*Antenna Tuning*
Mobile & Base
By: Eli
9-30-02

So you go down to your local Radio Slack and buy a new antenna. Throw it on the BOV, hook it up to the radio and you're ready to go....not quite. On the packaging of the antenna it shows how to put it together. It also has a SWR curve that tells you how long the whip needs to be and on what frequency for your optimal tune. The manufacturer tries to make you believe that assembling the antenna and mounting it is all that is "necessary". The instructions on the package say pretty much "for optimal performance antenna may be tuned using Radio Shack model # blah blah blah SWR/power meter" It doesn't say anything about if the SWR is too high it will cook your radio. Optimal performance is what you want, but you don't need a Radio Shack meter. Any SWR meter that is designed for the frequency range you are tuning will work.

What is SWR? SWR is Standing-Wave-Ratio. In English that means the ratio of your transmitter's driven power compared to the power reflected back down the feedline by the antenna. If the SWR is too high (too much power being reflected back into the radio) you are not getting full power out of the antenna and eventually you will most likely fry the final amplifier in your radio. Example: A radio is producing 50 watts of power. The antenna is sofar out of tune that 30 watts are being reflected back into the radio, you're actually only getting 20 watts out of the antenna. 30 watts is alot of power to reflect down a feedline!!!!!!!! It won't take long to smoke your radio. I saw that SWR a couple of weeks ago on one of the Sheriff's deputy's radio. Luckily he didn't fry it.

What is tuning? Every frequency has a certain electrical length. Tuning is simply matching the antenna's electrical length to the frequency range it will be used on. That doesn't mean that if you have a Cobra CB and a perfectly tuned antenna and then change to a Midland CB you have to retune the antenna. Some people have that belief. The antenna doesn't know what kind of radio it's hooked to. All it knows is what frequency it's tuned to. You can change radios 20 times a week and aslong as you operate in the same frequency range you don't have to retune.

For CB'ers. CB's by law are only permitted to produce 4 watts of RF(radio frequency) energy. If your antenna is not correctly tuned you may be driving 4 watts of power, reflecting 3 watts which means you are actually only getting 1 watt of power out of the antenna. 1 watt on CB freqs isn't much. Chances are you won't cook your radio due to the low power being reflected, but you won't be able to talk very far and an out of tune antenna also affects reception.

How do I tune a CB antenna?

Simple. From the beginning. Mount the antenna as near the middle of the roof of your vehicle as possible. This will give the antenna a nice even ground plane. Route the cable to the radio-avoid pinching it or running it next to electrical wires. The SWR meter can be installed anywhere between the transceiver and the antenna. Most mobile antennas have only one connection point. The easiest setup is to have the SWR meter directly after the radio.

Radio-SWR meter-antenna. Easy as that. Make sure the SWR meter is designed to work on the frequency range of 26-27 MHz. Hook it all up and pick a channel. I generally start on CH 20. Key the mic and watch the SWR...where's it at? Is it over 1:5:1? If it is, then go to CH 40, key the mic again..what is the reading? Remember it. Then go to CH 1, key the mic and check the reading. If the SWR reading is higher on CH 40 than it is on CH 1, the whip needs to be shortened. If the SWR reading is higher on CH 1 than it is on CH 40 the whip needs to be lengthened.

Easy way to remember that-Higher on the upper, shorten If the SWR is higher on the upper frequency, shorten the whip. Vice versa if it's the other way.

Adjusting the whip takes a little time & patience. Nomatter which way you're tuning it, do it in small incriments, approx 1/4 inch at a time. After each adjustment recheck the SWR on CH 1 & 40 until you find your correct tune. Ideally you should be able to get 4 watts driven with about 1/2 watt reflected. Equals about 1:5:1.

For tuning Ham antennas and all other antennas:

Same procedures apply for mounting and tuning. Make sure that your SWR meter is designed for the frequency range you are tuning. Be very very very cautious of RF energy while you are tuning. Holding onto a CB antenna pushing 4 watts ain't nothing. Holding onto a HF VHF or UHF etc antenna cranking 50 watts or more o' juice really hurts...bad burn....How do I know this?? ;o)

Base antennas:

BE CAREFUL AROUND POWERLINES!!!

If during the course of installing a base antenna it starts to fall over towards a powerline LET IT GO!!! Don't touch it!!

Since this article is on tuning I won't get into installations. Once your antenna is assembled, my recommendation is to tune it on the ground or at roof level. Standing on a roof and putting the antenna up and taking it down numerous times to tune it sucks especially if it's a heavy one.

Tuning procedures are the same. Higher on the upper, shorten, etc. After your antenna is tuned, then install it. After it's all secured in it's new home, recheck the SWR to ensure nothing changed while being installed.

Happy Tuning! ;o)
Eli

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