The rule of thumb for digital TV is if the antenna is over 10 years old it was not made to properly receive digital TV.
Cable is too old
The digital TV signal has to be protected from other signals. Other frequencies imposing themselves on the digital signal is called spurious interference. An example of this is when a light is switched on or the fridge is opened. Because the TV cable is in the roof with electrical cable when a light is switched on it creates a magnetic field which interferes with the digital signal. Having the right cable will minimise this effect considerably. We only use RG-6 Quad shielded cable.
Splitters, connectors and wall plates
I have done plenty of jobs and the common fault that households have is they have the old saddle type splitter, and wall plates. For digital TV it demands a higher degree of protection from spurious interference so we only use F-Type splitters and wall plates. These type of connections help prevent any interference from mixing with the digital TV signal.
Line of Sight
This is a crucial part of the installation. If there are large hills around or your house is at the bottom of a hill this can be harmful to achieving good quality digital TV signal. It is not all bad, all you have to do is call me and I will come around and give a free quote and 9 out of 10 times I can fix it.