Topband: Rx antennas
rudys at ordata.com
Wed May 19 14:17:00 EDT 2004
Mike Tope's comment this morning is dead on. After thinking a bit
about this last night I came to the same conclusion. The near field
characteristics of the receiving antenna when transmitting are not
particularly relevant. What matters is how the antenna behaves in the
of the noise source if the noise source is close enough to matter.
The far field transmitting pattern is the same as the receiving
pattern when the receiving antenna is illuminated with a plane wave.
This is because the far field when transmitting is a plane wave also.
This is the well known reciprocity. If however, the source is close
enough to the receiving antenna that the illumination is not a plane
wave but rather some arbitrary spherical wave then the response may be
You then have to see if, in a given situation, whether one type of
antenna is more appropriate than another. We do this experimentally
when we try different receiving antennas at a given location. We
should not be surprised if the results at other locations favor
different antennas and we get conflicting anecdotal advice. This
isn't just for near field noise sources. The far field stuff we are
also receiving is highly variable, with noise and signals coming from
varying directions, at varying angles and polarization. The best
antenna choice will vary day-to-day and site-to-site.
The best strategy is well known: have several different receiving
antennas with different characteristics and use the one that works
best at the moment. You may have to work quite a bit with your given
situation to arrive at a solution best suited to you.
Ethan, K8GU sent me a note this morning pointing out that the
equations for the field impedance graph are readily available in any
graduate EM text. He used Balanis and I went to Stratton. We both
reproduced the graph easily. There is however, a little more to it
but we don't need to thrash that on this reflector. If anybody is
interested contact me on the side.
73, Rudy N6LF
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