CB Radio

Reprint from the September 2000 Wanderer
From: Noel Roberts, Q13280, Yeppoon Qld

ABOUT C.B. RADIOS, UHF and 27 mHz H.F.

After talking to many people on the road, at Chapter meets and Rallies I decided to put together a few thoughts about C.B. Radios. Firstly why are they called C.B. (Citizens Band) radios.

Well in the beginning we had commercial radios e.g. Taxi, Government Departments, Commercial businesses etc who had a Base Station and Mobiles, for which they applied for an operating frequency which in most cases was allocated to that particular organisation and not to others in that town etc, so that it was a private channel, this concept did change in later times. Due to the efforts of persons like Mr Dick Smith and others, the Commonwealth Government set up a frequency band in the H.F. region of 27 MegaHertz using Amplitude Modulation similar to the American Citizens Band with most of the equipment being made in Japan with some imports from The United States of America.

Later The Philips Company in Australia designed and manufactured a U.H.F. Frequency Modulated Two-Way Radio, it was a simplified version of many of the then V.H.F. and U.H.F. commercial radios, it had 5 watts of Power and 40 channels. It was an immediate success. Repeaters soon followed and the system spread across Australia. Initially a license was charged, some years ago the licensing of both systems was changed to a Class Type of license without a fee.

An explanation of Bands:- The H.F. Band starts at 3.0 Megahetrtz and goes to 30.0 Megahertz, The V.H.F. Band starts at 30.0 Megahertz and extends to 300.0 Megahertz, the U.H.F. Band starts at 300.0 Megahetz and extends to 3000.0 Megahetz

As in the above there are two types of C.B. (Citizens Band) Two Way Radios in use in Australia and we Motor Homers use both types. They are generally called by some as C.B. and U.H.F. This not correct as they are both C.B. (Citizens Band) Radios and there are many other types of U.H.F. Systems including a new low power U.H.F. type for private use, so lets get it right and call them 27meg C.B. and U.H.F. C.B.

1/ U.H.F. (Ultra High Frequency) FREQUECY MODULATED C.B. RADIOS

This system some times incorrectly called U.H.F or Two-way Radio is an Australian developed system and it is the basis of most Australian private mobile radio communications. It uses 40 channel frequencies in the area of 476.425 to 477.400 Megahertz and with a maximum transmitter power of 5 watts. Range on the flat between two vehicles is about up to 20 kilometers, if one or both units are elevated, up to 100 kilometers may be obtained. If there is a hill or a mountain between the two units then range would be much less. Channels 1 to 8 and 31 to 38 are set aside for Repeater use. A Repeater is a Transmitter/Receiver usually located in a high situation, which provides extended coverage, by receiving you on say channel 1 and re-transmitting the speech on channel 31 to your mate. The system operates as follows = When you press your microphone button and you have the “Duplex or Repeater” button selected, your radio transmits to the repeater receiver on the repeater “Input” Channels 31 to 38, when received by the Repeater Receiver this causes the Repeater Transmitter to be activated and re-transmits on the Repeater “Output” channels 1 to 8.

If you transmit on Channels 31 to 38 and you are in a location to contact a repeater you will cause the Repeater to re-transmit your voice SO DO NOT use channels 31 to 38. You should also restrict your use of Channels 1 to 8 except when it is necessary to use a Repeater. The use of Channels 1 to 8 with the “Duplex Button” out is frowned upon. This U.H.F. C.B system works very well and does not suffer from interference from the vehicles’ electrical system or power lines or lightning storms.

Aerials are generally short, but Aerials with a high gain are quite popular, the High Gain types have a coil or coils wound into the steel rod or wound on a fibreglass rod and this type can be quite long. Gain is measured in Decibels (db) and this is an algorithmic measurement with 3db being 2 power times, 6db being 4 power times and 10 db being 10 power times. So that a 5 watt radio and a 6db antenna will perform like a 20 watt radio with the corollary advantage that it has the gain also on receive. Most U.H.F. Aerials require a Ground Plane i.e. they should be mounted on top of a metal surface. This is of course hard to obtain on a Motorhome, a good substitute is to run 3 wires from the base of the Aerial one forward one back and one across the vehicle. Some Ground Independent types are available.

