CB Radio
SAY AGAIN ............... OVER !
Part 1 - Radio Theory and Installation
One of the major things that can make or break a good 4WD trip is the quality of radio communications with your fellow travellers. Have a look around at the bush car park at Smoko and you will quickly notice that there are hardly two vehicles alike that have similar radio and /or antenna fits. It should come as no surprise then that performance across the air waves is similarly varied.
When it comes to the radio Receiver-Transmitter (RT), it’s like most things in life - you get what you pay for. If you buy bottom end of the market , then with few exceptions, you’ll get bottom end performance. For that reason, and a combination of not being rich, most folk settle for a mid-priced unit that does the job quite adequately for our requirements. Our requirements include communicating over relatively short distances (usually within 5km), through the trees, over hill and down dale - beyond line-of-sight. So we opt for relatively cheap 27MHZ CB (Citizen Band) radios that are able to work beyond line-of-sight by using the High Frequency (HF) radio propagation principle of skipping signals off layers in the ionosphere. Unfortunately, the skip distance is limited somewhat by the restricted power output of vehicular CBs which are limited by law to very low power levels.
With antenna selection, however, price is not the only governing factor over performance. Much more important is matching the antenna type (and there are many different types) with RT, coaxial cable, connections, mountings and very important, mounting location.
Delving back into the science of it for a moment. CB radios transmit and receive an amplitude modulated signal on a number of preset frequencies (channels - usually 40) incremented around 27MHZ - that’s a sine-wave shaped signal that peaks around 27,000,000 times per second. The distance between each successive peak is called the wavelength and knowing the speed that radio signals travel (186,000,000 miles per second), it can be equated to a distance on the ground. For an antenna to work properly - that is receive or transmit the full signal while the sine-wave is oscillating between minimum and maximum values - it must be related in length to the radio wavelength. In an ideal world an antenna to wavelength ratio of 1:1 would be best but at 27MHZ that would mean having an antenna some 36ft high - not real flash for getting through the drive-thru at McDonald’s unscathed.
The "boffins" did discover, however, that by using a shorter antenna length that was mathematically related to the wavelength (½ , ¼ , 5/8 etc), the antenna would still function properly, that is if there is an adequate ground plane - the other "unseen" half of the antenna - the car body (that is why the antenna is earthed by the outer shield of the coax which feeds from the RT unit). Even at a fraction of the wavelength it’s still not short enough for us though, so the antenna has been shortened even further with the difference being made up by "electrically lengthening" the antenna. Such antennas can be identified by having a noticeably thicker top section, middle section, bottom section or an extra wire spiralling around the antenna’s length. You will recognise some of the names for these antenna types - bottom loaded, top loaded, helical etc. The only problem with those type of antennae is that transmission and reception is biased toward the end with the extra electrical length. Same applies to an un-even ground plane i.e antenna mounted at one end of the vehicle instead of centrally. In both cases, the antenna behaves as if it were directional instead of omni-directional. The selection of an optimum mounting location is therefore highly desirable (but often, impractical).
So then, where do we mount the antenna? The best place is centrally mounted, up high, with no obstructions, and as far as possible from any self generated electrical interference. The latter is very difficult to exclude because every electrical and metal component in the vehicle is radiating electromagnetic energy (EMF) and the antenna is, of course, designed to pick up EMF. Not only that, but the wiring in the RT unit and the coax which feeds the antenna also picks up unwanted signals to some extent, although various noise suppressing features are available to lessen the impact of these. The worse place is down low, shielded by metal objects (the car body, roo bar etc) or near any major interference sources (engine electrics, fans etc). Worse still, is having the "loaded" end of the antenna heavily shielded or placed near the interference source.
So the choices, in order of preference are: smack in the middle of the roof (antenna loading type of less importance but higher is better), mounted on the roof rain channel, on the front guard (top loaded or helical), or on the roo bar (top loaded or helical). Least desirable is to have a base loaded antenna mounted on the bumper between the front grille and the roo bar (complete with any spot lights, winch or other paraphernalia hanging off) - been there, done that.
So now that we know there is a bit of science involved with antenna selection and placement, there are other things to consider. To feed the antenna, good quality coaxial cable should be used to get maximum shielding from stray EMF. Even then , the outer shield of the cable will still pick up some EMF, so avoid running it past electrics, particularly the engine, if you can. Use only the length required - no extra length curled up behind the dash for the day you transfer the radio to that semi-trailer you’re thinking of buying. All joins should be soldered, where applicable, to make good contact and exclude dust and water etc.
