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RE: [linrad] xpol question
- Subject: RE: [linrad] xpol question
- From: Leif Åsbrink <leif.asbrink@xxxxxxxxxxxxxxxxxxxx
- Date: Sat, 12 Jul 2003 14:14:11 +0200
> This weekend I like to finish my xpol linrad setup for 144MHz EME. I have
> some questions left:
> Do I need equal length of coax for H and V running from my
> preamps down to the shack ?
> As far as I can remember Linrad can run unequal lenghts. Anyone
> can confirm this?
You can run unequal lengths - and that is the only practical solution
because there may be some difference in the phase shift between
your preamplifiers that you might have to compensate for.
If you have a good pulse source and a bandpass filter you can
calibrate at the 144MHz input and then Linrad will incorporate
the phase shift differences from this point. I do not know
what hardware you have so I can not say if you could equally
well calibrate at some lower frequency. In case you use direct
conversion and if you do not need the "smart blanker" you do not
have to calibrate.
In case cable lengths differ or there is a phase error due to
incorrect or missing calibration, Linrad will work equally well
but the polarisation indicator will not show the polarisation
of the incoming wave. When the signal is present in both antennas
with equal amplitude the indicator will not show 45 degrees, it
will show something else, typically elliptical but even circular
is possible. It will be trivial to add a correction for this
but I have not done that yet. If you find circles rather than
45 degree lines in your polarisation indicator, add 0.25wl to
one of the cables until the phase shift is included in the
Leif / SM5BSZ