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Author Topic: 40M and 80M mobile antennas want to be shorter.  (Read 428 times)
KE6IGZ
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Posts: 5




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« on: February 26, 2005, 10:59:55 PM »

A few days ago I noticed that my 80M and 40M Hustler mobile antenna's had a very high SWR. This Saturday (today) I took apart the the entire antenna system, made sure all contacts were bright and shiny. This still did not fix my problem. Apon further inspecton I noticed that
my 40M antenna that was resonate in the phone section of the band had now shifted to the CW section of the band.
I the shortened the whip down as far as it would go without cutting it. The resonant point is now around 7.135 mhz. Still a bit low. I checked my 80M resonator and found it shifted as well.

My setup is a Hustler 54 inch mast with Super Resonators.
It is mounted on top of my truck cab with a tripple mag mount. I have added 1" copper braid from the mag mount ground to a seat bolt in the cab.

It has rained alot here recently. I have checked every possible connection and can't find a thing wrong. The only thing I can guess is it has somthing to do with the recent rain. I am stumped. Why do my antennas suddenly want to be shorter?
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K0BG
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« Reply #1 on: February 27, 2005, 06:12:52 AM »

The biggest drawback to the Hustler coils is the fact they are covered with vinyl plastic. Moisture gets under this cover and causes the resonant point to change.

I've heard of people placing them in an oven at low temperature for a few minutes, but you run the risk of melting the vinyl. Placing them under a 100 watt lamp is a little safer, but you have to make sure not to get them too hot. It takes several hours to "cook" out the moisture.

Alan, KØBG
www.k0bg.com
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AA4PB
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Posts: 12845




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« Reply #2 on: February 28, 2005, 06:11:32 AM »

One issue I see with the installation is a grounding problem. If the Hustler is on a mag mount on the roof and you have the strap grounded to a seat bolt, the length of the ground strap must be 6-feet or more. That's pretty long for an effective RF ground. An effective RF ground is especially important on 40M and 80M. Possibly the detuning is a result of a change in capacitance between the mag mount and the roof.
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N3ZKP
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Posts: 2008




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« Reply #3 on: February 28, 2005, 02:13:49 PM »

I'm surprised Alan didn't mention this ...

Aside from the performance considerations, I would be very concerned with an antenna that big on a magnet mount - even a triple one. This is a recipe for disaster if you have to do a panic stop.

A locked-brakes stop from 60mph will break that antenna loose and send it flying into whatever is in front of you, grounding strap or not. The inertia will break that strap in a fraction of a second. At the most it will change the antenna's direction of travel.

There are confirmed incidents of deaths from flying mag mount antennas, and these were not big HF antennas.

Mag mounts are for temporary installations, not full-time mobile-in-motion, especially HF.

Lon
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