Antenna issues

[yorkphotog]

Hey folks...

Trying to solve a bit of a dilemma and I've sort of run out of ideas.

I have 6 Larsen NMOs mounted to the roof rack of my SUV. In this particular situation, there is a tower that isn't far from me (VHF SmartZone) and I am unable to hear it. A friend sitting next to me has a Larsen 5/8th 2m whip mag mounted on his SUV. He gets a full signal on that same tower. I tested his whip on my Larsen NMO mount, and I still get no signal on that tower. If I use his mag mount w/ the 5/8 whip I get a full signal on my radio (BCT15). On my NMO mount I'm using a Larsen tri band antenna. I've also tried the same tower on my BCD996T, which is using a 1/4 wave VHF antenna. Still no signal.

Any thoughts?? If I hold on the control channel using my radio/antenna all I hear is static with a consistent "banging" type sound. If I move closer to the tower I get a signal, so its not a matter of programming. In general, he is getting a much better signal on almost everything...

I've posted an audio clip of the interference (or whatever it is that I'm getting): http://www.jrcnet.ca/public/radio_trouble.wav

Thx!

 

[rescue128]

bad coax on the mount?

 

[yorkphotog]

I was thinking that.... until I tested the antenna on a bunch of the other NMO mounts. Doesn't appear to be the wire. My friend thinks its a ground issue.... but I'm not sure how I would be able to determine that.

 

[bonus1331]

Was it raining?
Sounds like a wiper motor.

 

[fineshot1]

Possible receiver front end overload?

If its a VHF trunk system there will most likely be multiple channels(control and voice) transmitting at the same time. Also, if there are any paging channels operating at the same time that could be part of the problem.

 

[iMONITOR]

Maybe your antenna position is interacting with some of the other antennas on the roof. Similar to how director/reflector elements act on a Yagi antenna.

 

[yorkphotog]

Nope, wipers were off.

Thing is --- my friend was sitting next to me (he has god knows how many mag mounts on his roof)... he was able to hear perfectly and I had this static signal...

 

[iMONITOR]

Thing is --- my friend was sitting next to me (he has god knows how many mag mounts on his roof)... he was able to hear perfectly and I had this static signal...

It has a lot to do with the position and distance from the other antennas.

 

[prcguy]

I found antennas on a roof rack suffer because the bar they are mounted to makes a very poor ground plane and can also detune the antenna. Its gets worse with some of the mounting arrangements I have seen that place the antenna further away from the rack bar. To prove this try one of your antennas on your friends mag mount on your vehicle. Someone else mentioned the distance from other antennas and if you have all the other mounts loaded with antennas and they are all nestled together this will also degrade things. In this case remove all other antennas and start your testing with just one antenna. If some or all of your existing antennas work on a mag mount then you'll have to consider adding a an adequate size sheet metal ground plane to the roof rack, running with mag mounts or drilling holes in the roof.
prcguy

 

[zz0468]

I'm reading that as you having at least two bad mounts and cables - the tri-band antenna, and the VHF 1/4 wave. Have you checked for shorts or continuity with a meter?

As to the noise, I have no clue, but it's identical to a noise that I hear on 160 and 80 meters late at night.

 

[hoser147]

Might just simply try running a ground wire from a bolt on the rack mount to a grounded part of the body nearby to see if it makes a difference.

 

[prcguy]

Grounding to the car body is not the issue, the antennas want to see a flat sheet of conductive metal right at the feedpoint. A simple wire to the rack will generally not do anything but if you ran a few wires from the NMO mount outward they will start to make a ground plane.
prcguy

Might just simply try running a ground wire from a bolt on the rack mount to a grounded part of the body nearby to see if it makes a difference.

 

[CCHLLM]

Never mind. My original post took the rhetorical standpoint that the receiver and the antenna and its feedline were good and was just an attempt at proper antenna installation. It was verbose, off topic, and provided no relevant information whatsoever to the topic at hand: bad antenna and or feedline. It is now gone.

