My CB has been deterioriating steadily over the last couple years, so I tore it all down and tested the continuity of everything, tried a different CB, tuned the SWR, and it still needs help.

I've got the obligatory Cobra 18WXSTIII with a 4' firestik on the TJM bumper.

With everything hooked up, I was getting a resistance of 980 ohms between hot and ground everywhere in the antenna/mount/coax/radio path. When I disconnected the radio from the coax, I went to the open line I expected.

When I checked the radio by itself (still wired to the truck power-wise, but with no coax attached) by probing the coax connecter directly, it registered my nemesis 980 ohms.

Because I was in a hurry, and a dang fool, I didn't measure the voltage drop between them, but I'm not sure that would have helped me.

I tried retuning the SWR, and got a nice 1.2/1.2/1.3 on channels 1/19/40, but I couldn't pick up any transmissions. My Midland handheld was picking up a ton of stuff, from within the truck, without being attached to the external antenna! I then hooked up the Midland to the external antenna, and it was even better. (Rechecked SWR, still in the same range).

So, is my problem likely to be:
a) somewhere in the connections to constant power/accessory power/ground, there is a problem, a short, or other electrical snafu? I checked the constant power and accessory power leads, and they matched my battery voltage. The ground is a bit harder to get to (since it's tied into one of the dash grounds at this point), and I didn't feel like pulling my dash off at the time.

b) A piece of cre-ap radio

Any thoughts? I can use the handheld with the vehicle adapter for this weekend, but I'd really like to get this fixed.

Thanks!

 

Okay, you're talking about measuring the _impedance_ of the antenna circuit, which can't really be done with a nomral DC driven ohm meter. The only way that the measurement is valid (based on how you did it) is if you had specs from when you first installed the system.

The issue is that impedance isn't (really) resistance. It's a measurement of an active "ringing" AC circuit, and the measurement will change based on the freqs and power being transmitted.

This is an article with a lot of buzz words and general confusion unless you're a radio geek, but it demonstrates the complexity of trying to measure impedance.


All that being said, then given that your Midland handheld did "better" (which in itself is relative) hooked to the external antenna, I'd start looking at grounds at the antenna and radio.

The one-shot measurement you did looking for DC voltage isn't good enough in situations like this. The quality of the ground will come into play quicker than you think. All radio (and amp) grounds should be done against BARE metal, which usually means you're gonna have to fight with a file or sandpaper.


With the SWR being that low, then you know that the finals of the radio are working "efficiently" and there's minimal echo on the wire. That's a good thing and actually tells a LOT about the cabling and antenna situation.


If cleaning up the grounds doesn't make things better, then it's probably time to swap the radio. Reason being that the finals can degrade over time, caps can leak, power supply regulation can go wonky - especially when mounted in something that's bouncing around 10 hours/day on weekends and in the midst of all sorts of debris.

CB's are pretty inexpensive these days, so you shouldn't have to slice off an arm.


Have you ever run a kicker in this rig? If so, a high SWR at _some time_ in that life could have done pretty good damage to the radio. I've seen it happen...

 

Wait...

Do you mean the handheld or something else?

 

Thanks Mark...

Nope, never ran a kicker in this pup. Pretty stock setup. The "better" on the handheld may be relative, but the difference was huge... clear conversations from the freeway, a couple residential miles away vs. no, zero, zilch input.

(The conversation I was picking up was nothing I'd share with my mother, but hey, that's another story).

I'll try re-grounding the CB's ground wire, then just deep six the cobra and either replace it, or stay with the vehicle adapter and steal that DIN for another project I have in mind!

 

The handheld using a vehicle adapter kit (which I connected to the PL-259 that runs to the firestik on the TJM).

I *think* the only elements I have yet to isolate fully are the radio power/ground wires vs the CB itself. Easy enough to check. May tear it all down again for fun while hangin' out in the Desert this weekend.

 

Hmm... with a jump in reception like that (missed the dynamics the first read-through, sorry), I'm pointing at the radio being at fault. It wouldn't hurt to lookit the grounds, but your time could be worth more than just spending $50 on a new radio.