High Resistance on CB and 2M Mobile Antennas

[AndrewGS]

UPDATE: Spring, Texas

The mysterious impedance/resistance problem continues.

I took an old 5in. magnet mount I had laying around and stuck the 2M antenna on it. It worked SO much better than on my stake pocket mount. I then checked my CB antenna with the 2M removed and nothing changed, so I moved it to the magnet mount and while the SWR looked ok and I was able to hear the usual AM and CB noise, but the little red "ANT" light lit up, so I removed that immediately.

I put the antennas back on their stake pockets for now but decided to leave the Wilson's "matching transformer" disconnected. With it disconnected the SWR looks MUCH better on the radio's built-in meter and I can hear a little better, but it's still very, very quiet.

I didn't have time to move the ground to the S0239, but I still might try that this week.

If you have any information pertaining to this case, please, come forward.

 

[Robb]

I think you just proved two things:

1) The CB antenna is faulty/misadjusted
2) You need to put more space between them

Do you have another CB antenna that you can put on the mag mount - and put that assembly on the roof of the cab?
That is what I would do.
Two centavos...

 

[Beetle]

The decorative plate with the rubber pad underneath it should go away, and the antenna should be hard mounted to solid, clean, bare metal.

All metallic structures (bed, tailgate, doors, hood, etc) need to be bonded via solid, clean, bare metal to the frame/chassis.

This will usually mean drilling a hole for each antenna - which may be anathema to you, but if you want the best antenna installation possible, it's necessary.

 

[AndrewGS]

I think you just proved two things:

1) The CB antenna is faulty/misadjusted

The antenna is fine, it just works better with the matching transformer disconnected. It exhibits the same impedance/resistance problem as the 2M antenna on the same stake pocket mount.

2) You need to put more space between them

Removing the 2M antenna from the truck changed nothing with the CB antenna. Besides, lots of people run several antennas mounted closer together on trunks and roof racks.

Do you have another CB antenna that you can put on the mag mount - and put that assembly on the roof of the cab?
That is what I would do.
Two centavos...

I don't have another antenna to put on the magnet either for testing or mounting permanently. I think the magnet mount itself is a bit old and rusty, so I'm tossing it in the trash. I also don't want to use a magnet mount permanently because of the exposed feedline, paint and clearance issues. That's why I spent the money and time to mount it like I did.

The decorative plate with the rubber pad underneath it should go away, and the antenna should be hard mounted to solid, clean, bare metal.

It's not a decorative plate, it's the top side of the all-metal mount that secures to the bed rail through the stake pocket. It is mounted to clean, solid metal and well grounded.

All metallic structures (bed, tailgate, doors, hood, etc) need to be bonded via solid, clean, bare metal to the frame/chassis.

I've grounded the bed to the frame and the antenna mount is grounded to the frame and bed. The cab is well grounded to the frame and engine block at the factory while the doors and hood are also factory grounded to the cab. I'm assuming the tailgate is isolated due to paint and bushings and would be hard to ground.

This will usually mean drilling a hole for each antenna - which may be anathema to you, but if you want the best antenna installation possible, it's necessary.

I've thought about mounting an NMO to the top of the cab, but puling the interior apart is a major PITA. There's also no room for more than one antenna on the roof. I used these mounts because others have used them and other kinds on non-through a hole mounts with success.

 

[Moleculo]

The magmount test with the 2m antenna basically proves that the antenna is not seeing an effective counterpoise with the method that you originally had it installed. That means that either something isn't right about the stake pocket mount or the way it is grounded.

The same is probably true with the CB antenna. One thing to remember about that 3' Wilson is that it's about 1/3 the size of a 1/4 wave antenna for those frequencies. That means it's not going to be the best performer in the world. It also could be very directional because of the location of the install. I wouldn't expect miracles with that one.

 

[AndrewGS]

The magmount test with the 2m antenna basically proves that the antenna is not seeing an effective counterpoise with the method that you originally had it installed. That means that either something isn't right about the stake pocket mount or the way it is grounded.

I'm going to use the analyzer some time this week to try to set the SWR as best as possible on that antenna and my big gain Hustler as I have yet to get that far since I've been chasing this impedance/resistance issue. I've got my Larsen dual-band on an NMO magnet on the roof and it works grrreat! But I don't want magnets on the roof for long.

Using a repeater 30MI away as a reference I get an S-4 with the untuned antenna on the stake pocket and an S-7 on the tuned Larsen magnet.

The same is probably true with the CB antenna. One thing to remember about that 3' Wilson is that it's about 1/3 the size of a 1/4 wave antenna for those frequencies. That means it's not going to be the best performer in the world. It also could be very directional because of the location of the install. I wouldn't expect miracles with that one.

I've got the little matching transformer lead disconnected and the SWR, at least according to my Cobra 29's internal meter, is 1.3 at the ends and 1.1 on 19. I did a 360° test with my brother in-law and it is a little directional toward the front due to the ground plane characteristics. I'm going to test for distance tomorrow and if I can talk to him and his Cobra 19 DX IV/Wilson 1000 combo from 5 miles away with relative ease then I'll be happy.

As much as I'd love to run 108" of whip and spring I'd look like a bumper car in parking garages and drive-thrus. I'm not expecting true 1/4-wave performance out of the li'l Wilson as I have to compromise.