From PA0SE
Dave, G0MRF, wrote:
I recently installed a small broadcast transmitter on 1350kHz which was
licenced for a maximum antenna height of 10m.
After some time we managed to get a 10m glassfibre mast errected with a
small
top capacity hat, inductive loading at the centre (Litz), together with an
element of distributed loading in the top section between the loading coil
and the top hat.
When examined by the authorities the efficiency was around 7%.
A rough calculation would suggest that the Rrad of a 10m vertical is no
greater than 0.1R, on 136k. The computers perfect ground assumption could
be
affecting the ability to compare the effects of different arrangements for
top loading. Is it EZNEC which has poor / Avg / good / perfect / ground
models?
Dr. Dave,
The program AO by Brian Beezley, K6STI, indeed has the option of different
type of grounds (dielectric constant and conductivity). But that is only
used in computing radiation patterns and gain. And - as mentioned before -
these are not even applicable to the surface wave.
The impedance in the feed point is computed over perfect ground.
I have also the program NEC/WIRES by K6STI and this uses real ground also
for feed point impedance computation. But only for antennas that are not
fed angainst ground, dipoles etc. So not for Marconi's, inverted L and the
like.
I do not have EZNEC myself but I think that program can use real ground for
feed point impedance calculation of antennas fed against earth. .
Perhaps someone familiar with EZNEC can confirm.
73, Dick, PA0SE
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