Here are two photographs of television aerials. The top one was taken in Aviemore and the bottom one is my own TV aerial. Notice that the metal bars are at 90 degrees to each other. This is because TV signals are polarised when they are transmitted. Polarisation is when a transverse wave is made to vibrate in only one plane. If you polarise TV waves, you can send out twice as many from a transmitter without them interfering with each other. My aerial will only detect TV waves that oscillate in the horizontal plane. In Aviemore, the TV waves seem to be polarised to vibrate in the vertical plane. It is important to orientate your aerial appropriately.
