Weird issue after changing antenna mount

[snakecharmer]

I was using a mag mount with a 1/4 wave whip mounted to the top of a toolbox in the bed of my F150. I purchased a camper shell and removed the box necessitating relocation of the antenna. I decided to take the plunge and drill. Last night I installed an NMO mount just in front of my 3rd brake light on the cab roof. I used the same 1/4 whip off of the mag mount. After soldering on the connector and hooking it up, I fired up my radio and found that it would stop on a repeater where I would just hear static and my meter would be about half way up. I got out my multimeter and checked that I didn't short out the connector and as far as I can tell its good. I thought maybe it was because I was in a metal shop but it was working fine when I was doing some other stuff to the truck in the shop before I changed the antenna. It seems to be receiving fine but just stops on these random repeaters with just static. It doesn't seem to be any one repeater as I have seen 3 different ones last night and this morning. The signal is so strong that it will open the squelch even all the way up. What could I have possibly done wrong?

 

[SCPD]

Well the one difference .. is that the antenna will probably work alot better on top of the cab than on the mag mount.

Other than adjusting the squelch or maybe adding some attenuation to the memory channels I don't have much of an answer for you.

 

[fineshot1]

Does this reception problem happen every where you go?

I am thinking it was local noise in the area you were in at the time.

You also never mentioned what band you were on(frequency range).

 

[snakecharmer]

My bad, its 2 meters. I have only driven in about a 20 mile radius since I installed it last night but it doesn't seem to be localized. I actually had that same antenna in the same spot except on the mag mount for a while and don't remember it doing that but I suppose its possible its just pulling in a lot more than it was before. The weird thing is that it doesn't seem to do it all the time. I have just been afraid to key it up in case I screwed something up soldering the end on the coax.

 

[fineshot1]

When ever you make antenna or cable changes you should always check
the swr and wattage output with a quality swr/watt meter and not be in the
dark about what your work results are. It pays to be sure. It does not pay
to burn up your transmitter.

Its possible you are experiencing random vehicle computor noise on certain
frequencies and this would not be the first time I have heard of this complaint.

 

[JStemann]

have you tried this with the truck not running, key off? Also, you may want to recheck your ground (power) connections to the radio. Just considering the possibility that your new antenna mount is providing a better ground than the power ground is.

also check out this website for some ideas:

Welcome to KØBG's Web Site

Jeff.

 

[lmrtek]

Since you are now closer to the engine compartment and to cab electronics, you may be getting hash
off the vehicle electronics

It is usually better to locate antennas in the rear of the vehicle to avoid these issues.
Many auto manufacturers have opted to put their AM/FM radio antennas on the rear fender
to get away from such hash.

 

[snakecharmer]

have you tried this with the truck not running, key off? Also, you may want to recheck your ground (power) connections to the radio. Just considering the possibility that your new antenna mount is providing a better ground than the power ground is.

also check out this website for some ideas:

Welcome to KØBG's Web Site

Jeff.

The truck was actually off when it first happened. I have 10ga power and ground straight to the battery so I should be good there.

Since you are now closer to the engine compartment and to cab electronics, you may be getting hash
off the vehicle electronics

It is usually better to locate antennas in the rear of the vehicle to avoid these issues.
Many auto manufacturers have opted to put their AM/FM radio antennas on the rear fender
to get away from such hash.

Dont really think I am any closer, I originally had the mag mount in the same spot on the cab roof before I moved it to the box because I didn't like the cable hanging down my back window but the move was literally inches and this is a full size super cab truck so its about as far back as I can get it since it now has a fiberglass shell on it. Since it has been on there for several days now and I have paid a little more attention, I think the NMO just works way better and is picking up signals that it didn't before. I have noticed that the meter is almost always pegged where as before might have been 50-60%. I admit to being somewhat antenna stupid but since the NMO is drilled into and actually coupled to the sheet metal roof, does it act as more of a ground plane than the mag mount even mounted in the same spot? It also seems to be less frequent since I installed the shell, almost like I had signal bouncing out of the bed of the truck that the shell is blocking. No idea if that even makes any sense and could be just coincidence but I got the shell on Wednesday and haven't really noticed it since then except for on one repeater that has always done weird things. I have also noticed a couple of times where I would show full signal from the repeater and I thought it was just static but turning it up could hear very faint audio like someone was opening the repeater with a very weak signal and I was just hearing a 100 watts of nothing from the repeater. Does that make any sense?

 

[N8IAA]

It sounds like a bad ground on the antenna base, or, coax. Use a volt/ohm meter set on continuous and see if you have a short somewhere on the coax. You may need to make sure that you are getting a good ground at the NMO base where it bites into the metal on the inside of your roof.
Larry

 

[lmrtek]

If you are using a scanner, you are likely just getting intermod and interference.

You would have to have a VERY well designed scanner receiver in order to eliminate all the high power transmitters that are bombarding the front end.

Most newer vhf and uhf ham radios also have very poor receivers and are easily overloaded and subject to intermod.

I can't recall how many years it's been since I've seen a decent receiver on a ham rig.

I had a Kenwood TM-731 dual band mobile and it had a good front end on it.
But the TM-732 that replaced it was horrible

The BEST dual band mobile ham radio I ever saw was the Standard C-5608
It was totally imune to interference and produced 60 watts on vhf and 50 watts on uhf

 

[kb0nly]

If your inside the shop, flourescent lights is a good bet for interference of that nature also. The through hole mount provides a lot better ground plane and ground then the mag mount did, so basically the radio is getting overwhelmed by more than normal signal level to it.

Drive around town, hows it work then?

 

[Javi_M]

How do you ensure the NMO has a good ground? I have 2 and I think they may not be grounded correctly because I'm getting interference of some sort that is keeping my scanner from working properly, and my 2m is experiencing a "whine" on tx and rx.

 

[LtDoc]

I think what you are seeing is fairly 'normal'. If there was a problem, I think it was probably that the mag-mount wasn't doing the best it could before the change.
I think it would be a very good idea to check the tuning of that antenna, changing from a mag-mount to an NMO mount can certainly change the 'length' of that whip (not a lot, but some). If you know someone with a '259' or '269', that's a very nice way of doing that antenna checking. I also don't think that there would be a huge difference in that antenna's tune, so seeing how your radio 'likes' it isn't very dangerous, and I would do so.
Locally, we have a repeater or two that changes behavior with cold weather. Not much, but some, and it typically results in a bit of noises not present in warmer weather. (I make some different 'noises' when I'm that cold, don't see why that repeater wouldn't do the same thing, you know?)
- 'Doc

 

[ladn]

While I think this probably is a grounding issue or with the vehicle on board electronics, you might check your radio's user manual or online and see if the frequencies where it hangs up are known "birdies" or a harmonic of a birdie.

 

[kk5s]

you need a ground plane

I think you mentioned you have a fiberglass camper. If so, you will need at least a 1/4w ground plane - 2-meters requires 19-inches. A thin copper or aluminum sheet on the inside of the camper should do the trick for you.

'73,
Mikey kk5sc