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LF: Steep skywaves, polarisation, non-reciprocity

To: [email protected]
Subject: LF: Steep skywaves, polarisation, non-reciprocity
From: [email protected]
Date: Mon, 18 Jun 2001 20:36:17 EDT
Reply-to: [email protected]
Sender: <[email protected]>
Hi John, Alan, Mike, Rik and all,

I remember hearing slow QSB on the transmissions of G2AJV (when I was in GW) and GW4ALG, both at about 300km. They both used loops and I have not heard QSB at less than 450km with Marconi antennas.
I can confirm that Roger, G2AJV,  had a lot of QSB while othere G's at
about the same distance (but using verticals) had no QSB.
One point may be polarisation: While the groundwave must have vertical
E-field and transverse H to propagate, a skywave from above can have two
independent polarisations of its horizontal magnetic field (and no E-field
over conducting ground). We could surely think about re-using bandwidth by
polarisation multiplexing ;-)
In a coordinate system with z pointing vertical and x towards the receiver,
the skywave of a vertical Tx antenna can be received by its Ez and Hy fields.
The same would apply for a Tx loop oriented to the maximum (ie loop-plane in
x-z) or a horizontal x-dipole. In the minimum orientation (ie in y-z), these
would produce only Hx at the receiver and be undetectable by a vertical. And
in the in-between case of near-minimum orientation, the skywave would be
preferred and fading emphasized.
 But there is one thing that I never really understood :
 Even if a (transmitting) loop produces a lot of 'steep angle' radiation a
 vertical antenna (for receiving) should be rather insensitive for the
 incoming 'steep angle' signal. One would expect to notice the QSB only with
 both TX and RX side are using loops.
 But I could notice the QSB with a vertical as RX antenna.
 If the reciprocal principle works then any antenna that can receive 'steep
 angle' transmission should also transmit at these angles.
73, Rik ON7YD
Yes, even though theoretically a path may be non-reciprocal at a given
instant due to Faraday rotation in earth's field, on a long-term average the
swapping of Rx and Tx antennas should give equal results. For 300km
separation, the elevation would be around 35 deg, low enough for a vertical
to be effective.
How about that possible short-term non-reciprocity: Apparently it is common
in moonbounce work, but can it really be observed at LF? As a simple
experiment, one could (perhaps automatically) transmit periodic signal
reports in a fading situation, and look for discrepancies at the other end.
Regards
Markus, DF6NM


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