Thank you for your participation. That is all.

 

[zz0468]

I'm not buying it. I agree, of course, that a proper ground plane is essential to proper operation, but in a receive application, just hanging a piece of wire on the receiver should bring in SOMETHING. If that nearby system is putting out anything at all, he should hear it nearby regardless of the state of the ground, groundplane, or proximity to other antennas.

If the antenna is open to the receiver, or shorted, he should only hear it if he's on site next door to the transmitters.

 

[rfradioconsult]

1. Take your digital multimeter and put it on the lowest resistance scale; check the resistance from the center pin of your connector on the coax to the body of the connector that goes to the radio; you should read an OPEN (no resistance).
2. Take one of your test leads and hold it to the whip of the antenna you are testing, touch the other test lead to the tip of the connector at the radio end, you should read a short (ZERO resistance).
3. Finally touch one of your test leads to a good metal ground on your vehicle and the other lead to the shell of the coax connector; you should read ZERO ohms.
These tests will check for shorts or opens in the coax and connector.

 

[jim202]

I have to agree with some of the statements about the mounting on the luggage rack. However,
and I use the term loosely here, you are having receiver issues at this time.

The NMO style antenna mount is not designed to be out in the open with water having access to
the underside of the mount and coax cable. I would start looking at what type of electrical
contact you have on the base of the mount. Take the antenna off, (Coil and whip) measure
the coax continuity from shield to center conductor. You should get an infinity reading. Do
this on a high Ohm scale so you can see any leakage. Next, measure from the top center
conductor of the mount to the coax connector center conductor at the radio end. You should
see a very low Ohm reading of only a couple of Ohms. Next measure the outer mounting
metal ring of the mount to the outer shell of the coax connector. Again you should only
see a couple of Ohms. I really don't care about it being grounded to the vehicle right now.
However, the antennas being used, unless they are of the 5/8 length design, need an RF
ground to function correctly. They will give a high SWR measurement on TX.

My guess is that the weather has destroyed the electrical connections at the base of the NMO
mount. Water has migrated down the coax and destroyed it also. You will probably have to
replace both the mount and the coax.

Try sealing the base of the good antenna mounts with some good silicone. Let it dry before
you try to transmit on any mounts that you have sealed with the silicone. It is a little fussy
about RF going through it when it is not cured.

I expect that you will find your antenna problem with a couple of minutes and a good Ohm
meter. Have fun fixing them and let us know how you made out.

Jim

 

[yorkphotog]

I took a closer look at my antenna mounts today... the underneath of all the NMO mounts are rusted. Is this a problem? Should they have been siliconed?

Also, for some reason one of the mounts has a second (thin, black) wire underneath it. Can't tell where it goes from or to. What's up with that?

Thanks

 

[iMONITOR]

Quick note on using "Silicone" as a sealant. Make sure you use the non-acidic type (the stuff that doesn't smell like vinegar).

 

[W6KRU]

I took a closer look at my antenna mounts today... the underneath of all the NMO mounts are rusted. Is this a problem? Should they have been siliconed?

Also, for some reason one of the mounts has a second (thin, black) wire underneath it. Can't tell where it goes from or to. What's up with that?

Thanks

Rusty or corroded? What type of material is the luggage rack? Aluminum?

 

[yorkphotog]

Hey guys....

Some VERY helpful info you have provided!

I've also been in touch with one of the engineers @ Larsen. I tested the resistance between the mounts and the body of the vehicle; I got about 38 Ohms, and he says I should be getting under 1 Ohm. In other words, there is no ground plane. His suggestion (shy of drilling 6 holes in the roof) is to fender mount them. Larsen makes a high freq NMO mount that is identical to the regular NMO mounts, but the wiring is completely enclosed. He suggests that if/when I move everything to the fender that I replace the cable assemblies/mounts with these enclosed mounts.

Thoughts?