Topband: Rx antennas

[Rudy Severns]

As weak signal junkies, topbanders have a very practical interest in
the performance differences between different antenna options and this
subject certainly merits extensive discussion.

Obviously the signals we are trying to extract and much of the noise
that comes with them, originate in the very far field.  So the far
field patterns of prospective antennas are of first order importance.

We also need to recognize that in many locations there may be
significant noise sources much closer to home.  The question is "how
close to home matters?"  We know that close to any antenna the near
field response can be significantly different from the far field
response.  Generally "close" is considered to be of the order of a few
wavelengths.  You can quibble over the degree of attenuation of the
near field terms and the distance, etc, but given that we are
weak-signal fanatics the near field terms may have significance at a
greater distance if the local noise source is strong and we can't do
anything about it.

The graph on Tom's web site and his statements, appear to make a case
that "close" is more like 1/8-wavelength.  That's a bit startling but
very important if correct.  This is an important point which should be
verified and extended to more general comparisons.  I, and perhaps
others, would like to replicate the modeling/calculation behind the
graph.  I suspect that's not terribly difficult.  To do this we would
need to know in detail how it was derived; i.e. the modeling details
if NEC near field is used, etc, etc.  This might shed a good deal more
light on this discussion and perhaps provide some new insights.  I do
note that W7EL is noted on the graph, possibly the source of the
graph.

Tom, if you could post this information on your web site or share it
with us on the reflector, I would very much appreciate it.  We all
might just learn something or, if nothing else, it's good fun.

73, Rudy N6LF