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Dear John, > 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 agree. This method is also described
in the "antenna bible" (by Kraus). I once did the math exercise and the result is
correct.
> 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.
That makes
sense. But how to explain that on the one
end the loop benefits from the ground (doubling of the free space radiation
resistance while on the other hand it seems not (or far less) to suffer from
ground losses (compared to a vertical). 73, Rik ON7YD -
OR7T > ----- Original Message ----- From: "Rik
Strobbe"
> <[email protected]>
> To:
<[email protected]>
> 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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