SWRs off on 102" whip

[Biggie_Z]

Hello all, this is my first post on this forum and I'm a new CB radio user. Me and a friend set up CB radios in our trucks to communicate while we're traveling and offroading. We're both using 102" whips, mine is working great with good SWR reading, but we're having problems with my friend's. He mounted it on a steel pedestal on his truck bed, I'll post a picture. He is getting SWRs of 2.2 on ch 1 and 2.9 on channel 40. With the spring removed he's getting 1.8 on ch 1 and 2.2 ch 40. The ground is tested and good and I tested it with a known good radio and coax, so I'm thinking that the pedestal is acting as antenna length and throwing off the SWRs because they went down after shortening the antenna by taking off the spring. Can anyone confirm this or have any other ideas? Thank you.

Link to picture:

http://i.imgur.com/AynO4cK.jpg

 

[jonwienke]

That mount is crap. You need a large horizontal metal surface for the antenna to resonate properly, that must be directly attached to the antenna mount, not going down the pedestal and up the support pillar to the cab roof. That setup will never have good SWR, no matter what length you cut the whip, because you don't have a proper ground plane.

The best mount on a pickup is in the center of the cab roof. Move the antenna there and put the spring back, and it should perform awesomely.

 

[JayMojave]

Hello B_Z: That's a great Antenna Install, but placed in the wrong location, as I have seen this type of installation before and the antenna is too close to the cab throwing off the antenna matching.

Suggest trying the antenna mounted as far back on the bed away from the cab, or install as said on top of the cab. If you want a lower SWR. The existing SWR is NOT a big deal in performance, but the cab being so close is.

Jay in the Great Mojave Desert

 

[JayMojave]

Taking a second look at the antenna install it might be possible to install a ground wire from the antenna mount just below the antenna, to the cab allowing a connection that might lower the SWR, might be worth a try!?!?!?

 

[jonwienke]

Relocating the antenna to the center of the cab roof would be far better than cobbling a half-baked solution attempting to partially salvage a bad idea.

 

[prcguy]

I looked at the picture and raising the feedpoint of the antenna high above the vehicle with that mount will never work right. I have a 9ft long 80 through 10m (including CB) screwdriver antenna mounted about 6" behind the cab and mostly on top of the bed rail of my Tundra and it matches just fine on CB and 10m, so the cab interaction is not a concern.

I do have a wide braid ground strap on both sides of the top bed rail that bonds to the cab, which will help continue the ground plane effect from the top bed rail to the cab.
prcguy

 

[TheSpaceMann]

Use a ball mount to mount the antenna 90 degrees to the side of the truck further back from the cab. That should do the trick.

 

[wtp]

some math

the whip is 102 inches the spring is 6 inches.
108 inches is the length you are looking for.

 

[Biggie_Z]

Thanks everyone for the help. Here's some more pics to clarify the situation:

Imgur: The most awesome images on the Internet

 

[kf4eyr]

get rid of that cheap spring and get a real one,,,, that is used for the shorter fiberglass whips it would not be friendly with a 102 whip,,,,

http://www.new-tronics.com/main/html/mobile_accessories_page_1.html

 

[JayMojave]

Yo B_Z: Give that a try with the local stations it may just work alright, worth a try.

Jay in the Great Mojave Desert

 

[Biggie_Z]

Yo B_Z: Give that a try with the local stations it may just work alright, worth a try.

Jay in the Great Mojave Desert

Thanks for the suggestion, we tried running a ground strap from the pedestal to the frame but it didn't change anything. I was getting absolutely no ohms when testing resistance from the - terminal to the top of the pedestal with the multi-meter and it read the exact same voltage when measuring negative from the pedestal as it did at the negative terminal. I don't think ground is the problem here, I'm thinking maybe it's the long tubular shape of the ground plane itself that throwing off the SWRs. Is this possible? Everything else has been tested with known working units including the radio and the coax with the same results. The SWR lowered when the spring was removed which is would suggest that the antenna is too long which is what led me to believe the pedestal is acting as antenna length. But what throws me off is that the stud is mounted on the top of the pedestal with a plastic washer separating the stud and the pedestal. Would it count as antenna length regardless? Help is much appreciated. Thank you!

 

[prcguy]

I say get rid of the elevated pipe thing, replace the rectangular base of the pipe thing with a plate with the same hole pattern and stick a ball mount on it. Your matching problem is caused by the feedpoint of the antenna being up so high above the vehicle sheet metal.

The stud mount you are using is also prone to failure with a large whip and a good ball mount will last forever. If you do all this the antenna should match up fine.
prcguy