Adjusting CB SWR

[pcgb34]

New CBer here,

My setup:
Midland 75-822 with K40 Electronics Brand, K30 Model Magnetic Mount.
SWR for Channel 1 is great (~1.2), but Channel 40 is way high.
So the intarwebs tell me that I need to shorten the antenna.

Does that mean trim the whip? If so, how? And how much?

Thanks for any help.
Patrick

 

[goodtimes]

You are correct in that the antenna is too long.

Basically you need to trim the wire length (the whip -- assuming it is a steel whip, not fiberglass) -- I used to use a cut off wheel on a grinder to cut a little at a time (an inch or so before testing again).

But before you start cutting, make sure your vehicle is out in the open -- not in a garage, parked next to any tall walls, etc, and test it again out there. I tried to tune an antenna once while parked a few feet from the side of an apartment building -- didn't work out well in the end.

 

[SunTzuNephew]

Also, check the tuning around the middle of the band - not the ends, unless you transmit on a particular channel to the exclusion of others.

Normally you tune the antenna for the middle of the band (~chan 20) and hope that the SWR is acceptable on the ends.

 

[James86004]

Do some CB antennae come with some sort of thing on the end you turn to adjust SWR? I have a vague recollection about that.

 

[goodtimes]

Also, check the tuning around the middle of the band - not the ends, unless you transmit on a particular channel to the exclusion of others.

Normally you tune the antenna for the middle of the band (~chan 20) and hope that the SWR is acceptable on the ends.

True, but by checking at the two extreme ends, you can figure out if the antenna is to long or to short.

If the SWR is lower at the low frequencies (relative to the high frequencies), the antenna is too long (low, low, long).

 

[Sawyer]

Do some CB antennae come with some sort of thing on the end you turn to adjust SWR? I have a vague recollection about that.

Yes. for example the wilson flex.
http://www.rightchannelradios.com/wilson-cb-antenna-109/wilson-flex-cb-antenna-363.html

 

[pcgb34]

Thanks for the help, fellas!

I dug out the Dremmel and started trimming, going about 1/4" at time. Then drove out to a nearby parking lot to check the SWR again. Then back and forth a few more times.

I got ch40 down into the 1.6 neighborhood, but of course, that cost me on ch1 (as it also came up to around 1.6).

Ch19 checks around 1.1, so I called it good enough for now. I chose the magnet mount because it's a temporary installation anyway.

 

[SilverBullet]

That is a great match, I would be super excited to get mine that low.

 

[pcgb34]

I'm THRILLED being that low on Ch19, I had just hoped to be a little better across the board.

Also, this is my first CB, so I don't have any perspective on what is good. Most things I've read say that under 2.0 is acceptable, and under 1.5 is great. So I was hoping to get under that 1.5 mark.

 

[AxeAngel]

Getting it that low is very very good. I'm around the 2.0 mark, but then again my radio/wire/antenna cost me $40 total all new.

 

[PhulesAU]

Considering CB's are cheap junk to start with. You should bragg about that SWR...... Hop it lasts.

 

[DaveInDenver]

Thanks for the help, fellas!

I dug out the Dremmel and started trimming, going about 1/4" at time. Then drove out to a nearby parking lot to check the SWR again. Then back and forth a few more times.

I got ch40 down into the 1.6 neighborhood, but of course, that cost me on ch1 (as it also came up to around 1.6).

Ch19 checks around 1.1, so I called it good enough for now. I chose the magnet mount because it's a temporary installation anyway.

That is a perfect example of 'bandwidth'. This is an example bandwidth shown graphically, where the tuned frequency is 300MHz. You can see how the SWR increases off it's center.

View attachment 43972

Your channel 19 is 27.185MHz and the ends are 26.985MHz and 27.405MHz. Most times bandwidth is defined as the range over which 2.0:1 SWR can be achieved, but there is really no hard-fast rule. If your radio can tolerate 3.0:1 then your antenna bandwidth requirements can adjust accordingly.

So technically you have more than 420KHz of rated bandwidth using the 2:1 definition. That's not really bad at all for a compromised HF (mag mount = poor mount, physically very short, etc.) antenna. Not to mention a SWR of 2.0:1 represents about 0.5dB of loss relative to a perfect match, so at your 1.6:1 ends you are radiating about 95% of your radio's output power and getting about 99.75% at 1.1:1 at channel 19.

You in fact have a perfectly tuned CB antenna for general coverage, in that you really can't do much better without going to a physically longer antenna. I personally tuned mine for channel 9 and so my upper end is a poorer match than my lower end, but this was a calculated thing since I primarily use channel 4 and only if I have to. I could envision using channel 9 and 19 maybe, otherwise I just don't use CB. My HF radio can monitor CB and has a more sensitive receiver than any CB, so the only time I even put on a CB antenna is if I absolutely have to TX on CB, which is fairly rare anymore.