mkuzmuk

I moved my Cobra 75 from my old TJ to my new JK and am having problems getting the SWR below 3. When I ran it in my TJ I had a non-tuneable antenna and only used the CB for trail runs, where distance wasn't important, so I never tuned it. I have now bought an SWR meter and can't get the SWR below 3 on my JK, and am wondering if I damaged the radio by using it "untuned" on my TJ. I am running a Wilson 3' tuneable, a Teraflex mount (w/coating ground off at antenna mount, and studs), and a 18' wilson super mini 8 cable w/detachable FME & connector. I know my grounds are good, I have continuity between the body and the mount, as well as the body and the connector (below the mount)(not the stud above). I tested my SWR out in an open field with an inline radioshack meter. I changed the tip settings a bit, but with the SWR only slight lower (3.5 instead of pegged) on 40 then on 1, I don't think its an antenna tune issue. I'm running the cb power (pos and neg) to the 12volt cig plug in the dash, and have the module ziptied behind my glovebox. Could it be the detachable connector on my super mini 8 cable? Anyone have any ideas?

guitarguy

Ok, I did read somewhere recently (don't quote me on this, I read it on another jeep forum, but can't verify the truth), that with new solid state CB radios like the 75, high SWR should not damage them. So lets assume that isn't your problem for the moment.

If I were you, I'd check your coax. I was getting some higher than normal SWR readings (2.5 ch1 and over 3 ch40), and it turned out that my coax was bad, the insulation at the tip broke apart in my hands. Replaced the coax, and all was good.

For some good info, check on the Firestik website, either the FAQ or Tech Help section.

Hope this helps...

Mark Doiron

If you can talk to people, then the radio is probably okay. However, with an SWR of 3, you have 25% of your emitted RF power being returned to the radio transmitter. That's not particularly good, and with some radio designs could cause failure of the final amplifier (that power has to go somewhere, and it will be going into heat). Fortunately, with only 4 watts of power, there's not too much heat to dissipate.

But, with so little power you don't really have any to spare: I'd get it fixed. And don't forget that high SWR also reduces your receiver system sensitivity--it works both directions!

Try a new RF cable. Don't install it--just run it where ever and hook it up and see if the antenna can tune better. If so, then the problem is either your cable or your connectors. Visually inspect the cable. Has it been turned in a tight radius (kinked) anywhere? RF cable should be held to turns of no tighter than nine times the radius of the cable. If there's no visible damage, open up the connectors and inspect them for cold solder joints (or, if crimp-on, poor crimp connections). You can also try moving the cable about at suspicious points while transmitting and see if that affects your SWR--you might see it popping around if you have a weak or corroded connection, for example.

If you determine that the RF cable is good, try a new antenna. If that doesn't fix it try any other components in your RF path (feedthrough on the antenna mount?). And it that doesn't fix it ... ...

... try a different meter. LOL.

mkuzmuk

OK, I think I might be on to something here... I think the quick connector end of my cable is messed up. When the quick connector is connected, I am getting conductivity on the other end of the cable between the center pin and outer housing. When I take off the quick connector, no conductivity... Which is grounding out the upper stud above the mount... So much for paying extra for the quick disconnect end so it's easier to route thru the vehicle... Guess I'll cut it off and put on a new end... This could be my problem right?

armandov99

Definitely fix that end. Should be NO continuity between cable center pin and housing...that's bad.

mkuzmuk

OK, this is weird but... the tester only shows continuity between the core wire and the casing when the swr meter is hooked up.... If I disconnect the cable from the SWR meter, then the cable tests fine.. Is that supposed to happen?

Mark Doiron

When using an ohmmeter, always measure RF cables for DC resistance when disconnected at both ends. Otherwise, you might be measuring through whatever it is connected to.

Remember (or learn): Resistance to DC is not the same as resistance to AC (and RF, which is just higher frequencies of AC). So, since you're using a DC ohmmeter to test an RF (AC) circuit, be certain to remove anything from the test that could affect the results (like the SWR meter). If you wanted to do the measurement connected up, you'd need some very expensive and specialized equipment, such as a Time Domain Reflectometer (TDR-a really cool tool for troubleshooting AC cabling).

Mark Doiron

But I think that's what he says he has (no continuity when disconnected). As I explain above, ohmmeter measurements while connected are unreliable.

mkuzmuk

What I was trying to say was that I did all the standard ground and continuity checks. ie the mount is grounded, the housing is grounded to the mount and also the chassis ground. Both ends of the cable have conductivity with their respective parts (ie inner tests good with other end inner, and casing tests good with other end casing. However, when I connect the SWR meter in line ie (in between the antenna cable and the cb), the top of the antenna mount (stud) will test grounded to bottom casing as well as the chassis ground. I pulled the swr meter and put on tester on the threaded portion of the connecter, and the other tester inside, where the inner cable would be, and the meter is showing conductivity, which would explain why my inner and outter cable is showing conductivity when the swr meter is hooked up inline. Another test I did last night is I took a piece of old coax that has a kink in it, which I know from a previous setup is a BAD cable, and hooked it up just to see if the meter would read exactly the same or not, and it didnt. The meter read completly spiked for both CH1 and CH40. Hooked up the fancy wilson cable back up, and it measured maxed for CH1, but less than max (but >3) for CH40. So there was a difference. Not sure what else to check here...

Mark Doiron

You have to be careful about translating measurements with a DC ohmmeter to RF applications. For example, you might measure near zero ohms from the feedthrough/antenna adapter outer casing to the frame of your Jeep. However, that DOES NOT mean that the antenna is properly grounded. The reason is because your DC ohmmeter may be measuring down the outer shield of your RF cabling, through the connector at your radio, to the chassis of the radio, then through the negative power connection to the battery of your vehicle, then through the negative cable of your battery to the frame. Pretty darned close to zero ohms. And an absolutely worthless ground for your antenna because all of the unshielded line (including the outer shield of your RF cable and the power and battery wiring) is acting like a big, mismatched antenna. The result: High VSWR. So, do you have a good ground to the frame (IOW, a very short distance of wiring) by visual inspection, or by measurement? If the latter, I suggest that you try hooking up a shorter ground.