> Has anyone some experience with modelling antennas over
> real ground, at low heigths?
> Or in other words:
> Is 1/4 wavelength high enough not to have any major influence on the
> freq of the elements? Will modelling an ant at that low heigths give real
> Chris DL4YAO
Hi Chris and the group,
I did a similiar step developing my 3el-3band-Contest-Quad on a 4.5m (15ft)
Starting with a free space design, I've put it afterwards into 19m (63ft)
above ground. After optimization with AO in this height I've compared it with
NEC/Wires and found some major differences regarding feedpoint resistance and
resonance frequency. The reason is that AO is taking the real ground parameters
only for the calculation of the far field pattern, for the calculation of the
antenna structure (currents) it assumes ideal groundconditions.
NEC-2 takes always the given real ground into account and therefore provides
much more realistic results. Comparing the results of AO and NEC I've found that
I had to optimize my design in AO at higher frequencies, an offset between 50
120 kHz from the original target depending on the band. This values may be
different for another model, e.g. Yagi, and for lower heights.
At the end of the day the NEC/Wires results have been proven to be right. I've
built the antenna exactly as finally calculated and prooven with NEC and at
the real feedpoint resistance and resonance frequencies were measured exactly
as designed before. I have no capabilities to measure gain and F/B ratio, but
several contests I'm very satisfied with this antenna.
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