View Full Version : For the Newbies: How to tune a transmitting antenna

09-11-2013, 10:18 AM
This was written for a "New Guy" in the shop who kept asking how do you do this?
Testing should be done away from buildings and metal. Close your vehicle doors and if tuning a firestik be sure the cap is on. Keep helpers clear or in the vehicle when testing.

First be sure the installation of the coax is correct and none of the coax is cut, crimped, or looped around metal objects. Extra coax can be laid out in a zig zag pattern under the flooring material inside the car after tuning is complete. Never, ever, tune an antenna with a linear in the circuit because the harmonics are boosted and the reflected by the antenna and this gives a false indication of higher SWR. (I have proof of this with my spectrum analyzer)

Cutting the coax is not a good idea because the cable lengths are calculated at the factory for the lowest SWR for a given antenna. Some people think you can tune the antenna by trimming the coax, you affect the impedance of the transmission line but the antenna is not tuned. Antennas tuned this way will not be as efficient as the SWR can still be high at the antenna. Coax length is adjusted but only after the antenna is tuned. By doing that way you can get the SWR low but the antenna will not radiate effectively.

Loosen the stinger and set to the mid range of the adjustment range.
To start have your SWR meter in line with the jumper leads no longer the 12” in length. (Or as short as you can use) With no adjustments made to the length made test the SWR on channel 1 and channel 40. This first test is what will tell you what you need to do to tune the antenna.

Calibrate the SWR meter on channel number 1 take your reading.

Then repeat the entire process on channel 40.
If the SWR is higher on channel 40 than channel 1:
This indicates that the antenna is too long.

If the SWR is higher on channel 1 than channel 40:
This indicates the antenna is too short.

Now switch to channel 20 and make your adjustments to either lengthen or shorten the stinger. Tune for the lowest SWR on this channel and make small adjustments.
An eighth of an inch at time will give you results you can interpret. When you have it tuned to as low of SWR as you can get it, repeat the channel 1 and channel 40 processes to see if the antenna is tuned to the center of the CB band. If you still have a variance between channels 1 and 40 make smaller adjustments to try to center the tuning.

Some people feel that if the SWR is below 2:1 on the ends of the band that is good enough. But the lower the better.

Now that the antenna is tuned, adjusting the length of the coax can actually tune the entire circuit for maximum efficiency. This can be done by inserting a barrel connector and adding three feet of cable at a time. If you add 3 feet and the SWR goes up it is now too long. Remove your jumper. If it is too long cut out 1 foot and test and this will give you a good indication if tuning the coax will help. If the SWR goes down when you added the three foot jumper add another 3 feet and test.

Now you have a general idea of how it goes.

the caveman
09-12-2013, 08:28 AM
good information Tallman!