Date: Sun, 29 Aug 1999 02:15:20 -0400
From: "Dave Sergeant" <sergeantd@compuserve.com>
Subject: LF: Loops - current measurements
To: "rsgb_lf_group" <rsgb_lf_group@blacksheep.org>
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>From Dave G3YMC

This is a follow up on the mails earlier this month on this subject.

Today, assisted by Steve GW4ALG, we measured the rf current in my transmit
loop in a similar fashion to Steve's measurements on his loop.

For details of my loop see my web site: 
http://ourworld.compuserve.com/homepages/sergeantd/136.htm

I have previously done extensive simulation of my loop and its matching
network using a circuit analysis package and from the required values of
capacitors in the network to achieve correct match concluded that the
effective resistance of my loop at 136 is around 0.6ohms.

Measurements are as follows:

DC resistance     0.1ohms
Wire:  twin 79/0.2mm loudspeaker cable, the two pairs joined in parallel

Measured input power 5.2W
Loop Current    2.64A
DC resistance of RF meter   0.09 ohm
hence Loop resistance  0.66 ohm  (R=P/(I*I)

This result fits exactly with my previous simulations!

Note that at my normal transmit power of 35W the loop current is 8 A and if
I were to put 400W into it I would get 26 A - perhaps the caps would start
objecting!

At first I thought much of this loss was due to the series loss resistance
of the matching capacitors (Philips 376) but further calculations from the
datasheets shows this not to be the case, as this is only around 50
milliohms.  What I need is an accurate formula for calculating the skin
effect resistance of the wire.  The 98 ARRL handbook has a 'rough guide'
formula which would increase the resistance by a factor of around 3.5,
which is not enough, unless there is another effect I have overlooked.

As an aside, for those of you who remember the Cross Field Antenna
discussion on here a few months back, GM3HAT now has a paper describing the
CFA antennas in Egypt (http://www.antennex.com/preview/cfa/nab99cfa.htm)
which makes interesting, if amusing reading.  It sounds convincing until
you start asking yourself how the beasty really works and what it doesn't
say...  Enough said.

Cheers Dave G3YMC
sergeantd@compuserve.com