Now run away, get a cuppa and check all of the above.
Next part covers tuning the circuit (SWR adjustment), RF Gain and Squelch controls.
Part 2 - Antenna Tuning and Radio Controls
Once you have your radio and antenna installed there is one more thing to do - tune the system for optimum performance. All CB radios are designed to transmit through a 50 ohm coaxial cable into a 50 ohm antenna (ohm is an electrical measurement somewhat equivalent to resistance). The coax and the antenna are tailor made to be 50 ohms but as your vehicle is effectively the "unseen" bottom half of the antenna and all cars vary in shape mass metal content etc, that part needs to be adjusted. For that reason, CB antennas usually come with a grub screw adjustment at the base to allow the antenna to be lengthened or shortened in its mount, or else the antenna comes with a removable tip to allow you to trim off the end, usually in 1mm increments. What we are talking about here is adjusting the Standing Wave Ratio (SWR) of the antenna.
SWR is the ratio of the power transmitted to the antenna by the radio compared to the amount of power that can’t be radiated by the antenna due to the complete antenna system (including the unseen part) not being equal to 50 ohm. This unradiated power will reverse direction at the antenna and travel back to the radio. Excessive unradiated power will weaken your transmitted signal but in some cases, can also damage your radio’s final output transistor through overheating. To rectify this, a meter is used to measure the SWR and the antenna length is adjusted by use of the grub screw or trimming the tip of the antenna. While most antennas come allegedly "pre-tuned", this is really only an average setting. The antenna’s SWR still needs to be adjusted to suit your antenna/vehicle combination.
The procedure for checking and adjusting the SWR is relatively simple. Plug the SWR meter to the antenna output jack on the radio and coax feed to the antenna so that it is in the antenna feed line. Check SWR on CH1 and CH40. If the SWR is lower on CH1 than CH40, insert the antenna into the base about 5mm and check again. Note that on antennas without a grub screw adjustment it may be necessary to cut pieces off the antenna to achieve the same result. As there is no reversing this if you overdo it, it is best to do it very carefully in 1mm increments. Conversely, if the SWR is higher on CH1 and lower on CH40, pull the antenna out 5mm. The aim is to adjust the antenna length until the SWR reading is the same or as close as possible for both CH1 and CH40. It is important to have the doors of the vehicle shut while adjusting the SWR and the vehicle 10m clear of all metal and wire structures. There is no need to try to get a specific SWR reading on the meter as the actual ratio is largely out of your control, an equal CH1 and CH40 reading is the best we can achieve in the back yard. The SWR ratio should be in the 1.5:1 region though for CH1 and CH40. At CH20 it will read much lower, around 1:1. Any SWR that won’t balance at CH1 and CH40 or ratio readings that are way out should be referred to a radio specialist for advice.
Now that the hardware is installed and set up, there are another couple of practical things that need to be understood about CBs - user controls. There aren’t many, but they are important.
Most radios come with both an RF Gain and Squelch control. Both are receiver adjustments only and have no effect on transmission.
RF Gain adjusts the amount of incoming signal that is allowed through the radio circuits. Looking at the sine wave diagram, most of the signal we want is in the stronger peaked part of the wave. RF Gain adjusts how wide the window of frequencies are that radio processes either side of the peak. Increase the gain and you let in more wanted signal but also more unwanted noise which is mixed in with the weaker part of the signal. That is why you hear more static as you wind up the gain.
Squelch is nothing more than an adjustment which raises or lowers the automatic gain control of the receiver. There is always background noise over the radio waves (including weak radio signals from other users some distance away) so CB radios are designed to ignore a certain amount of the noise and remain quiet until confronted with a valid signal - your leader telling you to close the gate. The squelch control enables you to tell the radio to ignore more or less of the background noise (including the leader perhaps?) and wake me up when a real signal comes in. It basically moves the "ignore the noise" threshold up and down, relative to the wanted signal peak.
With these two controls, rudimentary as they are, it enables you to zero in on the desired signal and exclude a fair amount of the noise. In most cases, it is not necessary to have the RF full up and the squelch full off. The noisy hash coming from the guys radio next door tells you he either hasn’t installed and tuned his radio system correctly and has to compensate for its inefficiencies, or else he doesn’t understand the purpose of the user controls.
But now you do!
Danny Williams SC412