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Re: [TowerTalk] HFTA and diferent bands antennas

Subject: Re: [TowerTalk] HFTA and diferent bands antennas
From: David Gilbert <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Sat, 24 Apr 2010 20:32:29 -0700
List-post: <">>
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.

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 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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