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: [email protected]
[mailto:[email protected]] On Behalf Of Dick
Sent: 13 August 2006 09:24
To: [email protected]
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...
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