Editing Low-noise block downconverter (section)

==Polarization==

It is common to [[polarization (waves)|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 polarization. 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, [[direct broadcast satellite|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 linearly polarized signals so the converted signals can be treated the same as in systems that use linear polarization for transmission.

[[Image:old flange lnb.jpg|thumb|right|120px|A 1980s [[Ku band|K<sub>u</sub>-band]] LNB (2.18&nbsp;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&nbsp;V for vertical and 18&nbsp;V for horizontal) determines which polarization is passed on through the LNB for amplification and block-downconversion.

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.