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
>>
>>