Re: NEC-LIST: Comments on non-flat groundplane and NEC

John:

Here are a few comments regarding your inquiry.

The limitation of NEC to a flat groundplane was addressed by a small
group including Gerry Burke, Dick Adler, Jim Breakall and me in the
mid 1990's. We investigated a concept that went like this:

Assume that the groundplane under and adjacent to the antenna is flat
enough to a distance great enough to stabilize the antenna
impedance. This occurs when the currents are stabilized as the
groundplane gets larger around the antenna structure. Use NEC
(assuming an infinite groundplane) to solve for the currents. Put in
actual ground constants, and groundplane details, for this
solution. Then, having got the currents, use them as the input data
for the NEC-BSC (NEC-Basic Scattering Code) developed at Ohio State to
sovle for the far-field pattern. The irregular groundplane is modeled
in NEC-BSC as a set of plates (each at least one wavelength in
extent). The results of the NEC-BSC code approximates the actual
pattern quite well. The paper by Breakall, et al. in IEEE Trans. AP in
the mid-1990s (1995?) provides the details. Maybe Jim will also want
to respond to your inquiry.

Hope this helps.

George H. Hagn, P.E.
Consulting Engineer
4208 Sleepy Hollow Road
Annandale, VA 22003

Original Message:
-----------------
From: John Blackburn John.Blackburn_at_npl.co.uk
Date: Tue, 20 Mar 2001 11:40:53 -0000
To: nec-list_at_ee.ubc.ca
Subject: NEC-LIST: non-flat groundplane

I'm trying to model the behaviour of an antenna over a ground plane
which is finite in size and not perfectly flat (corresponding to the
actual groundplane that we have). My idea for doing this is to build
up the ground plane out of wires and possibly also have one of NEC's
perfect ground planes underneath to simulate the actual ground
underneath the wire groundplane. Could anyone give me advice on this,
for instance how far should the wires of the wire grid be from the
infinite ground plane underneath; and is it ok to attach the edges of
the wire grid to the ground plane (to avoid reflections from these
edges)?

John.
Received on Mon Mar 26 2001 - 01:01:29 EST