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Capacitive Grounding Mobile Antennas

from Richard E. Fusinski, K8NDS on February 2, 2019
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"Editor's Note: Due to the popularity of some of eHam's older articles, many of which you may not have read, the team has decided to rerun some of the best articles that we have received since eHam's inception. These articles will be reprinted to add to the quality of eHam's content and in a show of appreciation to the authors of these articles." This article was originally published on: 03/30/2009

Capacitive RF Grounding
Technique for Trunk Lip Antenna Mounts

Installation shown for Diamond

CR9800A - Quad Band Antenna

Non- Intrusive Mount (no paint intrusion)

  • 1/32” copper shim between set screws and paint for non-intrusive mount.

  • There is NO DC Ground connection from mount to trunk.

  • Notice split shrink tubing between paint and mount channels for non scratch.

  • 1 inch copper strap solders to 1/32” shim for RF ground and good skin effect.

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RF Coupling presents low Xc to vehicle body

with no actual DC contact, no paint removal.

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  • 1 inch copper strip soldered to 2 inch adhesive strip.

  • Diamond foam strip covered set screws and coaxial cable for better trunk seal.

Capacitive RF Grounding (Low Xc)

  • 2 inch Copper Foil Tape runs across entire underside of trunk lid.

  • Capacitance of 8000 pf was measured from mount to car body.

  • 8000pf is equal to approximately 1 ohm Capacitive Reactance @ 18 Mhz

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VSWR Measurements

< 1.2 to 1 on 2 Mtrs & 70 Cm

No ground plane is necessary on VHF/UHF

< 1.2 to 1 at resonance on 10 Mtrs

< 1.5 to 1 at resonance on 6 mtrs

Finished Mount

  • Finished Mount seals nicely against trunk seal.

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My particular Installation

Convertible Top Cable Track

  • Convertible top opens in both directions.

  • Supplied Diamond Coaxial Cable is tie rapped to existing scissor track.

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Trunk in Convertible Position

  • Trunk can open in both directions.

  • Coaxial cable follows scissor hinge

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Finished Mount

  • Diamond Mount presents a finished look.

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Cup Holder Mount

  • Cup holder mount presents an easy non-intrusive installation.

  • No holes necessary, easily removable.

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Rear View

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Side View

  • Constructed from one can of automotive bondo, mixed

and molder in a plastic cup with wood wedge epoxied

on top. Heavy material stays in place.

Cup Holder and Control head

  • Swivels easily for proper viewing angle

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The end of installation and the Beginning of some fun operating………………..

  • General Motors and Diamond!

  • A Great Automobile with a fine Antenna System…..


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By: Richard Fusinski

K8NDS Cottonwood, Arizona

Member Comments:
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Capacitive Grounding Mobile Antennas  
by NF6M on February 3, 2019 Mail this to a friend!
Nice clean installation Richard! That's a good use for some of that extra 2" copper from the helical loops too! ;-)
RE: Capacitive Grounding Mobile Antennas  
by K6AER on February 4, 2019 Mail this to a friend!
With the antenna grounded through a long copper strap to the frame and no ground plane around the antenna feed point, what is the inductive reactance of the ground plane (trunk lid and strap).
Capacitive Grounding Mobile Antennas  
by N2NH on February 7, 2019 Mail this to a friend!
Great article. Sadly it has outlived the Pontiac badge which was closed down in 2010. RIP.
Capacitive Grounding Mobile Antennas  
by W2DI on March 6, 2019 Mail this to a friend!
Excellent article. Going the extra distance makes a difference.

I’ve had a few mobile installations several years back, including a mobile QRP set-up with a Yaesu 817. Made many contacts with it until a rude net control op told me you can’t run QRP mobile and went on to the next log-in. Jerk.

But I wanted to add a comment on a strange phenomena I found while mobile. Well, maybe not so strange...

On a couple installations, I had a small mobile tuner to flatten the SWR. More OCD than anything else.

I would fire up the rig on the way home from work each day and I noticed that in two spots on the route the SWR would go up, then come down. Both very suddenly.

After a few times of this I also noticed that the street itself was a different shade at exactly where the SWR changed. At both spots where it went up, the street had been more recently repaved, as evidenced by the slightly darker and cleaner hue of the asphalt.

There was no denying the effect. As soon as I got to the newly paved area, the SWR went up. As soon as I left the segment of road, it went down... at both locations.

Why I don’t know. Maybe a different mix of material; maybe as the road wore it’s chacteristics changed?

It wasn’t a huge jump like from flat to 3:1. More like flat to 1.4:1 or so.

It did show the auto and antenna were capacitively coupled to the ground.

And don’t forget. Mobile QRP is impossible!

joe - w2di
Capacitive Grounding Mobile Antennas  
by YC2SAN on April 9, 2019 Mail this to a friend!
Nice and Clean Installation,
How about RFI effect to your electronic car
Ari Santoso de YC2SAN RR 73
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