Message-ID: <3AFD5932.9238FECE@alg.demon.co.uk>
Date: Sat, 12 May 2001 16:39:30 +0100
From: "Steve Rawlings" <steve@alg.demon.co.uk>
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To: "LF Group" <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: Measurement of antenna current
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Hi All,

Well, it's been drier in Chepstow this week, and we're
having a gloriously sunny day today.  So I've been doing some
more antenna current measurements this afternoon.  (If you're
getting bored with this topic, I suggest that you delete this
message now!)

Today's test was intended to find out whether having aluminium
foil under my loading coil makes any difference to the antenna
current.

For references purposes, my loading coil was set up in its usual 
place (no aluminium sheet): 1700 mm from the back wall of the
house, and raised 400 mm above the patio.
The current at the cold end of the loading coil was 2.0 A (0.2 A
less than last weekend), and the current at the hot end was 1.8 A
(same antenna current as last weekend).

I then moved the loading coil to one side, and put down two
strips
of aluminium foil, each measuring 450 mm  x  1600 mm.  I left a
gap of 30 mm between the strips, so the overall area covered
measured 930 mm  x  1600 mm.  Four lengths of timber were then
placed along the edges of the aluminium foil to hold it in
position.  I then replaced the loading coil in its original
position, at the centre of the screened area, and 1700 mm from
the wall, 400 mm above the aluminium.  The wall end of both 
aluminium strips was then connected to antenna ground (leaving 
the far ends of each strip open circuit). 
For a picture of the set-up (taken from an upstairs room), see:
http://www.alg.demon.co.uk/radio/136/pictures/screen_coil.jpg

With the aluminium foil in position, the current at the cold end 
was 2.0 A, the current at the hot end was 1.8 A.  (Interestingly,
the use of the aluminium foil appeared to have no significant 
affect upon system resonance.)

To recap from last week, I found no significant difference in the
antenna current (1.8 A) - whether excess turns at the hot end of
the coil were:
- left connected, but open circuit;
- left connected, but shorted;
- disconnected from main winding; or,
- physically removed from the coil former.

This weekend, I found that the use of grounded aluminium foil
under
my loading coil made no significant difference to the antenna
current.

Also with, or without the aluminium foil, the current into the
cold
end of my loading coil this weekend is 0.2 A less than last
weekend (was 2.2 A), while the antenna current remained the same
(1.8 A).  (A possible effect of the drier weather conditions?)


By the way, I heard good signals at around 10:13 UT this morning
from DF6NM (RST 539) and F6BWO (559).

Regards to all,
Steve GW4ALG