Re: Quarter-wave whip earth plane

Dave Platt <dplatt@coop.radagast.org> wrote:

In article <1r8zjnr.1exnyup1tqp24zN%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
 

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.

 
I'm not at all sure that this "balancing" capacitor is going to help
matters more than it hurts.  It will probably increase the series
(capacitive) reactance of the antenna system as a whole, raising
the SWR and making life tougher on the radio.

I've taken it out and cut the elements a bit shorter (they were too long
to start with), I can get VSWRs well below 1.5 on 144 Mc/s and 70 Mc/s
with two different elements.  There is a lockable sliding joint so I can
trim the length with the NanoVNA I have just bought.  The tuning of the
144 Mc/s quarter-wave whip is very broad and uncritical but the 70 Mc/s
whip is quite sharp (perhaps because of the capacitive effect in the
'earth' plane).

I experimented with ways of increasing the capacitance to the van roof
using aluminium cooking foil, but the difference it makes is not worth
the bother of cutting a more substantial capacitor plate from sheet
aluminium.

[...]

It could help you a lot if you can get access to a NanoVNA

It has made a huge difference, I can see all my mistakes instead of
blundering around in the dark.


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