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Below are extracts from a document created by Mike Ilsley, Chairman of the Land Rover Owners Club of South Africa. Please do not reproduce anything from these pages without the permission of the author.

29Mhz Radios Off Road

Section 1 (General)

Section 2 (Technical)

  • Radios
  • Electromagnetic Spectrum
  • Objectives of ORRA
  • The Radio Signal
  • The 29Mhz Band
  • Amplitude Modulation (AM)
  • Frequency Table
  • Single Side Band (SSB)
  • Kit
  • Antennas - General
  • Base Station Radios
  • Antennas - The 1/4 Wave
  • Mobile (Vehicle Mounted) Radios
  • The Loaded Antenna (Longer & Shorter than 1/4 wave.)
  • Portable (Hand Held) Radios
  • Antennas - Standing Wave Ratio (SWR)
  • Antennas
  • The Radio Itself.
  • Antenna Placement
  • Acronyms.
  • Antenna Fitment
  • Antenna Tuning
  • Multi-purpose Antennas
  • Magnetic Mount Antennas
  • Tips for Radio Fitment
  • Radio Frequency Interference (RFI)
  • Tips for Use
  • Radio Controls (AM & SSB)
  • Setting up your Transciever
  • Etiquette
  • [Back to Main Page]


    Section 1


    Radios

    The tendency in the Off-road Market is towards 29 MHz radios. These are relatively inexpensive and offer good communications in convoy situations. They are controlled by a body called the Off-road Radio Association and can be contacted via the SA 4x4 Club. The LROC is one of the three founder Clubs of ORRA. While some individuals still make use of the old CB radios on the 27 MHz frequencies, most of the Off-road clubs have adopted the 29 MHz standard. With the exception of Commercial Frequencies and Citizen Bands, a 29MHz radio License is the easiest to obtain and no proof of operator ability is required.

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    Objectives of ORRA

    The Association is formed for the purpose of:

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    The 29Mhz Band

    We have used these radios for some years now and found that they are very effective for short-range communications. By this I really mean line-of-sight. With some experimentation regarding types of antennas, and mounting locations we have been able to have very effective communications up to 15 and 20 Km, but in ideal conditions.

    The 29MHz band stretches from 29.700 MHz to 29.999 MHz. There are 23 discrete fixed frequency channels numbered 1 to 23. The Off Road Radio Association is licensed to use three frequencies, (See Frequency Table Below.) in both AM and SSB mode (See Section 2 for AM/SSB discussion). We should be aware that this band is not for the exclusive use of the ORRA. We share it with many other users.

    Other users of the 29mhz radio band include:

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    Frequency Table

    The 29Mhz ORRA frequencies are as follows:

    Recommended Channel Frequency Use
    1 or 14 29,8725 General Open ORRA
    2 or 16 29,8975 Open ORRA
    7 or 15 29,8850 General ORRA (New)

    The most used frequency seems to be 29,8975 - normally located on Channel 2 or 16.

    Other 29Mhz frequencies of interest are:

    Recommended Channel Frequency Use
    3 29,7725 Ski Boat Open "A"
    4 29,9350 Ski Boat Emergency
    5 29,8350 Mountain
    6 29,8475 Civil Defense
    8 29,9725 Ski Boat "B"

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    Kit

    The typical kit consists of a Radio Transceiver with Microphone, Antenna, power cable and RF Cable. The power of the Transceiver is specified by law and may not exceed 5 Watts on AM, 12 Watts PEP on SSB. There are several makes of radios, but they all fall into one of only three categories : base stations, vehicle mounted (termed Mobile) and hand-held (Portables).

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    Base Station Radios

    These are normally Mobile Radios fitted to peoples houses and powered by AC to DC transformers. They don’t really apply to the LROC Members (or Bush Club Members) as they are not mobile.

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    Mobile (Vehicle Mounted) Radios

    By far the most popular amongst are the Mobile Radios. They are normally mounted permanently in the vehicle and take their supply via a fused lead directly from the battery. Typically they will be about 25 cm wide, 6 cm high and 40 cm deep. Most of them offer 5 Watts Output power, to comply with the legislation, and operate on the frequencies indicated in the table. They are connected to some type of external antenna via a 50 Ohm Impedance Co-Axial Feeder cable. They are fitted with internal speakers and a microphone on a spiral cord.

    When in use the user only has to have a microphone in hand. The microphone includes a push-to-talk button, which the user is required to push to place the radio in transmit mode. Releasing the button, automatically swops the mode to receive. There are many makes, Kenwood, Dragon, Icom, Philips, Yaesu, just to name a few. They are programmed to receive only the allowed channels, and in most cases are AM only (See Section 2 for description of AM and SSB).

    Some of the Radios may be fitted with a SSB, Suppressed-Carrier Single Sideband option. However, one needs SSB functionality in both the transmitting and receiving sets to utilise this feature. SSB channels all the power into the voice signal (by removing the Carrier and one Side Band), thus can achieve up to 3 times the range of an AM set, for the same output power.

