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From: Joerg <news@analogconsultants.com>
Newsgroups: sci.electronics.design
Subject: Re: Quarter-wave whip earth plane
Date: Mon, 10 Mar 2025 13:52:48 -0700
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On 3/10/25 8:33 AM, Liz Tuddenham wrote:
> I've made a magnetic base for a quarter wave whip aerial; different
> length rods can be screwed onto it for the 2-metre and 4-metre bands and
> there is a sliding joint which allows a degree of length adjustment.
> 
> There are three ring magnets bolted to the underside of a 200mm diameter
> steel plate with thin polythene discs, to protect the paintwork, between
> the magnets and the roof of the van on which it will be used.  The total
> thickness of the polythene discs, the ceramic magnets and their back
> plates is enough to raise the underside of the steel plate about 11mm
> above the roof of the van.
> 
> The braid of the co-ax feeder is soldered to a tag on the steel plate,
> but of course this is not in direct contact with the metal of the van
> roof which forms the 'infinite' ground plane.  The capacitance between
> the steel plate and the van roof is about 120 pf, which has a reactance
> of about 18 ohms at 72 Mc/s and half as much at 145 Mc/s.  To balance
> the feed point I have inserted a 120pf capacitor between the centre
> conductor of the co-ax and the connection to the rod elements.
> 
> Is this going to cause a shift in resonance that can be corrected by
> adjusting the length of the elements (one of which is 'infinite' anyway)
> or is it liable to upset everything.  if the latter, is there a solution
> that doesn't involve butchering the van roof?
> 

9ohms isn't much, 18ohms is. But yes, that extra 120pf in series with 
the coax center will alter resonance which can be corrected by changing 
the length of the whip.

I don't see any "non-invasive" solution, assuming you don't even want a 
pointy contact screw into the metal because maybe the van is fairly new. 
That would also rule out sanding down the thickness of the paint :-)

Another option might be to just live with the asymmetry and use a common 
mode choke at the antenna to muffle the imbalance. That would have to be 
secured against wind, of course, but at least you don't have German 
autobahn speeds in the UK.

No chance to bolt to something conductive up there? I guess the roof 
rack channels on most modern verhicles aren't conductive anymore.

One thing you might consider is to drill or machine three big holes into 
the steel plate and sink the magnets into it so they are flush with the 
bottom surface. That way the whole steel plate could make surface 
contact with the roof (with a thin protective plastic layer in between, 
of course). That should increase the capacitance substantially.

-- 
Regards, Joerg

http://www.analogconsultants.com/