Having some difficulty with my CB setup... As per a thread I started about a week ago, I originally thought it was due to CB damage, but apparently its the antenna. Here's the question.

Doesn't adjusting the length of the antenna just fine tune the channel's between 1 and 40 so that they are optimal? I always thought that shortening/lengthing would just bring the SWR of CH1 and CH40 closer together from their original values, but would never lower both. So if CH1 is 3.0 and CH40 is 3.5, no matter what you do to the length you'll never get below 3.0. Is this right?

 

Here's a link to the K40 tech site, Some good info on SWR / Grounding exc.
ADD THE WWW. das-roadpro.com/k40/technical.asp

 

.....or should be in a perfect world.....

 

Not necessarily.

An antenna has a resonant frequency. Theoretically, at that frequency with all of the other components optimal, you should have a VSWR of 1.0. But, that just doesn't happen. However, if you have good components properly installed you should be able to get into the low 1s.

Now, if the resonant frequency of the antenna is between channels 1 and 40, then lengthening the antenna will reduce its resonant frequency (because frequency and wavelength are inversely related). That means that channel 1's VSWR SHOULD (see below*) come down, but channel 40's VSWR will go up. But, if your resonant frequency is NOT between channels 1 and 40 (IOW: It's higher than 40 or lower than 1), then shortening and lengthening the antenna will move all 40 channel's VSWR in the same direction--either higher or lower. So, you want to dial in channel 20, and then adjust the antenna at that frequency for optimal VSWR. Then check 1 and 40 and they should be within a few tenths of channel 20's VSWR.

*I say "should" because if you lengthen the antenna too much, you could push the resonant frequency way below channel 1 and cause the VSWR to start rising on all channels.

 

If you graphed the frequency on the horizontal axis, and SWR on the vertical axis, you'll get an inverted bell, or a shallow "U" shaped curve. When you adjust the antenna and the sWR changes, the curve just moves left or right across the frequency band. Typically you want the low point dip to be on the channel you use most, or around 19-20. The SWR will go up as you move either up channel, or down channel from there.

 

I agree, start with the ground. Make sure you have continuity between the antenna mount bracket and vehicle chassis. Also, make sure you ground the radio's black lead directly to the battery negative post versus some point on the dash.