CB swr adjustment - Page 3 - Toyota FJ Cruiser Forum
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post #21 of 25 (permalink) Old 07-10-2011, 08:48 PM
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Re: CB swr adjustment

Today I installed my Cobra 75 WSXT with a Wilson 4' 18 gauge antenna along with a heavy duty spring (no quick connect).
Right from the start I got a reading of 1.5 on ch 40 and 1.2 on ch 1. I tried to tweak it but I could not get a better reading so I left it at that. I think that if I did not have the spring I could probably tune it in a little better.
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post #22 of 25 (permalink) Old 07-10-2011, 09:19 PM
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Re: CB swr adjustment

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1911 previously said: View Post
Make sure you're reading the right scale on the meter; by definition SWR cannot be lower than 1.0.
when i read this from the Firestik site:
Now, switch your radio to the middle channel (19 on CB). Place the meter switch in the Forward (FWD) position, depress the microphone switch and adjust the meter to place the needle on the Set position of the meter face. Once in the set position, place the meter switch in the Reference (REF) position and note the reading. Release the microphone switch and write this value down to the nearest tenth of a point. Note: If your antenna system is closely matched to the radio you may get little or no movement from the meter needle on this channel. This is normal.

when i read this i thought the little movement i got would be normal or am i misinterpreting what they are saying. I tried to set ch 19 at 1.5 and then do ch 1 and ch 40 but I couldnt get a good enough reading on 1 and 40.
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post #23 of 25 (permalink) Old 07-11-2011, 11:50 AM
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Re: CB swr adjustment

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lmmmmm previously said: View Post
when i read this from the Firestik site:
Now, switch your radio to the middle channel (19 on CB). Place the meter switch in the Forward (FWD) position, depress the microphone switch and adjust the meter to place the needle on the Set position of the meter face. Once in the set position, place the meter switch in the Reference (REF) position and note the reading. Release the microphone switch and write this value down to the nearest tenth of a point. Note: If your antenna system is closely matched to the radio you may get little or no movement from the meter needle on this channel. This is normal.

when i read this i thought the little movement i got would be normal or am i misinterpreting what they are saying. I tried to set ch 19 at 1.5 and then do ch 1 and ch 40 but I couldnt get a good enough reading on 1 and 40.
I think you're doing it right. The problem is two-fold: 1) cheap SWR meters are inaccurate, and 2) some of them use the same meter to read power in Watts, and the scale is different. I've found other people here in the forum who were reading the Watts scale instead of the SWR scale by mistake, which is why I asked.

A simple single-band vertical antenna (like all cb antennas) is just a length of wire, where the exact length of that wire is some resonant multiple of the wavelength of the frequency it is tuned to. What this means is that the antenna can only be perfectly matched or tuned to one single frequency, in this case one single cb channel. All the other frequencies (cb channels) will be less than perfect, a compromise. If you made a graph of SWR on the y-axis and frequencies (cb channels) on the x-axis, the shape of the curve would be a parabola with the ideal frequency (channel) in the middle with the lowest SWR. So, the goal in tuning your antenna is to get that low, near-perfect SWR channel near the middle of the cb band, about channel 19. That way, all the other channels down to 1 and up to 40 should fall within acceptable SWR (less than 2.0) even though they are not perfect like the middle. Because the shape of the parabola is not always symmetrical (even on both sides), you are better off tuning the antenna so that SWR is the same on both channel 1 and channel 40, but the result is the same in that the "dip" of lowest near-perfect SWR will fall near the middle of that spread.

Testing SWR on channels 1 and 40 is also diagnostic of the length of the antenna needed to tune it. If the SWR on channel 1 is greater than the SWR on channel 40, the antenna is too short and needs to be lengthened. Conversely, if SWR on channel 1 is less than on channel 40, then the antenna is too long and needs to be shortened.

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post #24 of 25 (permalink) Old 07-11-2011, 01:10 PM
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Re: CB swr adjustment

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LaFJDude previously said: View Post
Ran up the ladder, adjust, then check the meter. Repeated until dialed in. Or thought it was dialed in. lol
If the ladder is metal, or has metal on it, be sure that it isn't too close to the antenna.
I like to check it on my driveway, away from the other car,
metal garage door, or whatever might influence the readings.
Even how you handle the SWR meter while setting it, will influence the
final reading that you get.

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post #25 of 25 (permalink) Old 07-13-2011, 07:16 PM
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Re: CB swr adjustment

I hear with a QD mount the antenna needs checked each time it's reinstalled. I use mine a few times a year so it's plug and play. I'd love to adjust the readings, but so far on the trails I've had no problems. Good job to those of you that have the time and money to do it right.
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