from wikipedia
It is common to
polarize satellite TV signals because it provides a way of transmitting more TV channels using a given block of frequencies. This approach requires the use of receiving equipment that can filter incoming signals based on their polarisation. Two satellite TV signals can then be transmitted on the same frequency (or, more usually, closely spaced frequencies) and provided that they are polarized differently, the receiving equipment can still separate them and display whichever one is currently required.
Throughout the world, most satellite TV transmissions use vertical and horizontal
linear polarization but in North America,
DBS transmissions use left and right hand
circular polarization. Within the waveguide of a North American DBS LNB a slab of
dielectric material is used to convert left and right circular polarized signals to vertical and horizontal linear polarized signals so the converted signals can be treated the same.
A 1980s
Ku-band LNB (2.18 dB
noise figure) without built-in polarization selection and with a WR75 fitting for separate feedhorn and polarizer
The probe inside the LNB waveguide collects signals that are polarized in the same plane as the probe. To maximise the strength of the wanted signals (and to minimise reception of unwanted signals of the opposite polarization), the probe is aligned with the polarization of the incoming signals. This is most simply achieved by adjusting the LNB's
skew; its rotation about the waveguide axis. To remotely select between the two polarizations, and to compensate for inaccuracies of the skew angle, it used to be common to fit a
polarizer in front of the LNB's waveguide mouth. This either rotates the incoming signal with an electromagnet around the waveguide (a magnetic polarizer) or rotates an intermediate probe within the waveguide using a servo motor (a mechanical polarizer) but such adjustable skew polarizers are rarely used today.
The simplification of antenna design that accompanied the first Astra DTH broadcast satellites in Europe to produce the LNBF extended to a simpler approach to the selection between vertical and horizontal polarized signals too. Astra type LNBFs incorporate two probes in the waveguide, at right angles to one another so that, once the LNB has been skewed in its mount to match the local polarization angle, one probe collects horizontal signals and the other vertical, and an electronic switch (controlled by the voltage of the LNB's power supply from the receiver: 13 V for vertical and 18 V for horizontal) determines which polarization is passed on through the LNB for amplification and block-down conversion.
Such LNBs can receive all the transmissions from a satellite with no moving parts and with just one cable connected to the receiver, and have since become the most common type of LNB produced.
The final alignment that needs to be set is the skew adjustment on the LNB. This is the angle that the LNB itself sits within the LNB holder. Unless the satellites that you wish to align your dish to are perfectly due south (or north down under!) then you are going to need to make an adjustment for the LNB skew. The reason this is so important is because satellite signals are broadcast in both horizontal and vertical polarisations and without setting the skew correctly the LNB and satellite dish will not be able to optimally identify the difference between these and they can interfere with one and another resulting in an unreliable signal, picture pixilation and possible complete loss of signal altogether.
in other words you got to tilt it in for the best signal.
