Re: NEC-LIST: When is the height equal to free space ?

Good day,

I think there's something wrong in your model. See "Small Antenna Design"
pp 60-66, especially figs. 3.10 and 3.12. This is a presentation of the
short dipole over perfect ground, both vertical and horizontal. You can see
that radiation resistance and directivity ripple around the free-space
values. The ripple begins to damp down by an altitude of 1 wavelength.
However, the greater the altitude, the more lobes the vertical plane
radiation pattern will have.

Regards,
Doug Miron

"wilfam" <wilfam_at_technet2000.com.au> writes:

Dear list,

I am interested in aircraft antennas and their performance as a function of
height.

I have used NEC to investigate the performance of these antennas at various
heights above ground and in free space.

I had expected that when the antenna was a certain number of wavelengths
above the ground then the antennas would approach the free space
performance but this does not seem to be the case. I have been
concentrating on horizontal antennas.

I went back to basics and moved a dipole to a great height using the GM
card. The theta plot looked ragged and non-physical.

I also looked at two-ray models and other analytic work but these didn't
seem to cover the range from ground to free space.

I'd appreciate you help, guidance and any references.

Regards,

Michael.

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