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Author Topic: Mag Mount Mobile Antenna  (Read 761 times)
N8CHR
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Posts: 34




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« on: October 01, 2012, 06:32:55 AM »

How does a mag mount complete the ground connection? Is it the magnetic field into the bed of my truck? Is it any where good or efficent?  How can I add another ground wire directly to the truck? Can't get into the base. Can I cut the coax, add 2 coax connectors and add a ground wire there. I'm in the early stages of adding HF, "20 meters" to my truck.
Thanks  Tom  N8CHR
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K3SF
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Posts: 51




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« Reply #1 on: October 01, 2012, 06:49:40 AM »

hi

my recommendation is to see website of K0BG at www.k0bg.com

a true expert on mobil setup..

i have used a multi-magnet mount and have run a seperate ground wire to point on the trunk lid as well as car body frame..have used a tarheel 200 as well as hustler mobil antennaes...

paul k3sf
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WX7G
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« Reply #2 on: October 01, 2012, 07:28:31 AM »

A mag mounts makes the ground connection through the capacitance between it and the automobile body. A 3 inch diameter mag mount setting flat on the automobile and having a paint/plastic thickness of 20 mils has a capacitance of 300 pF. The reactance is 38 ohms at 14 MHz and that is not a good ground connection.

So yes, a short ground wire is the thing to use. A 1 foot ground wire has an inductive reactance of 20 ohms and again that is not a good ground connection. The ground wire should be less than 3 inches.
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WB6DGN
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« Reply #3 on: October 02, 2012, 04:19:22 AM »

Quote
A 3 inch diameter mag mount setting flat on the automobile and having a paint/plastic thickness of 20 mils has a capacitance of 300 pF. The reactance is 38 ohms at 14 MHz and that is not a good ground connection. 
So yes, a short ground wire is the thing to use. A 1 foot ground wire has an inductive reactance of 20 ohms and again that is not a good ground connection. The ground wire should be less than 3 inches.

The best thing to do would be to drill a hole for a sheet metal screw within about an inch of the mag mount, then run that ground wire directly to that screw.  Don't forget to scratch off some paint, though, so the ground lug makes a good connection.  That should make the mag mount work about as good as it can.
OH! WAIT!  The whole idea was to not have to drill any holes!  Maybe an on-glass mount?  OH WELL!  I didn't want a radio in the car anyway!
Tom
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W5DXP
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« Reply #4 on: October 02, 2012, 06:27:38 AM »

A 3 inch diameter mag mount setting flat on the automobile and having a paint/plastic thickness of 20 mils has a capacitance of 300 pF. The reactance is 38 ohms at 14 MHz and that is not a good ground connection.

OTOH, my tri-mag-mount seems to provide an adequate ground (~10 ohms) on 17m.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
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