NEC-LIST: Small Loop Antennas Close To The Ground

I am modeling an application that involves electrically small loop
antennas close to the ground with NEC-4.1. Because I am new to the
world of NEC, I decided to do some validation test cases for my own
benefit. I have a question about an unusual observed NEC result for a
very small loop close to the ground.

Let 'k' be the wavenumber of an excitation in free space, 'a' is the
loop radius and 'h' is the height off the ground. I modeled two small
loops, a) ka ~ .1, b) ka << 1 (~ 1e-3, say), over a range of kh (from
5e-3 to 5, say). I used the Sommerfeld ground option for all cases
with |N1| ~ 5-10, eps_r ~ 10-20. For the first case, ka ~ .1, the NEC
results for change in resistance and reactance over all kh match well
the published results of J.R. Wait. For the second case, ka << 1, NEC
results for moderate kh ~ 1 agree with published results. However,
for kh << 1 the results are very poor. In fact, unphysical, since the
reported input resistance to the loop becomes negative. This did not
occur with the larger ka case, even with comparable kh.

My hypothesis is that numerical errors have accumulated to contaminate
the results in this situation (ka, kh << 1) due to the Sommerfeld
model (since the perfectly conducting ground case appears to be
accurate). Clearly, since I am interested in behavior very close to
the ground (1e-3 to 1e-2 wavelengths) I cannot use the RCA option.

Does anyone have corroborating evidence for this anomalous behavior or
can otherwise comment??

In a LLNL report by G. Burke circa '87 on small wire loops, he states
that NEC-2 and NEC-3 can be very inaccurate for a loop close to the
ground, due to many reasons. At that time, a possible improvement
involving loop basis/weighting function in the MoM solution technique
was suggested.

Does anyone know if these changes have been implemented in a more
recent version of NEC and if they improve the problem I am observing?

Finally, I am not an expert in the Sommerfeld method. Besides
possible numerical issues, are there any physical issues in the
modeling approach that limit its accuracy very close to the ground for
very small loops?

Regards,
-Chris Teixeira
 
Received on Fri Mar 10 2000 - 06:13:11 EST