Dear Stewart, Mike, LF Group,
But how can it work when the incoming
signal is from an unpredictable direction,
and the path length (and
hence phase) of a sky-wave signal is
varying all of the time?
The direction is predictable if you know where the
signal is coming from, and Stewart proposes to effectively keep the receivers
in sync by using the Loran noise received by all receivers as a common reference
to adjust the phase and amplitude at each receiver. Whether the vagaries of sky
wave propagation will cause too much unpredictable phase shift is something
that still needs to be determined, but it would be interesting to find out.
At best you will have diversity reception
(as described by Mal), but
how do you keep two sky-wave signals in
synch?
Mike, G3XDV
==========
With simple diversity
reception, SNR is unchanged, but you can select the signal with best SNR from a
number of receivers, as a way of combating fading. If you can coherently add
the signals, you get more than you would using diversity reception – ideally,
with N receivers, the signal-to-noise is N times better, assuming the noise at
each receiver is un-correlated. If the noise is correlated, in principle I
guess it ought to be possible to null it out completely
Something else which occurs to
me is about antennas – if you use a vertical antenna, the received signal
is in principle unaffected by direction of signal arrival. But if you use a
loop antenna, as well as the amplitude varying with direction, the phase of the
signal will also change by 180 degrees between the “front” and the “back”
of the loop. An omnidirectional system using a quadrature combiner to combine
the output of two loops at right angles, as used by DF6NM, will also produce a
phase shift that is a function of direction. Does this mean that effectively a
vertical must be used?
Cheers, Jim Moritz
73 de M0BMU
