Andy Luckman (AJL Electronics) wrote:
Four replies so far and not one of them anywhere near accurate. :-(
Quite. You beat me to it, but perhaps I can add some background.
The setting you describe is the "skew" setting. It is intended to match the
X & Y polarisation angles of the satellite. The default setting is position
3, but it is more accurate to peak the signal QUALITY. Ignore the strength,
it is of less importance.
Yes, it's polarisation skew or offset (not to be confused with the
concept of the offset feed).
As every schoolgirl knows, satellite TV uses the technique of
frequency-interleaved signals on orthogonal polarisations to double the
available spectrum. With satellites like the Astra fleet which use
plane (linear) polarisation it's most important to rotate the LNB to
ensure that signals on the 'unwanted' polarisation are nulled-out, so as
to achieve the full available cross-polar rejection performance. With a
traditional round clamp type of LNB mount you really need a spectrum
analyser to do this. Look at the LNB output and look in the gap between
two transponders - the best place to see the unwanted cross-polarised
signals - and rotate the LNB to minimise them.
To de-skill the installation process, and simplify the test equipment
required, Sky digital uses a click-stop approach to this skew adjustment
on the mini-dish. The correct setting is no. 3 for the great majority
of the UK. If you are in Eire, Devon, Cornwall or Pembrokeshire use no.
2. For NE Scotland - Aberdeen, Inverness, Orkneys and Shetlands - use
no. 4. For this to work it's most important that the dish is mounted
with its major axis truly horizontal, to which end the more recently
manufactured dishes have a built-in spirit level.
Incorrect skew will degrade the already marginal performance of
the minidish.
True, although the margin is not actually that bad if the dish is
correctly installed. Most of the problems in practice seem to relate to
bad installation, or microwave interference from certain illegal radar
jamming devices.
--
Andy
|