I could use some help/advice tuning my CB. I'm getting a SWR reading of 3.5 on channels 1, 20 and 40. All CB components are new and in good working condition and the antenna in question is a 4' Firestik.

Now, I have decided to make a big compromise and not install the antenna in the center of the roof (best groundplane), but rather installed it on my rear bumper. Shouldn't I be able to achieve at least a SWR of 2.0 in this location? Tell me what you think before I spend hours trying to find another location (without drilling new holes or buying yet another antenna.)

Here's how it is currently set up:


Thanks for any help you can provide.

 

Just make sure that the rubber gasket is in there to prevent the mount from grounding where your coax is.

That happened to me when I had it on my fender before ther ARB.
It was very hard to tell, but I put the gasket in wrong, and it allowed the coax to short out on the mount, thus I had two grounds going.

If indeed you are OK, I don't know why the match would be so high.
Perhaps Roger would have an answer for that.

 

YOu can "beep" it to see if it's making contact with the ground.

 

You can check for shorts with continuity tester or multimeter on the radio end of the coax. How long is your coax?

Also if your antenna is close to the fender you will get high SWR due to reflections from the metal fender or body panel.

It will help to have atleast half of the antenna above the fender or body.

 

Well, you may have just discovered the problem. I thought the mount is *supposed* to touch the metal, thus providing the groundplane.

I tested the SWR both with and without the groundstrap to the frame and it made no difference. Is the groundstrap required?

Obviously I know very little about CB tuning...:pat:

 

Oh yeah, Mr gasket is required

The gound strap should be as close to the mount as posible.
You can see mine here which is about 2" or so from where it exits the coax sheath.

It should be grounded very close.
The further away it could raise the SWR.

Check out some of the tech links to Firestik's site also on my CB page.

 

So basically I need to completely isolate the mount from the steel bumper.

IIRC, the CB coax has a copper wire through the center then has a braided copper wire over the outside. It's the outside braided wire that I should ground to the frame, right?

 

It's the outside braided wire that I should ground to the frame, right?

YES.

And the center conductor to the antenna.

What kind of mount do you have?

 

I am using the SS-184 connector as shown in the photo in my original post. Since RadioShack doesn't have any type of isolater in stock, I will likely wrap the connector bolt in rubber (aka bike tube) and use a plastic shim above and below the mount to isolate it from the bumper.

Now, keep in mind the antenna is mounted on the right rear corner of the bumper. The closest ground is about 1.5 feet away at the frame. Should I splice the coax at the antenna connection and run a groundstrap 1.5' to the frame, or should I splice coax 1.5' from the end, ground it to the frame, then continue running the coax 1.5' to the antenna connection? Does that make sense?

 

I would clean a spot next to the antenna mount and connect the shield to the bumper.

Your bumper should already be grounded to the chassis.

See pic.

 

You guys were a huge help; thank you very much!

 

I have my antenna setup the exact same way, and I just realized my antenna is not grounded. But my connection to the mount does not leave the ground wire exposed. There is a standard CB coax connector on the end of the wire that threads onto the bottom of the antenna mount. So would it make a difference if I just simply soldered the end of the ground wire to the outside body of the coax connector and then grounded the other end of the wire? I mean, the connector body is connected to the ground wire right?

 

If you have a PL-259 connector for your antenna connection this is the same connector that screws on to the back of your radio.

Yes, the body of the connector if properly attached to the coax will be grounded. And your mount should conduct the ground automatically.




BTW Corey has some very good info on the subject of CB's on his site.

 

If I'm not mistaken, the mount I have, which is simmilar to the ones pictured, has 2 plastic isolators that go on top and bottom of the mount to keep the metal touching the bumper, so nothing is touching the bumper, therefore not grounding. I'll have to double check tomorrow.

 

Okay, now I'm officially confused. I've read Firestik's entire FAQ section and it does not explain when the antenna connector (a PL259 in my case) should be isolated from its mount. If the above quote is true, then shouldn't the PL259 connector at the end of my coax "conduct the ground automatically" through my rear bumper?

 

OK, there is some confusion going on here.
I took the above pic and Photoshop drew on it.
Excuse the circle, I don't have PS mastered yet, and my good pic program I use to use is on my old PC and it won't run with XP

The shaft in the circle is what must be isolated from grounding.
If you ground that part to your chassis, or by the antenna mount itself, your SWR will short out and sky rocket.

That is what the two nylon washer are for.
When you righty tighty that bottom nut, it snugs up your mount to make it so it won't wobble.

But take note the bolt there that the nut goes on can either be the shaft of the antenna itself, or a spring as seen in my photo here.
You can see my two nylon washers isolate the shaft from the bumper mount entirely.

If either my bottom bolt or the bottom of the spring mount with the part you tighten up with a wrench were allowed to contact any metal of the bumper, it would ground the coax thing that is on the bottom bolt, thus grounding the antenna and shooting the SWR through the roof.

I think some were getting confused on this.
So just make sure your antenna shaft or spring mount if you are using one like mine touches no metal at all.
That's where the washers come into play.



I hope I explained this right.
Just keep your shaft from getting grounded

 

THANKS very much Corey. Perfectly clear explanation. I'll make these modifications this weekend and report back with my SWR readings.