On Mon, 21 Aug 2006 18:31:54 -0700, Roy Lewallen
wrote:
You should always run an average gain test when you have an unusually
high or low field strength. Using EZNEC, the average gain shows as
1.227, or 0.89 dB, and this same average gain should be reported by
NEC-2. That means that the actual gain is 0.89 dB less than what NEC-2
is reporting, or just about 2.0 dBi. If you don't understand what this
test is, consult the NEC-2 manual. EZNEC users will find it indexed in
the EZNEC manual.

You are correct--- an antenna cannot radiate more energy than was
input into it. Increasing the number of segments did not help the
accuracy of the model either. It appears that the angles are too
acute (40 degrees) for NEC-2 to model accurately. Changing the
geometry to avoid the smaller angles also produced an antenna with a
gain of 2 dBi as you suggested.

Perhaps you can advise me regarding the numerical stability of the
following omni-directional, which NEC indicates has a gain of 4 and a
VSWR1.2 when fed with a 50ohm feedline at 151.75 Mhz.

The geometry consists of a 54.5 inch vertical with 4 35.625 inch
radials that are bent upward by about 11 degrees. The vertical is
made from 14 AWG wire, while the radials are 1/8 inch brazing rod.

CM High Gain Omni for MURS
CE
GW 1 29 0 0 0 0 0 1.38375 0.000813863
GW 2 3 0 0 0 0.0508 0 0 0.0015875
GW 3 19 0.0508 0 0 0.938496 0 0.172104 0.0015875
GW 4 3 0 0 0 3.1105e-18 0.0508 0 0.0015875
GW 5 19 3.1105e-18 0.0508 0 5.74644e-17 0.938496 0.172104 0.0015875
GW 6 3 0 0 0 -0.0508 6.221e-18 0 0.0015875
GW 7 19 -0.0508 6.221e-18 0 -0.938496 1.14929e-16 0.172104 0.0015875
GW 8 3 0 0 0 -9.3315e-18 -0.0508 0 0.0015875
GW 9 19 -9.3315e-18 -0.0508 0 -1.72393e-16 -0.938496 0.172104 0.0015875
GE 0
FR 0 41 0 0 145 0.25
EX 0 1 1 0 1
RP 0 61 73 1001 0 0 3 5 10000, 0
GN -1 0 0 0 0.0 0.0 0.0 0.0 0.0 0.0
EN

With segment sizes of 0.05 and 0.025 lambda, the average power gain is
very close to 1. Changing the segment size to 0.0125, drops the
average power gain to 0.93, which indicates numerical instability.

Should I believe this model?

Thanks again,
--John