So bad that even an amplifier box only gave us a snowy and distorted picture. But we never really had the incentive to debug the original aerial until yesterday, when I decided to temporarily replace the scaffolding-blocked Sky system with a Freeview terrestrial digital TV set top box that we had lying around.
As expected signal strengths were poor, but one unusual effect was that the diagnostics on the DTV box inidcated that the digital signal quality was much lower than I'd have expected, even with the poor carrier signal strength. Even with out a signal analyser and a Smith chart I knew that there were some really nasty signal reflections somewhere in the system.
I knew there'd been a junction box outside the front window, where the aerial leads had been split between the upper and lower flats. Sometime before we'd moved in, a new antenna had been put up for the lower flat, and splitter had been made redundant - with our cable still using it as a junction box. What I hadn't realised was that the unused cable had just been cut - leaving a stub of cable which I now realise was probably shorting out, leaving an impedence mismatch in the splitter. I removed the stub, and rescanned for channels.
Bingo - 59 channels of TV and radio. And a clear picture on the old analogue services, too.
Yay. Go me.
And my L33T engineering skills.