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Re: LF: Measurement of antenna current

To: "LF Group" <[email protected]>
Subject: Re: LF: Measurement of antenna current
From: "Steve Rawlings" <[email protected]>
Date: Mon, 07 May 2001 17:52:19 +0100
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
Hi All,

Many thanks for the advice regarding my 'missing' milliamps, both
direct and via the Reflector.  Naturally, after getting so many
good ideas, I had no choice but to take advantage of the good
weather (but high QRN again) and carry out some further tests.

For reference, my original Email is included at the footer of
this one.  An illustration of the loading coil (prior to today's
modifications) can be seen at:
http://www.alg.demon.co.uk/radio/136/ldg_coil.htm#mark2

I was careful to check for system resonance before each
measurement.

1) The first thing I did was to confirm yesterday's measurement. No change there: still 2.2 A at the cold end; and 1.8 A at the
antenna  - a difference of 0.4 A.

2) The next thing I did was to short the excess turns at the hot
end of the loading coil (windings UE; UF; and UG in the
illustration).   Having done this, I then needed to add about
another 800 uH at the cold end to restore resonance before I
could make the measurements.  Once resonance was obtained, I
again measured 2.2 A at the cold end; and 1.8 A at the antenna.
3) Because the loading coil had been used on several occasions, I
also made a visual check for evidence of earlier corona discharge
around the connections to the excess turns (especially the top of
UG).  I saw no evidence that any arcing had taken place.

4) I then disconnected the connection to the excess turns (the
link between  UD and UE).  After resonating the antenna, there
was still 2.2 A at the cold end; and 1.8 A at the antenna

5) I then removed 15 turns from the cold end of the excess
winding (UE), leaving a gap between UD & UF.  There was still 2.2
A at the cold end; and 1.8 A at the antenna.

6) I then removed a further 21 turns (UF).  There was still 2.2 A
at the cold end; and 1.8 A at the antenna.

7) I now began to realise that what appeared to be all excess
turns at the outset was not really the case.  Evidently, the
capacitance of the excess turns had been helping to bring the
antenna to resonance, and I now found that I now needed to retain
the remaining 18 turns of 'excess' turns (UG) to resonate the 12
m vertical.

And that's how I've left it for now.
To conclude: I found no significant difference in the antenna
current - whether the excess turns were:
- left connected, but open circuit;
- left connected, but shorted; - disconnected from main winding; or,
- physically removed from the coil former.

But I'm not beat yet!  I still want to know who is stealing my
0.4 A!

From the responses received so far, remaining possibilities seem
to be:
i)    Radiation from the coil; or,
ii)   Losses due to stray capacitance.

The loading coil has a mean diameter of 470 mm; and is raised 400
mm off the patio slabs.  The distance between the back wall of
the house (brick construction) and the centre of the loading coil
is 1700 mm.

I hope to get some time this week to try out Laurie's
suggestions, using large sheets of aluminium, or equivalent.
Regards to all,
Steve GW4ALG



*******************  ORIGINAL TEXT  ***********************
Steve GW4ALG wrote:
Hi All,

Today, I made some more antenna current measurements on my 12 m
vertical (and my shiny new loading coil), using a thermocouple
meter.

At resonance, the current into the base of the loading coil, was
2.2 A.  But when measuring the current into the antenna at the
hot end of the coil, the meter only indicated 1.8 A.  I had
expected the reading to be the same at both ends of the coil.

The loading coil has many more turns than is required to resonate
the 12 m vertical, so the connection to the antenna uses a tap
which is several turns down from the top of the coil.  I've been
thinking that the excess turns at the top of the coil may be
causing the difference in current readings.  (This may sound
silly, but in my mind's eye, I can sort of picture some of the
current in the coil going into the capacitance of the antenna;
and some current going into the capacitance of the unused turns.)

Before I try removing the excess turns, I'd like to know whether
the current should indeed be the same at both ends of the loading
coil.  Can anyone help?

Regards to all,
Steve GW4ALG


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