No ... it doesn't really have anything to do about propagation
reciprocity through the ionosphere. It is all about non-symmetry of
terrain. As a simple example, picture a tall distant peak with a
shorter peak in the foreground. An outgoing signal would see both peaks
and the resultant takeoff angle profile would be the net result of ray
paths reflecting and refracting from both peaks. An incoming signal,
especially at very low arrival angles, could very possibly see only the
taller peak with the smaller peak totally in its shadow. I don't see
how such non-symmetry could possibly result in symmetrical path performance.
Here's what N6BV (the author of HFTA) says on page 3-29 of the 20th
edition of the ARRL Antenna Book:
"It is fascinating to reflect on the thought that received signals
coming down from the ionosphere to the receiver are having encounters
with the terrain, but from the opposite direction. It's not surprising,
given these kinds of interactions, that transmitting and receiving might
not be totally reciprocal."
I'm convinced that they are not reciprocal if the terrain is not
symmetrical. That seems intuitive to me, but so far I haven't figured
out any way to prove it. HFTA assumes a point source for the
transmitted signal, and at first I thought I could trick the program to
generate essentially parallel rays by defining a very high antenna and
looking at the response in the distant foreground, but HFTA does not
give any output for negative angles (i.e., below the plane of the
horizon) so that didn't work. If anyone has any clever thoughts on how
to skin this cat I'd be very pleased to hear them, but I'm guessing that
we will have to wait for someone like N6BV to actually do the math for us.
On 4/24/2010 5:52 PM, jimlux wrote:
> David Gilbert wrote:
>> The REAL Swiss Army knife for antenna modeling would, in my opinion,
>> combine the terrain analysis of HFTA with the antenna modeling
>> capability of NEC .... and do it for both transmit and receive. I
>> remain convinced that, especially for low angle signals, asymmetric
>> terrain that "looks different" to an outgoing signal than it does to an
>> incoming signal can cause dramatically different signal levels on the
>> two ends of a QSO even if all other considerations (power level,
>> antennas, noise level, ground conditions, etc) are equal.
> I don't know about that... That would imply a violation of reciprocity,
> wouldn't it?
> I would believe non-symmetric propagation via skywave, but not that the
> "effective antenna pattern" is different for transmit/receive.
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