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[TowerTalk] Physically Noisy mobile antenna

Subject: [TowerTalk] Physically Noisy mobile antenna
From: K8RI <>
Date: Tue, 06 Dec 2011 23:49:48 -0500
List-post: <">>
There are several rather tall duo band (144/440) mobile antennas that 
work very well be they on a mag mount of more conventional, but the top 
of the line are rather tall and they have a "fold over" feature in the 
base for getting into "those parking ramps".  I have one of then, the 
Diamond "super gainer SG7900 in front of me.  It works great BUT with 
time they develop some play at the base where the fold over mechanism is 
located.  This almost has to be from the normal, day-to-day movement of 
the antenna wearing where to two pieces come together. The surfaces are 
tapered and make a good solid connection...for a while.  Eventually I 
start hearing a knocking sound whenever going over an uneven surface, 
particularly at low speeds  As the car rocks the knocking can get quite 
loud and annoying.  Uneven parking lots or speed bumps not crossed 
perpendicular are particularly annoying.

Now I can take the base apart and lap the two surfaces into a very good 
fit and there is a fair amount of room to do this before I'd run out of 
material. I've also though of defeating the fold over feature with a bit 
small bit of epoxy, or even a sleeve of flooded heat shrink tubing over 
the junction. These would work but it would defeat the fold over and I 
just might need to pull it into the shop or garage to work on the car.  
Now this is a 5' antenna (155CM) on top of a 6-6 1/2foot tall SUV.  That 
means the thing sticks up about 11 feet and makes for a lot of places I 
can't go with the antenna up.  It also subjects the antennas to much 
more physical abuse than they'd see on a car at about half the height. 
When that SUV rocks the base of the antenna really whips it around. Once 
the base to antenna joint loosens to the point where the antenna rocks 
enough to hear it really starts to beat up that junction.   As it is a 
mag mount I can remove it if I eliminate the fold over feature, but that 
too is a royal PITA pulling the coax off the roof (it routes through the 
luggage rack and down the weather seal inside the door and involves a 
partial disassembling of that rack. It doesn't take long, but it's just 
one more thing that has to be done.

I think the lapping is probably the most effective and useful of the 
methods, but it is also the most work AND requires careful cleaning of 
the parts to get all of the abrasive out to prevent accelerated wear. I 
basically have to disassemble the base to thoroughly clean out any left 
over particles and getting that threaded pin back in with that spring 
under tension is also a good way to lose parts. Fortunately I use a 
water based lapping compound so it's much easier to rinse out using hot, 
soapy water than an oil based compound would be.

BTW Either the Comet or Diamond versions will allow me to run the 160 to 
200 watt amp when out in the boonies.  I never tried to see just how 
much the antennas would take before smoking.

Any one else ever run into this problem?
Any opinions or suggestions?


Roger (K8RI)


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