture is based on the starting point of the 1/4 wave radiator.
2. From this theory, the simplest antenna that can be manufactured is the 1/4 wave dipole or the 1/4 wave whip antenna.
3. On the 29 MHz band a 1/4 wavelength is approx. 2750mm.

1. A 1/4 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 onto their vehicle.
2. Using an antenna length that is shorter than this 1/4 wavelength (or longer for that matter) requires a matching circuit that will ensure proper radiation of RF energy, as well as correct matching of the antenna to the radio.
3. The simplest and most common matching technique is the introduction of a coil in the antenna’s length somewhere.
4. Most of the short antennas used by 29 MHz radio owners are the top loaded type, but centre and base loaded antennas are also available.
5. Top loaded antennas work very well, but are susceptible to damage in the field.
6. Base loaded antennas on the other hand are sensitive to the mounting location and can be difficult to tune.
7. Shorter "loaded" antennas used on 29 MHz suffer slightly on efficiency but this is not enough to outweigh the physical advantages of the installation in many instances.
8. The choice of a long or a short antenna is generally personal and vehicle mounting-dependant.

1. Because of the low transmitter powers used on 29 MHz, correct installation and tuning of the antenna is critical.
2. 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.
3. The base mounting point of the antenna must be earthed to the bodywork and / or the chassis of the vehicle.
4. As much of the antenna length as is possible must be clear of the vehicle bodywork.
5. The antenna must be adjusted for optimum SWR (see below) with the applicable test instrumentation, by someone skilled in its use.

1. 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.
2. 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.
3. 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.
4. This reflected power can do damage to the transmitter electronics and should be kept to a minimum at all times.
5. 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.

1. Various mounting positions on a vehicle have an effect on the radiation pattern of the transmission.
2. A similar receive sensitivity "pattern" is applicable to an installation as well.
3. It must be emphasised at this point that due to the low transmitter powers and the short ranges over which the 29 MHz system will work, other physical issues will generally compromise the communications before the effect of these patterns will be noticed.
4. It is however true that under certain circumstances, vehicle orientation with respect to . the other station can have an effect on the transmitted and the received signal.

1. Radio equipment is designed from the onset to operate in specific modulation modes and on specific frequencies. This means that equipment designed for a certain application cannot be easily converted or modified to work in other modes. 29 MHz equipment is no exception.
2. The design criteria for the 29 MHz system were for a low power, safe, cheap, short range communication system and it is in this role that it works very well.
3. With a power output of only about 4 to 5 watts on AM and around 12 watts PEP on sideband, the radios are relatively cheap and do not need heavy cabling for high current draw.
4. Unless used extensively in transmit mode a 29 MHz radio should not run down a car battery. If uncertain, it may be advisable to start the vehicle every half an hour or so and let it run at a fast idle for around five minutes.
5. Antennas are not very expensive or high tech and due again to the low power are not likely to cause RF injury if they accidentally come into contact with humans or animals while a radio is in transmit.
6. 29 MHz radios are usually channelised and programmed to have the operating frequencies on fixed channel switch locations.
7. A channelised radio makes operation very simple. But the user does need to know what frequency is programmed to what channel number, as this is not generally pre-defined.

1. Volume
   Adjusts the listener volume. Usually also includes the on / off switch.
2. Squelch
   Adjusts the threshold for incoming signals below which the audio circuits will mute, and above which the audio system will automatically switch on. Used to mute the radio when it is not receiving a signal.
3. Channel Selector
   Selects the operating channel, and thus the frequency.
4. RF gain / ATT
   Different radios either have a RF gain control or a RF attenuator switch. These controls are used to make the radio receiver more or less sensitive depending on the operating conditions.
5. Mic gain / DX mic.
   Adjust how much amplification is given to the microphone signal prior to the modulation process in a transmitter. Ideally used to set for maximum modulation of the carrier with no distortion.
6. Delta tune / Clarifier / BFO (Rotating control)
   Found only on a SSB transceiver. Used to clarify the received signal and take the "Donald Duck" out of the voice and make it intelligible.
7. Delta Tune "+ 0 - " (Switch).
   Sometimes found on a regular AM radio. Allows the frequency of operation to be shifted slightly above or below the standard operating frequency of a given fixed channel.
8. Noise limiter / noise blanker
   Filter circuits that can be switched in to either reduce spurious noise or remove pulsed type ignition noise from the received signal. When used it compromises receiver sensitivity.
9. Mode selector (USB, LSB, AM)
   Selects the mode of operation (Upper Sideband modulation, Lower Sideband modulation, Amplitude Modulation.)
10. Scan button
    On newer digitally controlled radios that have this function, this allows the radio to automatically scan through a range of frequencies or channels and look for the presence of a radio signal.

1. Although a 29 MHz set is a low power device it is strongly recommended that it be installed correctly to obtain optimum results.
2. 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.
3. Power cabling must be of sufficient gauge and preferably be connected directly to the vehicle battery. Do not connect into ignition switches or the vehicle fuse box.
4. The cabinet of the radio should be earthed to reduce spurious noise pickup.
5. If in doubt about a radio installation, get someone with the right skills to assist you or have it done by a professional. Do not take chances, as a bad installation can become a fire hazard.

1. Radios are susceptible to interference from a great variety of sources.
2. These sources are broadly divided up into two categories, natural and man made.
3. Natural interference is caused by electrical storms, varying atmospheric conditions and sun spot activity.
4. Man made interference comes from a great variety of sources including power lines, neon and other fluorescent lighting systems, in car electronic equipment, cell phone etc.
5. It should also be noted that Radio transmitters can also interfere with other devices - as an example GPS receivers, cell phones and some medical electronic equipment are extremely sensitive to RF energy.

Presented 26 October 2002

By Eric P. Lawrenson.
ZS6 EPL / T60