On Wed, 11 Oct 2006 16:32:50 GMT, "Frank's"
wrote:

....

One thing that has bothered me about small loop modeling. When
driven with a voltage source; loops of length 0.1 wavelengths
exhibit a radiated power ,and efficiency, of 0. In other words
the structure loss is than the input power. Removing
the wire conductivity loading corrects the problem. In your
particular case, at 5 MHz, the real input impedance is
more than three orders of magnitude greater than the radiation resistance.
I guess it could just be numerical rounding errors, but have gotten
identical results when using double precision.

Hi Frank,

That is interesting.

I haven't pursued modelling the loop being excited directly, but I
accept your observations.

I have no idea what causes the problem. Perhaps a NEC guru may have an
explanation?

I thinking about the impact of error in the estimate of either
radiation resistance or loss resistance of the conductor in the
scenario that I did model, it should be low, probably insignificant.
The magnitude of both components is small relative to the load
impedance and the loop's own reactance, and does not influence the
loop current (and therefore the load voltage) much.

Whilst it would be interesting to know if it is a fault in modelling,
or just numerical stability in the computation engine, I suspect the
effect you observed probable does not give great concern for the
accuracy of my receive loop models.

I would like to have modelled insulated wire in the loops, but don't
have NEC-4... so I will build the loops from bare wire, although I
think it would make very little difference, even more so at the lower
frequency side of the range.

Thanks, appreciate the thoughts.

Owen
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