RE: NEC-LIST:MSAs in NEC

The short answer is that microstrip antennas can't be simulated in NEC or
its variants. There's no way to model the dielectric slab.

NEC is based on the free-space Green's function, which means that the
physical problem must be expressed as an unknown surface current
distribution radiating in free space. The user models all 'matter' in the
problem as discrete wire segments.

NEC allows you to add a dielectric coating to wires (I haven't used that
feature) but that can't be used to model a chunk of dielectric material.

I'm not familiar with Ouray, but there are a number of commercial moment
method codes for microstrip antennas. They're based on "spectral domain
Green's functions" for multilayer environments, which are derived by a
process similar to transmission-line analysis. These moment method codes
are more accurate than the cavity approximation.

Methods like finite difference time domain and finite elements can also be
applied to microstrip geometries.

Depending on your needs and expertise, you could even write your own moment
method code for a simple microstrip dipole or patch. There are many papers
by D. Pozar and his group at UMass (N. K. Das and D. M. Pozar, "A
Generalized Spectral Domain Green's Function for Multilayer Dielectric
Substrates with Application to Multilayer Transmission Lines," IEEE Trans.
on Microwave Theory and Techniques, Vol. MTT-35, pp. 326-335, March 1987 is
a reasonable place to start.)

Hope this helps,

Brian

-----Original Message-----
From: Graham Kinch [mailto:graham_kinch_at_hotmail.com]
Sent: Friday, April 12, 2002 7:17 AM
To: nec-list_at_gweep.ca
Subject: NEC-LIST:MSAs in NEC

I know I'm over looking the obvious here, but if someone could point out
the obvious to me it would be much appreciated!I have simulated a 435MHz
microstrip antenna in 'Ouray Microstrip Antenna Designer' software. This
software is based on the Cavity Model. When I try and simulate the
dimensions suggested in Ouray in NEC software it simply doesn't work, I
get very strange results such as positive return loss! The software I'm
using is Win NEC Pro and Super NEC, both NEC2 based programs. I'm also
trying to simulate a plate antenna in both and getting very similar
results. The solid plates in both the microstrip and the plate antennas
are simulated in both pieces of software as wire meshes, with the wire
spacing of the mesh less than 1/10th of a wavelength. To the software
this appears to be solid. The problem is that in both antennas the
distance between my ground plane and the plate is much less than 1/10th
of a wavelength, approximately 10mm. Could the problem be that the NEC
software assumes that the ground plane and top plate are one solid
structure with no gap between them? Can microstrip antennas even be
simulated in NEC?Any suggestions would be much appreciated. Graham
KinchDublin
Institute of Technology.

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