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

Dear list Members,

Many thanks to all who responded to my question concerning height and free
space.

I have learnt that "Over a flat, infinite ground plane, there is no height
above it for which free space behaviour is approached". This is due to the
lack of curvature in the NEC ground plane.

I was kindly directed to the work of Bullington and the literature on VHF
propagation.

I was also kindly directed to the following article for an explanation of
my "ragged and non-physical" patterns:
http://www.cebik.com/amod/amod40.html
It seems I wasn't generating patterns with sufficient resolution.

Most references that I had found discuss the effects of antenna height on
the impedance. I did find on reference that said:
When the transmitting and receiving antennas are more than 35*lambda2/3
above the ground, the effect of the Earth's surface becomes small and thus
the space wave predominates.
There was, however, no supporting evidence.

The question of when an antenna becomes free of ground effects is more
difficult than it sounds due to the number of effects than can be factored
into this calculation (ground parameters, polarisation, curvature,
attenuation, terrain etc).

Thanks for everyone's help.

Regards,

Michael.

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