Dear Rik,
This is my theory - At 0 degrees elevation, the far-field radiation from a
rectangular loop antenna is the same as that produced by 2 vertical elements
of the same dimensions as the vertical conductors in the loops and carrying
the same currents (other angles/shapes would be similar, but would require
resolving the horizontally and vertically directed components of the
current). In this 2-element vertical array, both are carrying large
currents, and the current in one is equal to the other, but flowing in the
opposite direction, and the spacing between them is a very small fraction of
a wavelength. The radiated field at a large distance is the result of
superimposing the seperate contributions of these two vertical elements
which, because of the small spacing, almost but not quite cancel each other
out (they do cancel out at right angles to the plane of the loop of course).
So the radiated field, and so the radiation resistance, is a result of a
small difference between two relatively large numbers - the smaller the
length of the loop in wavelengths, the smaller the difference gets. This is
why you need much more current in a loop than a vertical of a similar size.
I think the discrepancy in the simulated radiation resistance may be caused
by the limited precision of the calculations done by the simulator - you
would expect the effect of this to get worse as the sides of the loop became
smaller fractions of a wavelength. When I have tried to simulate small loops
in the past (using an old EZNEC), I have had similar problems, and also
simulation errors possibly for the same kind of reason. I don't think NEC
likes small loops of conductor much - I have also had problems in the past
when simulating a vertical with a multi-wire top-load where the ends of the
wires were connected together - the simulation results had 1 amp in the
uplead, but many amps circulating through the top-loading wires.
An experiment you could try is simulating larger loops to see if there is a
more accurate agreement between theory and simulation.
There is also the effect of the ground plane - a small loop close to a
ground plane should have double the radiation resistance of one in free
space due to the effect of the "image" loop reflected in the ground plane -
I think this implies large currents flowing in the ground plane under the
loop, so it is another matter to what extent this actually happens with a
real ground plane.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.be>
To: <rsgb_lf_group@blacksheep.org>
Sent: Friday, May 29, 2009 1:14 PM
Subject: LF: modeling a loop
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
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