Ground Plane/SWR Antenna Tuning

[Porkchop]

This is going to be complicated, I know, but try to hang in here with me on this one.

Ok. I've got a dump truck with fiberglass doors but a metal cab. I've been running a tri-mag setup on the roof with a Wilson 5000 Trucker antenna but it's got to leave here. It's ruining the paint on my truck, which I bought brand new in 04 and it's about to drive me crazy looking at those scratches.

So, you can't just plop a mirror mount on a Sterling. You'll get a high SWR cause there is no ground plane or counterpoise, whichever one it is. So...following the advice from Wilson and FireStik's sites, I added ground cables. My cab to frame ground is a huge braided strap, about 2 inches wide. All connections are soldered, nothing crimped or nothing like that.

The best SWR that I can get is a 2.5 across the band. I've tried my Wilson 5000 Trucker and I have a Predator 10K double coil antenna, both give the same results. I ground the antenna bracket itself to the lowest bolt on the mirror bracket, ran a ground inside the door, grounded that to the cab. Then ran a ground from the cab to the frame. No ground is over about 8 inches or so...so I know it's not a problem of the signal trying to radiate off the ground line.

Anybody have any idea what's going on or how to get the ground proper? I can reach out the window and grab the antenna and the mount and the SWR will fall to around 1.1 or so.

[Chipster]

Try using a fiberglass firestick mounted upside down on the same bracket as a ground plane. I saw this on a mid to late 90's model Volvo and had to ask the driver what it was about. He told me he had high SWR due to no metal anywhere. He had twin Firestick Fiberglass on the mirrors and made his own bracket. He took a slightly longer mirror mount bracket, drilled a 3/8" hole right beside the existing hole and bolted a 3/8" Firestick on the underside of it. He said the Firestick pointing downward provided the groundplane effect for the actual radiating Firestick placed on the top. He said he patterned the idea from when he use to make base antenna with conduit tubing. It makes sense in theory and he said it works great.

I don't know if you have enough space to point a 3' antenna downward, but it owuld be worth a try to see.

[prcguy]

The upside down grounded antenna sounds like a great idea and should fix the problem. There will be RF current on the grounded antenna so the larger it is the better the entire antenna system will radiate, just like making the main antenna larger.
prcguy

[Porkchop]

So you don't actually connect it to anything? You just put the adapter there like you'd hook coax to and install the antenna upside down? I assume you'd short the antenna out so it does act as a ground, by removing the nylon washer, is this right?

[prcguy]

You would need to ground the upside down antenna at the mount. If there is enough room on the bracket you can drill a 3/8 hole and use a nut on the antenna threads or use a second mount next to the existing and leave out the insulating washers.
prcguy

Originally Posted by Porkchop

So you don't actually connect it to anything? You just put the adapter there like you'd hook coax to and install the antenna upside down? I assume you'd short the antenna out so it does act as a ground, by removing the nylon washer, is this right?

[N_Jay]

We just invented the dipole!

[prcguy]

30yrs ago I worked in a CB shop that offered free SWR checks and antenna tuning. In addition to the 1,000+ radios and antennas I installed and probably another thousand more antennas I've tuned and repaired, I have never seen or thought of the simple solution of a second grounded antenna upside down for problem vehicles.
prcguy

Originally Posted by N_Jay

We just invented the dipole!

[Porkchop]

Well, later today when it cools off some outside I'll try this and let you know how it works out for me. I do appreciate this tid-bit cause it's about to drive me crazy.

[Porkchop]

I can't believe this...but I should have expected it...it didn't work. I tried two or three different antennas just to make sure. There is something about Sterling trucks and antennas that just don't go together. This thing is really getting the best of me... Tomorrow, I'm going to ground strap that truck like there is no tomorrow. I'm not expecting that to work either...so at that point...I don't know what I'll do. I don't want to fry a radio on this thing but the mag mount has to go. Of course, on a mag mount, I get a pretty good SWR...

[N_Jay]

make sure your cable and all is good.

The two antenna trick should work with nothing around. Do a double check in the open (not in the truck).

[Porkchop]

You mean try it off the truck...like set up a dummy situation?

I can put a dummy load on the end of the coax and I get a 1.1 SWR. So that says my coax is good, right? Even if it works off the truck...that's no good when it won't work on the truck...so what do I try next? What should I be looking for?

[N_Jay]

Maybe the antenna is bad, eliminate problems one at a time.

[Porkchop]

Tried 3. Factory whip, Wilson 5000 trucker and Predator 10K. Same result on all 3 antennas. For the ground plane antenna, upside down, tried 2...factory whip and Firestik with tunable tip. Same results.

Changed coax, same problem. Checked coax on dummy load, good SWR, still bad on the antenna.

Factory, the truck had dual whips, fiberglass pre-tuned. The coax also was ran into the stereo... Testing the factory coax...you get a good SWR. But when you put your own mount out there and your own coax, it goes to hades. You can't use the factory coax unless you run dual antennas, which I don't want to do...one antenna beating and knocking around in trees and over-passes is enough. Plus, most of the time...your radio will feedback into the stereo...which isn't any good.

I'm lost as to what to do...

[N_Jay]

Either play the trim the coax to match game or get a match-box.

[Porkchop]

So you think it's an issue of coax length then? I suppose you're saying that if the coax is 18 feet, the length of the antenna (electrically) needs to match the electric length of the coax? I was thinking this but I was also thinking I was crazy. I guess trying a shorter run of coax or maybe a longer run should change the SWR then...?

Should the back of the radio be grounded to the frame also? I know the coax should technically tie the radio and antenna together...but I'm hunting for causes for my problem.

[N_Jay]

Trimming the coax can hide the mismatch when all else fails

[Porkchop]

Hide the mismatch or cure it? If it's still there, reflected power that is, then I'm still in the same boat, right?

But I'll try shorter and longer runs tomorrow. I know there is a solution to this problem but I have yet to find it. The factory figures a way out to get a low SWR...how they do it, I don't know for sure. I think if they'd use better coax and leave the stereo out of the equation, then things would be good.

[prcguy]

The original antenna will tune differently with the upside grounded antenna nearby and the upside grounded antenna needs to be tuned with the original. Both antennas together now make up your antenna system. The upside down antenna tuning will also be skewed a bit by whatever its mounted to and if its running parallel and close to the metal door or body it will further influence things. It may take an antenna analyzer to rough things in like finding the resonant frequency of each antenna separately and together. This will help determine if the whips need pruning or lengthening.
prcguy

[Porkchop]

Alright. But I noticed that it didn't matter which antenna I put up top and with one I put upside down, the SWR remained the same. I don't have an analyzer, wish I did...I'm sure it would make this easier. I didn't expect to see any change at all by putting the antenna upside down, and I didn't. Not because I don't believe it works...but because nothing else has worked so far on this truck. I'm still thinking it's a ground issue.

Also, another question...I don't assume the height of the coil would matter much, would it? I know it should be above the highest point of the vehicle...but would that effect this? Would changing the length of the loaded section of the antenna help here or make the problem worse?

[N_Jay]

Explain what you are doing with the coil?