From: "Gary - G4WGT" <g4wgt@tiscali.co.uk>
To: <rsgb_lf_group@blacksheep.org>
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Date: Mon, 21 Jul 2008 13:02:51 +0100
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Subject: LF: RE: Loop Conundrum
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Andy, LF,

You wrote :-

"Now, I take two identical such loops and mount then on the same centre line
but at right angles to each other so there should be no coupling
between them, whatsoever.   Now, I connect the two loops in series and
resonate the combination with a single capacitor of half the original
value."

I have been pondering about something similar as I have problems remotely
rotating yet another antenna.

My idea is to make the loops in the same way as you describe but feed the 2
loops to the two inputs of a balanced pre-amp like the G3LNP loop pre-amp
shown in the "LF Experimenters Handbook"

What are your thoughts on that please.

Gary - G4WGT.

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Andy Talbot
Sent: 21 July 2008 12:30
To: rsgb_lf_group@blacksheep.org
Subject: LF: Loop Conundrum

Was pondering this while out walking the other day, and couldn't come
to a satisfactory conclusion either way...

A small magnetic loop mounted vertically has a defined radiation
resistance that is a function of its diameter, a loss that is function
of its conductor and hence a loss or efficiency that is the ratio of
the two. It is resonated with a good quality vacuum capacitor, and
fed/matched by any suitable metrhod.  Lets also leave aside all the
myth and folklore about small loops, and also ignore the environment
for now.   It also as a radiation pattern with nulls.

Now, I take two identical such loops and mount then on the same centre
line but at right angles to eachother so there should be no coupling
between them, whatsoever.   Now, I connect the two loops in series and
resonate the combination with a single capacitor of half the original
value.   The resulting radiation pattern should have the nulls filled
in, and be a reasonable approximation to omnidirectional in azimuth.

BUT...
What is the resulting change in efficiency?

Argument 1:
Two identical loops = two times the loss R,  but also two times the
radiation resistance (since they don't couple) so net efficiency
remains the same.

Argument 2 :
Chu-Harrington relates efficiency / Q / bandwidth / volume enclosed.
Therefore, as the enclosed volume has increased, the effciency ought
to rise.

Both arguments developed little side trendrils & thoughts as I walked
and pondered, and both appear valid in their own way.  So
the floor is open for discussion :-

And where does the net radiation pattern fit into the equation?  Does
it, at all ?
-- 
Andy G4JNT
www.scrbg.org/g4jnt

ps.  Fascinating paper on EMP btw. - I was up way past midnight last
night  reading it.