If you think about your example of the two ground peaks at different
heights, what would the shadow of the larger one have to do with direction?
If the receive path has the small peak in the shadow of the larger peak so
that it isn't seen by the incoming signal then the small peak will not be
seen by the outgoing signal either.
Yes it could be in the path of the outgoing signal for a particular angle
but not for the same angle that clears the larger peak.
Outside of some amount of refraction over the peaks, a low angle outgoing
signal is not going to bounce off the smaller peak and combine with the
energy in a slightly higher angle wave that clears the higher peak. Any
signal that bounces off the lower peak is going to keep going up at the
reflection angle and right thru the higher angle energy wave that is
clearing the higher peak. The reflected wave from the low peak is not going
to bend and combine with the higher angle wave as it intercepts it.
If I understand what you are getting at in your non-reciprocal path
> -----Original Message-----
> From: firstname.lastname@example.org [mailto:towertalk-
> email@example.com] On Behalf Of David Gilbert
> Sent: Saturday, April 24, 2010 11:32 PM
> To: firstname.lastname@example.org
> Subject: Re: [TowerTalk] HFTA and diferent bands antennas
> 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
> 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
> Dave AB7E
> 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
> >> 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
> > wouldn't it?
> > I would believe non-symmetric propagation via skywave, but not that
> > "effective antenna pattern" is different for transmit/receive.
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