New mount changes SWR

If everything else is the same including the antenna
It must be in the mount. Did you use one of those
gumdrop mounts from Wilson? They tend to show
some continuity, that is what i would look for first.
Use a continuity meter and look for a weak short
between bottom ground side and top (antenna) hot
side. After that make sure you have continuity to the
frame from the bracket. Then adjust the length of the
antenna if needed.

 

You need to cut the whip if it was a ground problem swr would be high like 3 and on 40 and 1im guessing the new mount changed the increased the total length which is why your swrs went up.

 

I got rid of my stainless mount w/ the allen head on the bottom, it didn't seem like it held the center conductor of the coax in there firmly.

Also, on a Peterbilt mirror bracket, the closer I moved the antenna bracket to the cab, the higher the SWR got.

 

moving the antenna closer to something big like the cab will cause a slight jump in SWRs. that is normal.

 

I have the stainless mount with the allen head........It is working but i thought that was a poor design for the center conductor of the coax to go into the allen head hole.

I noticed too what cadillacdude stated....moved the mounts away from the cab as far as possible and SWR came down.

 

Yes, the whip is too long, as evidenced by the lower SWR on the lower channels. As you go up in channels from 1, does it simply go up, or does it dip and then go up?

The SWR increase is not at all surprising. When you changed the mount, you changed the ground resistance, which is a factor in the SWR.

A lot of people don't know that a resonant vertical over a perfect ground will show a 1.5:1 SWR; the vertical has a 37Ω impedance. That's tripped up a lot of hams after installing radials under their ground mounted verticals; the SWR went up, but the antenna worked better.

You need to shorten the whip a little bit. If your antenna has an adjustment with a set screw, see if the whip is "bottoming out" in the adjustment range. If it is, then cut the antenna right below where the set screw is currently holding it; that way, you can put it back to its current length if shortening it doesn't work. As you shorten it, you should see the channel 1 SWR go up and the channel 40 SWR go down. I like to tune my CB antennas so that the lowest SWR is on channel 19; close enough to the middle of the band for me.

Or, you can always have a shop trim it for you.

 

Yea, verily and forsooth, MsJ hath divined the truest of answers yet again.

It's for that reason that I tell folks to ignore the "absolute" reading, i.e., the actual number shown on an SWR meter, and simply tune for the lowest reflected voltage on the SWR meter. Once you've got that, *then* write down the numeric value in a maintenance log or put on the back of the rig with a Sharpie® or something similar, and use it for a quick check of changes if/when they occur. Also to make sure that the minimum SWR isn't *already* somewhere between what's shown on Ch 1 & Ch 40 before cutting anything; it *may* need to be longer to hit its minimum.

It doesn't hurt to compare the reading with more of a lab standard, like the ubiquitous Bird 43 series, to see what actual value is shown, just for peace of mind. By the same token, if the reflected voltage show solidly "in the red" after an install, or changes to a really high apparent value sometime during the lifetime of the install, stop using the radio until the apparent short (or open) in the coax or antenna can be found & corrected.

A good example of why SWR by itself shouldn't be taken as the Holy Grail of antenna measurements. I sweep installed antennas with an analyzer to determine where they're resonant, although I realize they're a pretty big investment for someone with only one or two antenna to keep track of, too.

Toodles & 73