Grounding problem with Firestik CB antenna

I installed my Cobra 75 with a 4' firestik on my Taco this weekend. My idea was to build an antenna mount for the bed rails so I can take it off when not in use. It all looks great. Problem is (I hope) grounding the antenna. Because the bed is plastic I have to jerry-rig something.

First I tried running a wire down to the frame (about 2.5 feet). I was skeptical about this since it's so long. My cheap, crappy Radio Shack SWR meter pegs out when I key the mic.



Next I tried a shorter wire (about 8") to a bolt on the bed. It's the closest thing I could find. It's tough to get to because it's between the bed and cab. SWR still pegs out.



I know I have a true ground on both. I checked it with an ohm meter. There are no kinks or damage to the coax. The "plug box" for the Cobra 75 is mounted and grounded. The power is correctly grounded. I get a strong, loud signal on the weather radio. So it seems to be all hooked up right.



What else could I be missing?

 

Long story short...Firesticks suck. Cut your losses and just buy a k40 now and it will tune out just fine.

 

X2.

I personally bridged a wire between the side rail (as it is a good ground) to the front rail. Rail bolt to rail bolt.

 

I have found the problem. The stud is grounded!



I took it apart and after removing the washer from the ground side, it was correctly insulated. The washer I got with the mount was too small and was touching the middle (insulated core) of the adapter stud.

I've got it mounted now and getting an SWR of 2.0 with the shorter (8 inch) ground line. (The frame ground was still over 3.0.) I'm hoping to tune it better when I get home.

 

Drunktaco is having the same issue right now with a Firestik II in same location, way high SWR readings, not sure if he has worked it out yet.

As far as the Firstik II goes, I have installed two recently, a 3' on my Taco and a 4' on a FJ.

Taco used a tailgate/bed mount, the FJ was the Botec (sic?) rear door hing mount.

Both vehicles are using the Cobra 75.

No problem whatsoever with SWR on either, highest reading on either radio is 1.3

 

You're the shit Jerry haha thanks for the link to here.

I'm going to look and try this now and see what happens. Thanks op for a killer find!

 

awww shucks


Good Luck

 

I was following advice from the firestik website. I started checking for a short in the coax and, just on a whim, tried the stud too. They list a whole mess of possible problems.

http://www.firestik.com/Tech_Docs/swr-causes.htm

 

So I just got a 3' firestik with a 3-way mount and was planning on installing it in the same location, but I'm a little confused as to what you had to change to fix the grounding issue. The underside of the mount (where it attaches to the coax) looks the same as yours, and the top has a plastic spacer, a flat washer, and then a lock washer. Is this what you were talking about? Sorry if this is a dumb question haha...gotta work on my wiring knowledge. Thanks!

 

The bed rails and end caps are a poor ground, yes a multimeter may show a ground but it's not good. The 6 bed bolts are all that attach the bed to the frame to the battery and they all touch "fiberglas". I've had lights installed and they worked sometimes. Ground your antenna to the frame.

 

Thanks for the post, the image really makes things clear.

 

In my case, the flat washer (between the lock washer and the rubber gasket, pink in the diagram above), had too small of an inside diameter. It was touching the threaded stud and grounding out. I may have had the washers i the wrong order.

As for finding a good ground, I ended up running a wire to a bolt on the bed between the cab and bed. My SRW is still not very good, but good enough for trail groups.

 

Bed "grounds" are not good grounds.

If that washer was grounding out you had something in the wrong order.

 

These are ground-plane antennas. That means they need a large plane of metal at the base of the antenna for best performance. In mobile installations that is provided by the body of the vehicle. It doesn't need to return to the battery. If your running a wire of any length to get to a "ground" you could be compromising the performance or making it difficult to tune. Even straight wire has inductance. At radio frequencies the inductive reactance of the wire may be significant, even if it looks like a low resistance on a DC Ohm meter. If you have to use wire make it as short and heavy as possible, from the base of the antenna to the body. Unless you are drilling a hole and mounting directly to the body, a metal mounting bracket bolted to the body or frame is best.

 

In short... Unless you drill a hole in the roof of your cab to mount your antenna, you need a good ground plane. In most people's case that ground plane will be the vehicle frame. A bed rail, which is not electrically connected to the frame, is not a good (large enough) ground plane.

 

Here's a shot of my REP Design Toyota Trac Mount, I've since added an additional ground strap between the trac mount and the metal pillar that comes up right behind the tailgate (metal exposed).

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I get acceptable performance upto 40m, any lower gets tough though.

I'm switching over to a different type of mount though as I find the rail mount to be a bit unstable even for the Wolf River Coil.

 

Nice! I ended up mounting to the bed rails and grounding it directly to the frame.