TUNING YOUR CB SYSTEM

Well, now that you have inspected, serviced or replaced your CB's Coax, (see last issue - Communications Corner), it's time to test your set up to see just how good it is. This is done by placing a Standing Wave Ratio (SWR) meter in line between the CB and the Coax.

An SWR meter is a piece of relatively inexpensive test equipment that really makes or breaks a CB system. It checks how well your antenna and coax is matched to your CB. If the antenna is exactly the right length, grounded properly and not located near any parallel metal surfaces, you should get a good SWR meter reading.

Having a tuned system is very important for getting all the power out of your CB, through the coax and out the antenna. Unfortunately, if the antenna is not the correct length, it reflects some of the power back into the CB. This reflection is proportional to how far the antenna is off its correct length. The SWR meter measures the ratio of power available to get out of the antenna and the reflected power not able to get out of the antenna. A high SWR reading, say 3 or more, means power that should be going out of the antenna is returning back into the CB. This can damage the CB, ruining the transmitter section. Caution should be taken if buying a used CB, that the unit transmits well.

The long and short of it (ha ha) is that unless you use an SWR meter to tune your antenna to the correct length, you are not getting all of the available power out of your CB, no matter how good your radio is or how much you paid for it. SWR meters are very easy to operate. As they measure the transmitted power supplied by the CB, no external power is required. An SWR meter does not require batteries, unless it is to run a light bulb that illuminates the meter.

To test your system with an SWR meter, undo the coax at the rear of your CB and place the meter in line by plugging it into the back of the radio. You may need a short jumper coax if one is not attached to the meter. The SWR meter will have two jacks, input (from CB) and output (to antenna). Once in line, place the switch on the front of the meter to FWD or CAL position. Turn on your CB and select a clear channel, near the middle of the band (i.e. - channel 20). Now key up the Mic and turn the adjustment knob on the SWR meter so that the needle falls in line with the SET or CAL mark. This mark is usually at the extreme right side of the scale. Once adjusted, unkey the radio. You have now calibrated the SWR meter for the power of your CB. Change the switch on the SWR meter to the REF position and re-key the mic. Do not readjust the knob. Now read the scale and if the needle is under 2, things are well. If under 1.5, very well. And if over 3, and into the red, STOP transmitting.

So, your CB reads 3+ and into the red on the SWR meter. This is not good as it is showing a very high ratio of power being reflected back into the radio. You will be happy to hear that the problem is not with your CB. It will be a matter of repair or replacement of your Coax, or repair, tuning or moving your antenna. Since the coax was previously discussed, let's deal with the antenna.

How critical is the length of the antenna? Well, the shorter it is, the more critical the adjustments. 1/4" off of a wire wound, 4ft fiberglass antenna can be the difference from good to junk! Please note that the fiberglass rod, in a fiberglass antenna has minimal electrical affect on the antenna, as it is just a carrier for the wire.

If you have cut your antenna 3 inches or more, so it could fit into your garage better, throw it away. If you have cut it less than 3 inches, you may be able to restore it to its proper length by installing a spring and/or quick disconnect coupling.

In a future article, I will explain why I like a 102 inch stainless steel whip but if you have to buy a shorty (to fit into the garage), please get one that adjusts, with a tip/rod and a set screw.

Use the adjustment screw to raise or lower the adjustable rod until the SWR is lower on the meter. You may have to re-calibrate the meter every time you change the length of the rod in 1/16" increments or less. Which direction, up or down, do you go? Well, if the SWR is high on channel 1 and lower on channel 40, the antenna has to be longer. If the SWR is low on channel 1 and high on channel 40, it has to be shorter. An acceptable and expected reading for a short style antenna would be 2 on channels 1 & 40, with a dip lower on channel 20 to 1.4 or less. For a full length stainless steel whip, a reading of 1.4 or less across the entire 40 channels should be obtainable.

Before you go trimming your antenna, make sure the antenna itself is electrically isolated from the vehicle body, and the antenna mount is well grounded to the metal of the body.

The location of the antenna with respect to the body also has a big affect on the antenna performance. The metal of the body actually forms the other half of the antenna and is used in conjunction with the antenna to transmit the signal. The antenna wants to see the metal of the vehicle under it, as a horizontal surface, not beside it. The best location would be to mount the antenna on top of the centre of the roof. However, that's not practical for those low tree limbs while off roading or soft-tops!

If your antenna is mounted too close to a vertical metal surface, this can have an ill effect on your SWR. For instance, if your whip is mounted on the rear bumper of your pickup truck, it must be at least 6-8 inches away from the metal, vertical and parallel surface of the body. Or worse yet, if your antenna is on the front bumper and bends back towards the front of the vehicle, this will have a drastic shorting affect, as the SWR will rise when the antenna approaches the vehicle body.

To demonstrate how the antenna and the body act together, while checking the SWR, get someone to whip the antenna and let go, so that it goes back and forth. Then key up and watch the SWR meter's needle do the same. SAFETY NOTE: never touch any transmitting antenna with your body or you may be burned.

Keep in mind that when mounting your CB, you should be able to get at the connectors on the back of your radio easily, as the SWR should be checked every 6 months or less.

If you have bought a CB with all the bells and whistles, look close, you may have a SWR meter built-in. Actually, other than a public address function, a built-in SWR meter is the only gadget built into a CB that might justify spending more money.

SWR meters are available at your local radio store for about $35.00. Don't go all out on an expensive unit. I would rather have a small meter with me than a fancy one at home. An SWR meter could also make for a good club purchase, probably less than a buck a vehicle!

Other Issues: Coax cables | SWR Meters | Antenna Mounting
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