Re: Quarter-wave whip earth plane

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/