Help with CB SWR

[EricG]

I recently mounted a CB antenna on the front bumper of my 80. It's a 3' Wilson Flex 5/8 wave. Mount is just a generic plate-mount. There is a factory cable from the antenna to just forward of the firewall. I cut the factory end off of the coax coming from the radio and fed it through an rubber grommet on the firewall, then stripped and put a new end on it to join to the cable from the antenna.

On a run a few weeks ago a guy in our club hooked up an antenna tuner. We had to jury-rig some things to make the connection, but were convinced that the SWR was to high. So I went and got a Radio Shack CB SWR meter and checked it out tonight. SWR was way over 3?!!?

So I started checking things.

- No continuity from the antenna core/top of the antenna mount to chassis ground
- Good continuity from bottom of the mount & outer shell of connectors to chassis ground.
- No conductivity from inner conductor to outer shell

There is also a separate ground lead from the antenna - I tried screwing this down to its own hole as well. No significant change.

I had a spring between the mount and the antenna. I took that off and screwed the antenna directly to the mount. Again, no significant change.

Now I haven't actually played with the tunable tip on the antenna yet - ran out of daylight. But it's my understanding that this is for fine tuning and won't take you from well over 3 to something reasonable.

Any suggestions what else I should look at? Not the greatest picture, but you can see the antenna on the passenger side of the bumper.

FWIW, I got decent reception on that run, but didn't transmit much as I understand I could kill my radio trying to push power through a bad antenna.

 

[1911]

Not enough ground plane on the bumper for the Wilson, try a different antenna.

 

[EricG]

Not enough ground plane on the bumper for the Wilson, try a different antenna.

Really? the choice of antenna make can completely FUBAR the SWR like that? What do you recommend for this location?

 

[EricG]

Ok, I've been doing some more reading and one issue is that I've been doing the testing with the hood up next to the garage. So whatever ground plane the hood might provide is completely screwed up. So I've got to find a way to check it out with the hood down.

 

[1911]

Really? the choice of antenna make can completely FUBAR the SWR like that? What do you recommend for this location?

Yes it can; a 5/8-wave antenna needs a decent ground plane to work. Try a 1/2-wave instead (if you still don't get good SWR's with the hood down).

Ok, I've been doing some more reading and one issue is that I've been doing the testing with the hood up next to the garage. So whatever ground plane the hood might provide is completely screwed up. So I've got to find a way to check it out with the hood down.

Absolutely! You should be testing SWR from the coax where it connects at the back of the radio, so there shouldn't be any problem having the hood down. Also, it would be better if you moved away from your garage/house/any buildings to test.

 

[EricG]

Absolutely! You should be testing SWR from the coax where it connects at the back of the radio, so there shouldn't be any problem having the hood down. Also, it would be better if you moved away from your garage/house/any buildings to test.

I've been testing where the coax comes through the firewall, since I have a joint there already. It's an in-dash install so somewhat of a a PITA to get to the back of the radio. Particularly when it's cold, as I worry about breaking plastic bits. But I'll work something out.

 

[1911]

I've been testing where the coax comes through the firewall, since I have a joint there already. It's an in-dash install so somewhat of a a PITA to get to the back of the radio.

At the very least, I would add a jumper (extension) to the coax and drop it down through the engine bay and test from there, but you've got to get that hood down or you will never get an accurate measurement.

 

[96r50]

I have had bad luck with two 4' Wilson Flex antennas breaking, even when mounted on a spring. The material inside the antenna snaps where it connects to the threaded base and it becomes impossible to tune. You wouldn't know it from looking at it, but you can check the resistance between the tip and the mounting screw to verify (I forget whether you're looking for continuity or not though ).

 

[EricG]

I have had bad luck with two 4' Wilson Flex antennas breaking, even when mounted on a spring. The material inside the antenna snaps where it connects to the threaded base and it becomes impossible to tune. You wouldn't know it from looking at it, but you can check the resistance between the tip and the mounting screw to verify (I forget whether you're looking for continuity or not though ).

Thanks - I'll take a look at that. I've only been checking the continuity at the base.

 

[Kofoed]

IMHO that mounting location is poor. Even if you can get the SWR to a reasonable level, it is out the window once the antenna starts moving around and bouncing off the fender.....

This is a better solution, and maintains overhead clearance

CATALOG Main Page (Frame Setup)

Go to hood channel mount.

 

[EricG]

Ok, I got a chance to play with it some more tonight. With the hood closed (Doh!) the SWR immedietely dropped to about 2.5 Adjusting the tip got it down to about 1.4

This was with the tip all the way down, which actually left about 1/2" sticking out. Since it was trending down as you decreased the length, I figured I'd cut off 1/2" so I could put it all the way down. Didn't get any lower though. Then I put the little plastic cap back on and was up around 2 again! Ended up around 1.6 with the tip all the way out - wish I had that 1/2" back!

In any case, I'm pretty happy with 1.6

 

[1911]

Ok, I got a chance to play with it some more tonight. With the hood closed (Doh!) the SWR immedietely dropped to about 2.5 Adjusting the tip got it down to about 1.4

This was with the tip all the way down, which actually left about 1/2" sticking out. Since it was trending down as you decreased the length, I figured I'd cut off 1/2" so I could put it all the way down. Didn't get any lower though. Then I put the little plastic cap back on and was up around 2 again! Ended up around 1.6 with the tip all the way out - wish I had that 1/2" back!

In any case, I'm pretty happy with 1.6

1.6 on which channel/frequency? What you really want is to get the same SWR on Channel 1 and Channel 40, and the lowest possible SWR will fall in between somewhere. The difference between readings on Channel 1 and Channel 40 will tell you if your antenna is too long or too short. Glad that closing your hood got you in the ball park though. 1.6 is good enough for these radios, as long as it's not too high on either end (1 and 40).

 

[EricG]

1.6 on which channel/frequency? What you really want is to get the same SWR on Channel 1 and Channel 40, and the lowest possible SWR will fall in between somewhere. The difference between readings on Channel 1 and Channel 40 will tell you if your antenna is too long or too short. Glad that closing your hood got you in the ball park though. 1.6 is good enough for these radios, as long as it's not too high on either end (1 and 40).

I tuned it on channel 4, since that's what we use in the club & the back country around here. Ran out of daylight to play with it more then that. Maybe tomorrow.