I have seen quite a long Aerial (about 1.2 metres long) on one of the “wing” mirrors which worked well. When power is applied from the transmitter to the aerial, unless the Aerial is correctly tuned, some of that power is fed back to the transmitter where it is dissipated in heat. This is called the V.S.W.R. (Voltage Standing Wave Ratio) it should be checked and adjusted so that the amount of reflected power is nil or nearly so. A V.S.W.R of One to 0ne being perfect. Good Communication service shops have such an adjusting meter.

As U.H.F. communication is line of sight plus a margin, the Aerial should be mounted as high and as clear as possible.

2/ H.F. 27 MEGAHERTZ AMPLITUDE MODULATED or SINGLE SIDEBAND C.B. RADIOS.

This system incorrectly called C.B. was pioneered in the United States of America and at the time of its inception into Australia it certainly filled a need. It uses 40 channel frequencies in the H.F. area (3 to 30 Megahertz) of 26.965 to 27.405 Megahertz and a maximum transmitter power of 5 watts for A.M. and an equivalent peak power in the S.S.B. mode. Though becoming lesser used today, many Motorhomers have these units fitted. The system suffers from variation of range due to atmospheric and weather conditions; Range between units can vary from a few kilometers to the other side of Australia, with no predictability that in say 10 minutes time you may get the same range. It is very susceptible to electrical interference from vehicle electrical systems. Overseas Signals and Power Line Interference. Most of the radios used by motor homers are the A.M. (Amplitude Modulated) versions.

The Aerial used for this system is usually a Black Fiberglass rod about 1 to 1.5 meters long. All types of H.F. 27 Meg Aerials need a ground plane, as losses are much less at this lower frequency a poorer Ground Plane will normally suffice and mounting the Aerial on top of a Rear vision Mirror generally works well, however the V.S.W.R. must be checked.

Correct usage of the squelch (mute) control. 27 mhz and u.h.f. systems.

Liken the squelch control to a noise “gate”, when “open” the noise will rush through and when “closed” the noise is stopped, but when a carrier signal is received, the “gate” will open.

To adjust! set the volume control to the level you prefer. Adjust the squelch knob so that there is noise when no speech or signal is present, turn the knob back until the noise just stops, turn back about 3 mm and leave there. If you turn the knob right back you will cut out weak signals.

Microphone technique

All mobile radio Microphones are designed so that for correct modulation of the radio, you must talk close to the microphone Talking across the microphone is better than talking directly into it, to prevent the air from your mouth impinging on the microphone diaphragm.

Power connection

It is preferred that the Positive wire of all Two Way Radios be directly connected via a fuse to the Battery, This gives a low impedance connection to help eliminate interference from the vehicle electrics. When connecting the Negative wire it should be connected to the chassis and not to the battery negative terminal. The reason is that if a bad connection is made in other wires to the negative terminal all other current will try to be derived via the radio and probably through the Aerial circuit with subsequent damage to the radio.

Channels to be used.

Some Channels are set aside for special purposes. On 27 mHz Channel 9 and on U.H.F. 5/35 are accepted as the Emergency Channels. On 27 mHz Channel 11 and on U.H.F. 11are accepted as the Calling Channels. On 27 mHz. Channel 8 and on U.H.F. Channel 40 are accepted as the Truckies Channel

Some time ago our Club decided to use Channel 20 on both 27 mHz and U.H.F. and this was confirmed at the 14 th Anniversary Rally at Warragul, where Signs for your vehicle were available for purchase!! I sincerely urge you to get a sign on the back and possibly on the front of your Unit and use Channel 20, the semi’s will call you if needed and maybe you will answer me when we see one another on the road.

Article from April 2000:

AM

For many years the humble AM CB (Amplitude Modulation) was king, generally small in size, low cost and easy to install with very few controls to learn, made it popular. As it was the first CB radio available, it's popularity was enhanced, not by performance but by numbers, if you wanted to travel with in company with others and they had AM CB, then you had to have the same.

With the use of small antennas for mobile operation, the reliable range is quite small, 1 – 3km in average terrain, vehicle noise can be a problem making the speech hard to understand and while longer distances, even worldwide, are possible it is not common and should not be relied upon.

SSB

The correct terminology is Suppressed-carrier Single Side Band but shortened for general usage to Single Side Band. An SSB CB has three modes AM, LSB (lower side band) and USB (upper side band). It can be used in AM mode over all 40 channels making them compatible with the AM CB in every respect. In SSB mode, because the power is being directed to one sideband, the effective radiated power is increased they therefore have a longer range and better performance. In conditions where weak signals are common, SSB can provide intelligible audio through a background of considerable noise.

