Why do signals to tv aerials need to be polarised?

[Giada]

is it because by polarising the em waves, it restricts the area needed to receive them, which makes it easier, otherwise you'd need to create moving tv aerials which wouldn't be convenient?

 

[CWatters]

Why would they need to move?

 

[sophiecentaur]

is it because by polarising the em waves, it restricts the area needed to receive them, which makes it easier, otherwise you'd need to create moving tv aerials which wouldn't be convenient?

Conventional aerials - like most of the ones you see on rooftops - consist of a set of wires that lie in a particular plane (vertical or horizontal, usually). They are only sensitive to signals with either horizontal or vertically polarised Electric Fields and will not pick up signals with the opposite polarisation. So the transmitting aerial will also consist, basically, of vertical or horizontal wires. All aerials, tansmititing and receiving, in a particular area have the same polarisation and it is often arranged that (TV) aerials in nearby areas work on a different polarisation. This gives some protection agains interference from nearby stations and allows better use to be made of the available channels.
If you rotate an aerial about the axis of the 'boom', you can cut out reception of a station quite effectively. This protection works well when your are situated away from high metal structures which cause reflections and upset the polarisation of some signals.