Quarter-wave whip earth plane

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?

--
~ Liz Tuddenham ~
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www.poppyrecords.co.uk