EZNEC 5 shows the 1200 fence (3ft hi) antenna to have remarkable receive
properties
If terminated with 400ohms it produces a polar diagram not unlike a
3-element Yagi with a maximum gain of -17dBi and a F/B of 14 dB.
The maximum elevation gain is 37 degrees although the elevation gain is
within 2dB from 20 to 50degrees.
The antenna current shows a classic corkscrew current distribution of a
travelling wave antenna.
If unterminated it loses its travelling wave characteristics and
directivity.
EZNEC can use real ground or MiniNEC. I have used real ground with a
conductivity of 0.005S/m and a dielectric constant of 13.
It would need matching. The calculated Z is R11 -j518.
The 2000m antenna is something else!
Regards
Peter, G3LDO
Rik Strobbe wrote:
It seems that (at least in theory) even 1200 foot is too short to be
long on 500kHz ...
Or does MMANA not work properly ?
Hello Rik,
MMANA works very well for free space models but since it is based on
MiniNEC, it does not properly model how the "real world" ground
interacts with the near field. As far as I know, it uses the ground
parameters for the far field calculations only. In other words, I
don't think MMANA is able to make meaningful performance predictions
for small antennas close to non-perfect ground.
I just put a 2000m horizontal wire into MMANA and tried different
heights from 1cm to 1m on 500kHz with real ground. Both gain and
impedance figures were ridiculous, >600dB gain difference and
sometimes the real part of the impedance was negative! :-D
Beverages rely on non-perfect ground to work so I saw no reason to
trust the radiation patterns either.
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