Re: Quarter-wave whip earth plane

On 3/10/25 2:33 PM, legg wrote:

On Mon, 10 Mar 2025 15:33:33 +0000, liz@poppyrecords.invalid.invalid
(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?

 Most of the capacitance (11mm distance) will be developed across
the polythene discs.( ~ < 1mm ). Would you be happier if it was
just paint? (~ < 0.2mm )
 You can bypass this with wide, soft copper strap, or replace the
plastic with the strap alone, providing it's soft copper and the
magnets actually fit the painted surface's flatness, with no
sharp edges. Copper may harden with use/reuse.
 

Just be careful if you meant copper tape. I've had that slightly "bleed through" over several years. The paint it was affixed to had a minor but seemingly permanent discoloration. Could be a bummer if Liz ever wants to sell the van.

Grease will reduce oxidation, but you've got to keep the mating
interface clean, between and for the duration of installs.
Paint 'fog' can be buffed, if noticeable. Same problem if just
plastic spacers are used. (dirt and vibration)
 Resulting 'L'is reduced by strap width and number of bypass
locations; to be ignored considering the poor quality of infinity.
Capacitance increased by a factor of 25 (using numbers above).
 

"L" is actually an idea. What if Liz made a series inductor that resonates with the 120pF and form a series resonant circuit? Of course it would have to be a diffent inductor if she changes bands and a switch up there in the typical UK weather won't be so great.
--
Regards, Joerg (AJ6QL)
http://www.analogconsultants.com/