Weird Antenna Stuff (5/8 Mobile and Rubber Duckie)

[johnnymitch74]

All,

Today I was doing some railfanning in Southern Wisconsin as I normally do, and I took my Larsen NMO-150 along to use with my Icom IC-V82, plus my RH77CA rubber duck for pedestrian mobile use. My 5/8 wave mobile seemed to be acting especially poor, with minimal signal reception occurring. What's weird, is that my RH77CA was receiving much much better. It was the difference between hearing trains well, and not hearing them at all. I checked the SWR of my Mobile when I got home, which was fine when I bought it, only to find that it was now totally out of whack (over 5:1, although I think something may be wrong with my meter). I took apart my mount to inspect it for water or broken coax and found nothing. Anybody know what could be happening? Everything on the antenna, mount, and coax seems fine.

Thanks,
KC9TSZ

 

[KB7MIB]

Wirelessly posted (Mozilla/5.0 (X11; Linux i686; U; en-US) Gecko/20081217 Vision-Browser/8.1 301x200 LG VN530)

Physically check the entire length of the coax for damage. Check for electrical continuity from the connector end to the antenna end as well.

 

[fineshot1]

possible water infiltration

i am not saying this is your problem but in the past i have had some larsen
nmo mount coils get water infiltration in them.

once that happens the coil is useless and worthless.

long ago i started replacing my larsen antennas with hustler
and never had that problem again.

don't take me wrong on this. i am not saying larsen antennas are
bad. perhaps i just had some bad luck with them.

 

[LtDoc]

It's evident that something changed. No idea what, but something had to change or the antenna would be working as it did before. So, what can you do? Inspect everything dealing with that antenna. Does everything you look at seem like it was to start with? Then look a little 'deeper', at the things you normally can't see. One of those thingys is that coil inside the base. But that means disassembling the thing and probably ruining it? But then, it isn't working as it should anyway, right? If you can do that disassembling without breaking anything, that's good! It also helps if you know what you are looking at, not always a 'sure thing' (for me too).
I can't say what the problem is. I have a feeling it's the antenna (or part of it), but that's certainly not a 'sure thing'!
Good luck.
- 'Doc

(That's just basic 'trouble-shooting'.)

 

[johnnymitch74]

All,
Figured out the problem! Larsen ships all of their antennas with a weather sealing gasket that goes inbetween the loading coil and the mount. Apparently, however, with my magnet mount (MFJ), it actually insulates the grounding portion of the antenna from the mount and therefore the car, providing the antenna with no ground plane! a simple removal of this gasket was the difference between a flat SWR across the entire band and a SWR that was off the charts. Thanks all for your help and guidance!

p.s. for what it's worth, the Larsen NMO-150 is the best mobile out there! When I took this gasket off I couldn't believe what I was hearing. Working simplex with .5W on the HT is good stuff!

 

[willgrah]

Are you referring to the rubber cup that goes over the magnet to protect the paint or the rubber gasket that seals the NMO connection. Just curious.

 

[johnnymitch74]

The gasket that seals the nmo connection.

This bad boy here (attached):

 

[jhooten]

Since the NMO 150 coil is not DC grounded and all the brass insert in the base does is give a threads for the coil to screw on to the base, what probably happened is the tab in the base of the coil got compressed and was not making good contact with the center pin of the mount. When you removed the gasket it allowed the coil to screw down far enough to make contact.

 

[fineshot1]

Since the NMO 150 coil is not DC grounded and all the brass insert in the base does is give a threads for the coil to screw on to the base, what probably happened is the tab in the base of the coil got compressed and was not making good contact with the center pin of the mount. When you removed the gasket it allowed the coil to screw down far enough to make contact.

I think Jerry is right on target with this as i too have experienced that.
You always have to take care of the contact tabs and make sure they
are bent outward in the proper configuration to make contact with the
NMO mount center button.

Also an FYI for ya.

The NMO 150 is also resonant on the 6 meter band as well.