Date: Fri, 29 May 2009 14:14:38 +0200
To: rsgb_lf_group@blacksheep.org
From: Rik Strobbe <rik.strobbe@fys.kuleuven.be>
In-Reply-To: <c38.554797c5.37500d0f@aol.com>
References: <c38.554797c5.37500d0f@aol.com>
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Message-Id: <20090529121437.120FDF3862@smtps02.kuleuven.be>
Subject: LF: modeling a loop
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Dear all,

confronted with the seasonal rise of the loss resistance of my 500kHz 
antenna (from 35 Ohm in winter to 80 Ohm in summer) I would 
eventually consider to try a loop antenna.
Dimensions of the loop would be limited to "closing" the existing 
lazy L-antenna that is 11m high and 22m long.
One of the reasons for considering a loop is the fact that the 
radiation resistance of a loop raises with the 4th power of the 
frequency (for a Marconi antenna this is only 2nd power).
So, comparing 502kHz to 137kHz, in theory the radiation resistance of 
a vertical improves by a factor of 13 (11dB). For a loop it is a 
factor of 180 (22.5dB).

When I model a 11m high and 22m long lazy-L antenna (using MMANA-GAL) 
the modeling result seem to agree with theory: 0.027 Ohm at 137kHz 
and 0.359 Ohm at 502kHz.
But when I model a 10.5m high and 22m long loop, 0.5m above ground:
- 0.0013 Ohm at 137kHz and 0.033 Ohm at 502kHz, a ratio of only 25 
instead of the expected 180
- very different from the theoretical values: 0.00007 Ohm at 137kHz 
and 0.013 Ohm at 502kHz
- modeling in free space at 502kHz confirms the theoretical 0.013 
Ohm, but at 137kHz even in free space it is 0.0012 Ohm. If the claim 
that loop losses are not affected by the ground I would expect that 
the radiation resistance is not affected either.

I guess MMANA-GAL is not suited for modeling very small loops, is 
there other software that can scope with this ?
Before going into the effort of putting op the loop I would like to 
have an idea of what to expect.

73, Rik  ON7YD - OR7T