Hi cdstack,
In short you need to find the correct aerial for your transmitter.
I Posted this information on here a few years ago but as the links have changed I will post it again.
Have a look at this map to help figure out which transmitters you can receive, click on the dot nearest to you.
BBC - Help receiving TV and radio - TV Transmitters
this gives you the Aerial group to help you chose the correct aerial for your transmitter
For example if it says under aerial group A/H then you need a group A aerial in the horizontal position
Main transmitters are horizontally polarized, so the elements should be horizontal.
Relay transmitters are vertically polarized, so the elements should be vertical
To help you, if you are buying from local shops, take a look at the end of the aerial
the aerials have Colour coded caps on the ends check this list against your transmitter
Channels - Group - Colour
21-37 - A - red
35-53 - B - yellow
48-68 - C/D - green
35-68 - E - brown
21-48 - K - grey
21-68 - W - black........this is wideband (not usually optimal)
My personal preference of brand is Antiference and the 20 element version should be long enough for most poor reception areas
Darcy
EDIT: if you don,t know which transmitter you should be receiving try typing your postcode in
Digital UK - Home (tick the "i am in the installation trade box), it should give you some local information