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    Portable (Hand Held) Radios

    While they seem to be a good alternative, we have found them to be very impractical for in-vehicle use. Their transmission power is considerably less (normally 2 Watts) than their vehicle mounted counterparts, and their stubby “rubber” antennas compound the problem. The batteries do not last nearly long enough and the end result is that they are invariably connected to the cigarette lighter socket with a cable, and then to an external antenna with another cable. Instead of the driver only having to pick up a microphone to operate the radio, you end up with the entire radio connected by wires that unplug themselves at the wrong moments, in your hand. Even the dealers will advise against their in-vehicle use.
    Out of the vehicle of course is a different matter. Here they are excellent and are often used by farmers, game rangers and the like, who require mobility.

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    Antennas

    Several types of vehicle-mounted antennas are available and I will discuss the pros and cons of each type.
    They are named according to their physical characteristics, from the basic Whip antenna, which is normally ¼ Wavelength long or roughly 2.75 meters. The other antennas are shorter, but to achieve the required length, the designers include a small coil either at the base of the antenna (Base Loaded) or in the centre of the antenna (Middle Loaded) or at the top (Top Loaded).

    Type Pro's Con's
    Whip Antenna Best Reception. Excessive length, sways excessively and, due to it's strength, removes lights in parking garages.
    Base Loaded Good reception, does not sway excessively, fairly robust. Longer than Top Loaded, so still tends to hit lights in parking garages.
    Middle Loaded Shorter Antenna, good reception. Sways around badly due to weight of coil, low hanging branches tend to damage the coil.
    Top Loaded Shorter Antenna, excellent reception. Sways around badly due to weight of coil, low hanging branches tend to damage the coil.

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    Antenna Placement

    This is a very emotional issue as it normally means drilling holes in your vehicle. For the antenna to function effectively a “ground plane” is required. This is any conductive grid, to which the unshielded ground cable can be attached. A non-technical explanation says that the antenna propagates the radio waves and “bounces” them off this ground plane into the atmosphere. In our case, the vehicle surfaces form this “ground plane”, so the best place for the antenna is in the centre of the roof at the highest point. With military vehicles or dedicated Off-road vehicles, the placement of the antenna in this position is fairly easy. However for our use, it is not, so some compromises are required. Here are the pointers.

    The antenna should be mounted:

    Some of these are achievable even on our vehicles, but then we hit practical problems such as the antenna removing lights in parking garages, or getting wiped out by passing trees.

    This really leaves us with three options.

    The first option and also the worst from a reception point of view, is a base/middle or top loaded antenna fitted to one of the front fenders. In this case the top loaded antenna is normally the shortest and is short enough to miss most overhead lights etc, but sways around very badly. The reception is also affected by which fender it is located on, as the “ground plane” will be missing on one side thus giving worse reception to one side. Having said all that, this remains the most practical option for permanent mounting for recreational purposes. This will normally work for general convoy use, but severely restricts the range.

    The second favoured option is to mount the antenna on the roof of the vehicle using a “Gutter Clamp”. The Gutter clamp makes the antenna removable and does not require holes to be drilled. A problem is that antennas are often destroyed by low hanging bushes and branches when fitted to roof racks and gutters. Here the choice of Antenna itself is open, but probably the best is the base loaded antenna due to its reduced “sprung” weight. This means that it will not sway as much and is mechanically more able to take the knocks.

    This option will also provide different reception depending on which gutter is chosen, unless the vehicle is fitted with a gutter over the windscreen. Mounting the antenna in the centre of this gutter provides the almost ultimate spot. Some “Gutter Mounts” incorporate a method by which the antenna can be tilted without removing them from the gutter, should you wish to go into places with lower clearance.

    The third option is to mount the Antenna on the Bush bar. This is a very practical mounting, does not require holes in the bodywork, but has severe restrictions in forward transmissions due to the lack of Ground Plane. Antennas should always be mounted so that the minimum length of the antenna is shielded by the vehicle’s bodywork.

    Very Important: Never key the radio with the antenna tilted, removed, disconnected or touching any metallic parts of the vehicle, as this will most likely blow the output transistors of the radio.

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    Antenna Fitment

    Once we have decided where to fit the antenna, the following tips will help you achieve maximum performance for your chosen spot.

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    Antenna Tuning

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    Multi-purpose Antennas

    While there are some multi-purpose antennas on the market such as combined FM/27MHz or FM/29 MHz antennas, they are not recommended as their performance is a compromise and neither operation is optimised. FM should preferably have it’s own antenna. Two choices are offered for those who wish to use multiple frequency radios in their vehicles.

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    Magnetic Mount Antennas

    Although they look like a blessing in disguise, as they can be used at will, please remember the following traits:
    They do not earth to the bodywork, so power output / ground plane is not as good as other mounting options.
    They tend to scratch the paintwork especially if they get dust in-between the mounting and the bodywork.
    They don’t adhere to aluminium-bodied vehicles at all.
    But they can be removed when not in use, so no unsightly holes, mounting brackets and antennas when using the Landy to the shopping mall.
    They are easily removed by passing trees and members of the “plaaslike bevolking”.

