Odd Knowledge: Old Technology
(CB Radio...)

How to Tune a CB Antenna
Version 1.2

Install the CB. Make sure the ground is a good one (some recommend running a wire from the vehicle's chassis) & that you are using the same power line as runs to your car stereo (for noise filteration) or perhaps a direct line from the noise suppression unit in the engine. Inspect the coax cable for damage. On the end that attaches to the antenna base, you may want to use pliers to (gently) tighten the coax connector to the base. Be sure the coax is hand tight on the back of the CB radio (put some effort into it! You don't want it falling off.). Attach the microphone.

INSTALL the antenna. Be sure the antenna is insulated from ground. Recall that coax cable has a sheathed internal wire and a surrounding, but separate "hollow cable" which provides ground running around the sheathed internal wire. (The ground "hollow cable" is not visible because it, too, is shielded). Remember that the sheathed internal wire must only share conductivity with the female part of the coax hookup on the back of the CB and, on the other end, the antenna itself. So, be absolutely certain there is a non-conductive buffer between the base attachment for the antenna (which looks like a cylindrical nut, and will be grounded) and the antenna itself. (With a magnetic mount this is not an issue, since the company supplies an appropriate base.) On a do-it-yourself-permanent installation, be aware that the modified cylindrical "nut" that accepts the "bolt" part of the bottom of the antenna (or the "bolt" part of the spring on the base of the antenna) must sit on a teflon disk that allows the cylindrical "nut" to have no conductivity with the base attachment of the antenna; that is, the vehicle's ground. In order from top to bottom you should see: antenna, possibly a spring, a bolt with threads, a cylindrical nut that accepts the "bolt" part of the antenna, then a non-cunductive buffer, then the vehicle's sheet metal. Under the sheet metal, you'll have a lock nut, an attachment for the end of the coax to screw into, then finally the coax cable end. In summary, the inside center of the cylindrical "nut" that accepts the base of the antenna should have conductivity ONLY to the center wire of the coax cable, which has conductivity ONLY to the inside coax fitting on the back of the CB. If you get this wrong, your SWR meter will let you know immediately.

Don't use the CB without the antenna attached! Moving the antenna changes its tune, so once you install it, leave it (or at least the base) where it is. The best place is center of the vehicle, as high as you can get it. For a pickup, that's in the middle of the top of the cab. If you mount the antenna elsewhere, you will broadcast toward the area of the most metal. Ie, if you mount the antenna on the rear right bumper (worst place), you will broadcast in a conical pattern in the direction of the left front fender, which is the direction of the most metal in your vehicle.

Ask yourself, what channels will I be using the most? Remember:
More people frequent 1-20
(never use 9 except in an emergency!)
Busiest (trucker's) channel: 19
SSB use is usually 30-40, especially 35-40.

Tuning the antenna (below) is an issue only for broadcasting, not receiving.

Turn the engine on and idle (don't just turn the accessories on--you have 13.8 volts when you idle, but only 12 when you turn on accessories only.) Choose the channel in the middle of the range you'll use (say ch 20 if you want to compromise and tune for all 40 channels). Switch the CB's meter to the SWR position, and "key up" briefly (ie, transmit for a second or so). Watch the needle & the scale that says SWR--Standing Wave Ratio (ignore RF scale and the CAL scale for now). The needle should stay low, somewhere between 1 and 1.5 on the SWR scale. If it gets up to 2, you need to do some tuning. If it gets much higher than 2, beware, you could harm the CB: broadcast in less than one second intervals.

Tuning entails adjusting your antenna so the SWR meter reads as low as possible for the channel you've selected. To tune your antenna, you may slide a cuff along the shaft, turn a bolt at the tip, or in the case of a steel whip, trim the whip at the bottom till you get the lowest reading you can. You must tune the antenna so the SWR is as low as possible for the channel you select in the condition the antenna will actually be used. For instance, a bolt-top tuning antenna has a plastic cap over the bolt. You must tune the antenna for the lowest SWR reading with the cap on. (Yes! this entails a lot of trial and error!) The SWR reading should be between 1 (the lowest it gets) and 1.5 for any given channel.

Now calibrate your SWR meter. Switch the CB's meter to CAL, key up briefly (a second or two), and rotate the CAL dial to where it says CALIBRATE (perhaps just a red dot) on the dial. Now check the tune of your antenna again. You may wish to repeat the tuning process with a calibrated meter (not an option with a wire whip antenna! With a wire whip, it may be best to calibrate first and risk a high SWR reading if you think you can calibrate very quickly.) Tuning is now done.

Now switch the CB's meter back to RF, which is where you will leave it most of the time. RF is the way you measure someone else's power and your own output power. Often, the RF scale is labeled S. Transmitting power uses a metric called "S". It's common to ask another CB-er how high you are registering on the S meter (they may reply in slang: you're coming in at 20 pounds--means you're about as high as it gets, around the 20 mark on the S meter. The red part of the dial is sometimes referred to as the cherry orchard, which is where you want other people's S meters to be when you broadcast--meaning your antenna is well-tuned and you're broadcasting at maximum power.) To get someone to tell you how far you are broadcasting, ask for a "radio check." They'll tell you where they are and how many S units you're sending.

You may want to check your broadcasting power (RF & the S-scale) on different channels. You will notice you have more power when the engine is on and you are in a certain channel range (hopefully, the channels you tuned for!) For maximum range, use a channel upon which you output best. You will also notice that the SWR readings will mirror the RF/S readings. Ie, channels you are weak on will probably show a higher SWR reading.

Now to set the dials. If you see a RF GAIN dial, set it at about 75% strength. This helps you hear others better, but degrades the sound as it gets really high. If you're talking close to someone, turn it down; you don't need the "receiving power" or the concomitant noise it brings. Find a station where no one is broadcasting, and turn up the SQUELCH just until the background noise is silent. (You can squelch out louder stuff too; use more squelch if you are in close caravan so you won't be bothered by other people on the channel). If you see switches labeled NB (noise blocker) or ANL (automatic noise limiter), turn them on to cut electrical engine noise. Of course, if the CB is switched to PA, it won't transmit or receive at all, that's "public address," which you won't ever use. If you have a fine-tuning knob, set it at 50% and experiment from there. If the other radio is broadcasting somewhat off frequency, you can improve the reception a little by using the fine-tuner.

You may have a RX (receive) or TX (transmit) light. They simply flash when either is occurring. If you have an SWR light (or SWR ALERT light), it will light up if the antenna is unattached, or badly tuned. I think it simply gives the same info as the SWR meter, only in an idiot light form. If you have a MOD (modulation) light, you should speak into the microphone so that it flashes in accordance with your voice. When it is not flashing, you are not speaking loudly enough or closely enough.

Copyright © 2004 K Rhoads