[linrad] Re: Polarization question

['Zaba' OH1ZAA <dlc.fi; zaba@xxxxxxxxxxxxxxxx>]

Sir Joe!

As far as I have understood Linrad tries to combine the incoming 
spectrum of two
antennas in order to reach a signal vector maximum for each spectral component.
Thus it does not seem to matter (at least for that summation process) what the
absolute length of feed lines or relative signal amplitude in each 
channel is.

I have also the full WSE-set since 2005 or so, but I have never even 
attempted to
power it up (that will be one of this Summer's projects). So far I 
have run Linrad as
a home-brew single-channel I/Q direct conversion receiver, and man-made noise
suppression has already been quite impressive without calibration. I 
have plans
to expand to a multi-path take-off angle analyzer with the two 
channels activated.

It is probably already 7 years ago (at the 2000 VHF/UHF/SHF-meeting 
in Finland)
that the three of us: SM5BSZ/OZ1RH/OH1ZAA discussed all night (probably up to
4 - 5 AM) about the early Linrad and other properties of weak signal 
detection. It was
one of the most interesting discussions that I have ever experienced 
during my ham
career. Most probably we did not define everything very accurately at 
that time, as
at least one of the topics was "how deep we could go into the noise" 
with Linrad,
and I had the impression at that time that it went beyond certain 
physical barriers
(my perceived limits). But maybe we were not too clear about the distinction
between noise and interference... well it is too long ago anyway to recollect.

I think that I had a question about the two-channel case for a 
situation where the
two pre-amps would have a markedly different noise figure, and my understanding
was that Linrad could do some miracles in overcoming the noise issue probably
by correlating. Leif may have some good additional remarks on these matters.

                                                 73, "Zaba"  OH1ZAA / NNoY


P.S. These issues deal with the summation of (un)correlated noise



At 17:02 10.6.2007, K1JT wrote:

> Hi Leif (or anyone else who knows),
>
> Ideally, I suppose, with an xpol antenna one would like to have 
> phase-matched preamps for the two polarizations and equal electrical 
> lengths for the two Rx feedlines.
>
> Suppose they are not well matched.  In other words, suppose that the 
> complex gains and signal delays in the two polarization channels are 
> not equal.  Will Linrad's polarization-matching capability be 
> compromised?  As far as I can see, it still works well even with 
> poorly matched feedlines.  I suppose this must mean that Linrad 
> solves for a differential complex gain, and that over a fairly 
> narrow bandwidth a different delay can be treated as a phase shift.
>
> Is this the correct way to look at it?  Can you enlighten me any further?
>
>         -- 73, Joe, K1JT
>
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