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    Tips for Radio Fitment

    The following installation tips will ensure that you achieve optimum use of your radio.

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    Radio Frequency Interference (RFI)

    Radios are susceptible to interference from a great variety of sources. These sources are broadly divided up into two categories, natural and man made. It should also be noted that Radio transmitters can also interfere with other devices - as an example GPS receivers, vehicle alarm systems, cell phones and some medical electronic equipment are extremely sensitive to RF energy.

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    Tips for Use

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    Radio Controls (AM & SSB)

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    Setting up your Transciever

    Once you have installed the components, get a friend with a radio to assist you in setting up your set. All sets will not have all of these controls, but check anyway.
    1. Switch the set on and before keying the microphone, do the following:
    2. Turn the Volume knob to about 75% Volume.
    3. Select the desired channel, by turning the channel adjustment. The channel number should be displayed on the LED panel.
    4. Set the Squelch knob to its lowest possible position. (We will set the squelch properly later.) You should hear a hissing sound.
    5. Turn the Volume Knob until the volume of the hissing is audible but acceptable.
    6. Set the power to the highest position, normally marked in Decibels (db). Most sets will have a 20 db and 40 db setting. The 40 db setting will provide the best transmission over a long distance, while for short distance convoy work, the 20 db setting will be fine. It will also prevent you “blasting” the other peoples radios, also called “over modulating”
    7. If fitted with a Public Address (PA) option, switch it off, or to the Transmit (TX) position. The PA option is only used if the set has an external loudspeaker attached.
    8. If you key the Transmit button on the microphone, the Transmit LED should light up indicating that you are transmitting and the level on the meter should indicate 5 Watts. Your friend should be able to receive your message and should respond to you audibly.
    9. Now to the Squelch Adjustment. (Note: some radios are fitted with Automatic Squelch and thus the user cannot do the following adjustments.) While not transmitting and not receiving any signals, the “static” on the frequency is always present as the hissing noise. To stop the noise, turn the Squelch Knob slowly clockwise until it disappears. If you turn the knob back a fraction, it should re-appear.
      The correct setting is so that the static just disappears. If you now start the engine, the static levels will normally increase and thus the noise will reappear. Again, carefully adjust the Squelch higher until it disappears.
      While driving, the levels of static will vary from time to time and it may be necessary to re-adjust the Squelch accordingly. Some items that will interfere are overhead power cables and steel bridges. They are evidenced by a sudden “burst” of static, which disappears when you are clear of them.
      Do not reset the squelch for these occasions.
    10. If you suddenly seem to have no reception, it is normally due to the Squelch being too high, so turn it fully off (normally counter-clockwise) and try again.
    11. If the other station is almost out of range, you may have to turn the Squelch adjustment down, so that you can attempt to decipher the other vehicle’s transmission in-between the static noise.

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    Etiquette

    As previously discussed, the 29MHz band is a shared band and therefore we may encounter other users on the air from time to time. If you follow the following guidelines, all the users will be able to enjoy the communication, with little effort.

    Although these guidelines may seem like a lot of effort, they make for easier communications and can make those long stretches on the road a lot of fun.

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    Section 2


    Electromagnetic Spectrum

    The electromagnetic spectrum is the term used to define the phenomena of electromagnetic radiation. There are many various types of radiation within this spectrum including; visible light, infrared light, ultraviolet light, microwave, gamma, X-ray, and also radio waves. Within that small portion called radio waves, there is an even smaller portion called the 29MHz citizen band, which is shared between a number of different users.

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    The Radio Signal

    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.) The information being carried is mixed onto the carrier wave by a technique called modulation. There are many different types of modulation used for different applications. Some common ones are:

    This paper will only deal with the two forms of modulation used in 29MHz equipment.

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    Amplitude Modulation (AM)

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    Single Side Band (SSB)

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    Antennas - General

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    Antennas - The 1/4 Wave

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    The Loaded Antenna (Longer & Shorter than 1/4 wave.)

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    Antennas - Standing Wave Ratio (SWR)

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    The Radio Itself.

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    Acronyms.

    AAWDC Association of All Wheel Drive Clubs of SA
    AM Amplitude Modulation
    CB Citizens Band (27Mhz)
    FM Frequency Modulation
    GPS Global Positioning System
    HAM Radio Amateur
    HF High Frequency
    LROC Land Rover Owners Club of Southern Africa
    MHz MegaHertz (or Million Hertz)
    Mic Microphone or Mike
    ORRA Off-Road Radio Association
    PA Public Address
    PCM Pulse Code Modulation
    PEP Peak Envelope POwer
    PWM Pulse Width Modulation
    RF Radio Frequency
    Rx Recieve
    SSB Single Side Band (Suppressed Carrier)
    SWR Standing Wave Ratio
    Tx Transmit

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