how install a new ourdoor digital aerial
There's no such thing as a Digital Antenna, the radio wave being intercepted by the antenna doesn't care what it looks like, only that its electrical specs capture the signal. Those fancy cut weird antennas that are made to look digital are just a sales ploy to increase the selling price. 
You probably need a new antenna with clean connections to the active elements and more gain that the old one you're replacing.
Digital TV is very sensitive to signal strength. Passive splitters decrease signal strength, long cable runs reduce signal strength. If you're going to have long cable runs, use a distribution amplifier instead of a passive splitter and use low-loss quad-shielded RG-6 for all your runs to reduce signal loss both before and after the amplifier.