Gamma-type matches consist of a small loop containing a source. This
type of geometry can be a problem for NEC, and often produces the
symptom you describe. You should have the progam report the average gain
(see the NEC documentation on the RP "card"), and use it to correct
the gain, unless you can rearrange the loop to get the average gain
close to 1. MININEC-based programs don't have this problem with small
loops -- they have other problems instead.

EZNEC users will see the average gain reported at the bottom of the main
window after every 3D calculation, and its meaning and use described in
the manual. However, note that EZNEC normalizes the correct average gain
to a standard value of 1 with or without ground, while NEC will report a
value of 0.5 (or 2, I don't recall for certain) when everything is ok
but a ground is present.

Roy Lewallen, W7EL

Brian Howie wrote:
Hi,

I've been trying to model the Clemens match ( it's a sort of off-set
gamma match) on NEC (specifically 4NEC2 and NEC2D). I find that the
addition of this reduces the calculated antenna gain by about 2dB. If I
model it using MMANA, the gain is unaffected, which is what I expect,
Has anyone had any experience of modelling this sort of match ?

73 Brian GM4DIJ