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From: "james moritz" <j.r.moritz@herts.ac.uk>
To: <rsgb_lf_group@blacksheep.org>
Date: Mon, 14 Aug 2006 14:09:20 +0100
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Subject: LF: RE: PA4VHF tests/problems?
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Dear Dick, LF Group,

If the antenna current really is 200mA, then with around 50W of RF, an
antenna resistance of over 1k ohm is implied - obviously there is a big
disagreement with your antenna impedance measurement. 1k ohm seems like =
a
very high loss resistance, even for a small antenna.

The extra 30 - 40pF of capacity is quite likely to be due to the stray
capacitance of the coil and its connections, and would be a reasonable =
value
for a fairly large diameter coil.

Even a single ground rod will have much less than 1k resistance, unless
perhaps it is going into desert sand or solid rock or something similar =
- in
my clay soil, going from several parallel ground rods to a single rod =
led to
a rise in loss resistance of about 20 ohms.

If the resistance really is 1k, and losses actually are occurring in the
antenna, earth system, or antenna tuner, you would have a very low Q for =
the
overall antenna system - about 6, instead of a more normal 50 - 100 or
thereabouts. This would make the tuning very "flat". Normally, a change =
in
the loading inductance of less than 1% would lead to a big mismatch, =
leading
to very sharp antenna tuning. It would also be hard to achieve a low SWR
with 1k or more of loss resistance - if using a separate matching
transformer and loading coil, the transformer ratio would have to be =
about
1:5, while with a tapped loading coil you would need a big diameter with
thin wire to get a match. So if you are using fairly "normal" matching
arrangements, and you get a sharp adjustment of resonance, suspicion =
must
fall on the antenna current meter. You could check this by connecting it =
in
series between the TX output and dummy load - with 50W into 50R, you =
should
get 1A RMS of RF current.

Clicking sounds from ferrite cored transformers are not unusual - they =
are
caused my magnetostriction, which is a sort of magnetic version of the
piezoelectric effect. A change in flux in the core causes a slight =
change in
dimensions of the ferrite, producing a sound - SMPS transformers =
sometimes
"squeal" for this reason.

Cheers, Jim Moritz
73 de M0BMU

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Dick
Sent: 13 August 2006 09:24
To: rsgb_lf_group@blacksheep.org
Subject: LF: PA4VHF tests/problems?

Thanks Alan for the suggestion.

Well while tuning the variometer everything seemed working as it should =
be.
The SWR nice dropped to zero, while the antennacurrent raised.
With help of a separate receiver I was able to figure out that I was
certainly not tuning on a harmonic.

So, there really might be some extra capacitance produced by the
coilwindings etc.

About the antenna current, one reason might be an insufficient earth, as =
I
just connected the ground tap
to a groundrod with 5m wire to my groundrod 'somewhere' in the garden...