Jeep Antenna Tuning Problems

[MdntRdrZJ]

Bought a 1998 Jeep Grand Cherokee with a Cobra 75WXST installed and having major SWR problems.

Antenna mount is screwed into the body of the vehicle by the taillight, with a 3' Firestik on it. Hooked upto a meter the needle goes all the way to max when TX.



Ideas?

 

[hockeyshrink]

Bought a 1998 Jeep Grand Cherokee with a Cobra 75WXST installed and having major SWR problems.

Antenna mount is screwed into the body of the vehicle by the taillight, with a 3' Firestik on it. Hooked upto a meter the needle goes all the way to max when TX.



Ideas?

Barring a short in the coax and with no further information, I would guess that the whip simply does not clear the vehicle body sufficiently to avoid having the rf being radiated back into the system. There is probably about an inch between the whip and the body, for most of the whip's length, right?

I have also learned the hard way that any Cobra radio with a "WX" in the model is probably a piece of junk. Just sayin......

Edit: Is a tunable Firestik with the tuning screw under the red top cap? (Pull the red cap off to access the tuning screw)

 

[gewecke]

Get rid of the fire stik,plug the hole then drill a hole in the center of the roof,mount an NMO mount and screw a LARSEN NMO/27S onto it. No more problems.
If you want it to work right then it needs to be installed right!
n9zas

 

[MdntRdrZJ]

Barring a short in the coax and with no further information, I would guess that the whip simply does not clear the vehicle body sufficiently to avoid having the rf being radiated back into the system. There is probably about an inch between the whip and the body, for most of the whip's length, right?

I have also learned the hard way that any Cobra radio with a "WX" in the model is probably a piece of junk. Just sayin......

Edit: Is a tunable Firestik with the tuning screw under the red top cap? (Pull the red cap off to access the tuning screw)

It is tunable but even a full down and full up its still of the charts. I think my ground is junk but cant seem to find out where. It's probably 14" above the roofline as it sits on a HD Spring and the mount is at the top of the taillight.

 

[MdntRdrZJ]

Get rid of the fire stik,plug the hole then drill a hole in the center of the roof,mount an NMO mount and screw a LARSEN NMO/27S onto it. No more problems.
If you want it to work right then it needs to be installed right!
n9zas

It's installed like most Grand Cherokee's are. In case you don't realize, having a huge whip on the center of the roof doesn't work well on a trail. A tree would eat that thing alive.

 

[rfradioconsult]

Take the spring out and try it again. How are you calibrating your SWR meter?

 

[knightrider]

First thing to check, is the coax, and it is REAL simple.... Disconnect the radio, and take the antenna off. place the leads of a V.O.M. across the PL-259 connector that attaches to the radio. Have the V.O.M. in the Ohms range. If it shows 0 (Zero) Ohms, the coax is shorted, if it has an "infinite reading ( the same as not having the leads connected to anything) it might be the antenna or it's mounting. The next test is to short the antenna end of the coax with a jumper lead, and check it again with the V.O.M. this time, you should get a 0 (Zero) reading. If not, the coax is "open", meaning it has a break in it. This sounds difficult, but it's really not. If the coax does check out OK, then again, it may be the antenna, orthe location, and mounting. You can loosen the mount, and check for rust or dirt. If it's there, then clean it, and try again. The other alternative, is as mentioned before, use a roof mount. You can use the NMO mounts which are considered a permanent mount. I have a mag mount for 10m, and a mag mount for 2m. Many will tell you this is not a good system, but has worked good for me. I have made several 10m contacts going to and from work, in the Central U.S. area from Central Texas. My 2m contacts are simplex over about 20-40 miles, and to a wide area repeater about 70 miles away. My roof is still intact, because the antennas will fall over when stuck by a tree. Permanent mounts don't allow that... Guess you can put a spring in there someplace, but then the antenna likes to lay back a LOT when driving at highway speeds. Good luck, and try to find a Ham, CB'er, or someone who understands radio and electronics to help you trouble shoot, and get it working.
Knightrider...

 

[freqs]

You most likely dont have a good ground try to run a ground wire to the frame

 

[prcguy]

I agree if no other obvious problems are found. On a ball mount system when most of the antenna is running parallel and only an inch or two from the meta body its more of a transmission line than an antenna.

To test for this problem pull the antenna well away from the body with string and measure the SWR.
prcguy

Barring a short in the coax and with no further information, I would guess that the whip simply does not clear the vehicle body sufficiently to avoid having the rf being radiated back into the system. There is probably about an inch between the whip and the body, for most of the whip's length, right?

I have also learned the hard way that any Cobra radio with a "WX" in the model is probably a piece of junk. Just sayin......

Edit: Is a tunable Firestik with the tuning screw under the red top cap? (Pull the red cap off to access the tuning screw)