Beckley mast - plane polarised TV waves

Here's the view from Debenham's café in Oxford. Between  the church tower and the tree you might just be able to pick out Beckley mast, the TV transmitter for Oxford. It is a primary transmitter and the TV electromagnetic waves from it are horizontally plane polarised. The TV aerials on houses will need to have the metal rods in the horizontal plane to detect them. There will be places hidden by hills where you cannot see the mast. TV waves probably have too short a wavelength to diffract round the hills and into the valleys - I need to check this. So they use repeater antennae called relay stations. These take in the signal from Beckley and re-broadcast it into the valleys. The re-broadcast signal is sent out with vertical polarisation. I have not seen this said anywhere, but I imagine that is so that there is no interference between the two versions of the same signal. I found a marvellous website about all of these issues http://www.aerialsandtv.com/aerials.html so expect more!