Another feature of some importance in today’s crowded radio spectrum, two conversations can be held on one channel, one in LSB and one in USB. Because there is no carrier present, there is also no howls and whistles when two SSB stations are on the same channel. This does not mean they won’t be there, it means they are not created by the SSB sets, and it is one of the reason for the use of SSB in HF radios.

When the CB is in AM mode an SSB transmission sounds like duck talk, and impossible to understand. The CB should be placed in either LSB or USB whichever is the clearer and the transmission tuned in with the aid of the clarifier, to give acceptable speech quality.

It is important to note that while a CB can have a range of many thousands of kilometres, this is not reliable and should not be considered as a safety device. For a more detailed explanation please read up on propagation. If you intend to travel into remote country, don't "bet your life" and those of your companions, that your CB will get you out of trouble.

UHF

FM or Frequency Modulation is the mode of operation, and like your car radio the sound or speech is very clear and not affected by electrical or atmospheric noise, and the strongest signal is the one heard.

The antenna and the terrain govern the distance of reliable reception. On flat plains 20 – 30km is not unreasonable, in hilly country 5 -10Km may be the limit. Repeaters greatly enhanced the range. A repeater is a UHF radio, located on a tower or mountaintop with a very high gain antenna. Channels 1 - 8 (receive) and 31 - 38 (transmit) are devoted to repeaters; this will extend the range in some cases out to 200Km.

Most farms and outback stations use UHF CB for their day-to-day communication and will generally answer a call for assistance. Just place your CB in scan mode to hear what channels are being used.

 Transport drivers also use UHF CB and generally monitor channel 40, the "truckies channel".

ANTENNAS, POWER & SIGNAL STRENGTH

AM and SSB systems need an antenna mounted clear of any metal body and the higher the better the range. If mounted on a bull bar, a good idea is to connect a braided wire from the antenna base to the vehicle body to make sure there is a good earth.

Because the frequency of a UHF CB is in the Ultra High Frequency range, the antennas are much smaller that AM or SSB types. The higher the frequency the smaller the antenna, have a look at your cell phone, their frequency is nearly twice that of a UHF CB. Therefore an antenna can be made to have gain, which will increase the Effective Radiated Power (ERP) of the CB.

Mobile antennas with 3db, 6db, or 9db are common. A decibel abbreviated to db is defined, as the logarithm of a power ratio and its unit is the bel. It gives a way to compare power levels with each other and with some reference power. An antenna with a gain of 3db, doubles the ERP of a UHF CB from 12 watts to 24 watts, 6db is 4 times the power, and 10db is 10 times the power. As you can see antenna performance can assist your reception and transmission greatly. While on this subject reference to signal strength can be heard when comparing various signals. Signal strength is recognised as starting at 1 for an almost unintelligible signal to 9 for a very loud signal or 9+ for an even louder signal. One S point, that is the step from say strength 5 to 6 is equivalent to 6db. So all things being equal if you change your antenna from a 3db gain to a 6db gain you will appear one half an S point louder at the receiving end. This does not sound much, but it all helps and could be the difference between being heard and breaking up.

Breaking up is a term used when a signal is not strong enough to keep the squelch open fully all the time, and in cases like this it is wise to turn the squelch off if the signal is of importance. The background noise will be overcome by the signal in varying degrees however the signal should be intelligible most of the time.

With AM & SSB CB radio's it is necessary to tune the antenna to the radio, most antennas are supplied longer than required so that they can be trimmed during installation. This tuning is done with an SWR (Standing Wave Ratio) meter. Tuning of the antenna will have very little effect on reception in most cases, however transmission will be very poor with a high SWR. If others are having trouble hearing you, check the connections to the antenna base, they may be corroded or broken. Make sure that the antenna is clear of any metal like stone guards or roof racks.

Have we a preference?

If you intend to fit one CB generally the FM unit is it. Although more expensive, more people will are likely to hear a call for help. The CMCA members generally use channel 20, on all CB radio's Australia wide if in convoy or in built up areas, on the open road going solo, channel 40 is common with the added advantage (?) that you may expand your vocabulary.

Link to Australian UHF Repeaters Listing