Television & Radio Hardware - Polarisation
Some aerials are mounted horizontally and some are vertically. This is known as the polarisation and for best performance, must be correct for the transmitter being used. In strong signal areas, an incorrectly polarised aerial will probably work, however for performance reasons it is obviously not recommended.
Generally, main transmitters such as Winter Hill, Belmont, and Emley moor are horizontally polarised. The smaller repeaters are often (but not always) vertically polarised.
Polarisation generally prevents co-channel interference. This is the case of two transmitters broadcasting on the same channel causing interference on the receiving tuner. However, situations can occur with the polarisation is swapped (such as a signal bouncing off a roof), which means that the polarisation of an aerial isn’t a perfect method of rejecting an unwanted signal.