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Author Topic: Solving high swr problem  (Read 14074 times)

Posts: 18

« on: June 21, 2009, 08:24:55 PM »

I need ideas on how to lower swr right now it is at about 3.1:1-3.5:1 the high swr is because the antenna is mounted right behind the cab of a truck on the passenger side of the box. I have cut the antenna down about 5 inch's and that is how I got it down to the current level. Out of the package it read off my meter. The simple answer is move the antenna back away form the cab. Unfortanetly he allready drilled the hole in his brand new truck and does not want to drill another or just leave a hole there. Is there anyway to get the swr to a useable range with the antenna in its current place? Would an antenna tuner work? I hate to have him buy one and it not work. If it matters the antenna is a base loaded 48" (factory) now 43" CB antenna used for 10 meters and i beleive his radio does around 100 watts so if we can't get it down I don't think the finals are going to last long. Also one the opposite side of the truck in the same place he has a base loaded 2 meter antenna (the antennas look alike other then coils) I was able to tune that one down to about 2.0:1 by taking around 3-4 inch's off so with his 50 watt radio I told him he should be fine on that one but the high swr on the 10 meter radio worries me. Any thoughts or ideas would be great!

Posts: 10248


« Reply #1 on: June 22, 2009, 05:13:20 AM »

You're working with an unknown quantity. What you need to do, is beg, borrow, or steal an MFJ-259B and find the true resonant point. That is, when X=Ø, not the lowest SWR!!!!! Remember, SWR Means nothing, if the reactance is way off.

The fact it is mounted to close to the body certainly adds to the overall losses, but that fact doesn't preclude matching it, and you do not need a tuner to do it.

Go to my web site, and look under Antenna Matching.

Alan, KØBG


Posts: 15066

« Reply #2 on: June 22, 2009, 06:02:24 AM »

Alan is right. You need more info than you can get from an SWR meter. Tuning for minimum SWR only works when you know that the antenna is designed to present a 50 ohm load when it is resonant. You need to do two things: 1) make the antenna resonant (zero reactance) and 2) make it match 50 ohms. Quite often with HF antennas it is necessary to tune the antenna loading coil to resonance and then tune a matching coil at the base to get 50 ohms and a low SWR.

A tuner can provide a nice match for the radio but it won't necessarily make the antenna system efficient. It can wind up masking the real problem.

Bob  AA4PB
Garrisonville, VA

Posts: 18

« Reply #3 on: June 22, 2009, 08:33:46 AM »

I am still not sure whats all going on that I am getting the high reading but just to see how his antenna is working we hooked his old cobra cb up to it (better to burn that up then the 10 meter radio) and I have a uniden 68 elite hooked to a shakespear big stick as my base radio. So to test his antenna he drove away from my house and we were able to talk all of 7 miles with out lossing each other at all signal got a little weaker at the bottom of hills but the signal never became unusable. At 7 miles I decided that was far enough because that is about the same average range my brother has and he is running the same radio i have in the house but with a wilson 500. So aparently even thou I am getting a high reading his antenna seems to be performing like it should.

Posts: 18

« Reply #4 on: June 22, 2009, 08:40:21 AM »

K0BG, Thanks for the link to your site! I read a few things you have on there and have to admit I don't understand 1/2 the things I glanced threw so will be revisiting that site and see what I can learn or better understand. There is a cb shop in town and i know he has an antenna analizer I seen him use it to test what band an antenna was for. I didn't realize what he was doing at the time just remembered it now but I think I will go give him a visit and see if he will let me use it. I have bought a few diffrent things from him over the years so hopefully he is in a good mood.

Posts: 1458

« Reply #5 on: July 01, 2009, 06:32:29 AM »

It sounds to me like you have defective antennas, mounts or cables. I have never experienced SWR that high from a new mobile antenna nor did I have to trim one. Do not transmit until you find the problem! Try to find someone who is more experienced in mobile antenna installation at your local ham club before you burn up a transmitter. If you are having this much trouble with a basic mobile antenna installation, an antenna analyzer is not going to help.

My antenna is mounted on the tool box behind the cab. It is a cheap air coil 19" VHF/UHF antenna made by Pro Am that I paid $12.50 plus tax for at a ham fest. The SWR is 1.2, after I added a stainless steel washer as a elevating spacer on the threaded stud mount. Previously, It was 1.5, even behind the cab. The location and other ham antenna is not the problem. Something is defective!
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