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From: "Lee Hudson" <hudsonville@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
Date: Mon, 21 Jul 2008 17:03:21 +0100
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Subject: LF: RE: Loop Conundrum
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Hi Andy,

Interesting conundrum that had me scratching my head. 
These are the conclusions I've come to.

Firstly the efficiency has nothing to do with the radiation pattern as you
suggest. Since you are now filling in the nulls with radiation; this will
steal radiation from the lobes of the single loop system. IE The net
radiated power is the same, but the peak field strength has reduced (by say
3dB).

Secondly for a small loop you must phase each loop at 90 degrees to obtain
the desired omni-directional pattern, as suggested by DJ1ZB.

Thirdly the efficiency should be the same as a single loop if you feed them
in series or in parallel with 90 degree phase shift.

So far it appears that Argument 1 is holding up. 

Not being conversant with the works of Argument 2 all I can suggest here is
that the efficiency does not increase since only one loop is giving it's
full radiation every 90 degrees with the other loop contributing nothing.
Since the phasing will split power evenly between the 2 loops. Net result
same efficiency, hence the Argument 2 model is actually a single loop.

Finally since the world is never perfect some loss will occur in the 90
degree phasing so the answer as I see it is the same efficiency as a single
loop less the efficiency of the phasing required to feed both loops to give
you an omni-direction pattern.

73,
Lee M0LMH.



-----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.

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