I moved into a house with an old satellite dish parked up the roof next to a new one. Can I use the old dish as a backup aerial antenna (to receive normal broadcast signals) when my satellite reception goes on the fritz? If so, how do I do it?

Nope, satellite dishes ONLY receive satellite signals!

Terrestrial transmissions are on a complete different frequency and need the special antennas designed for them (VHF-UHF frequencies).

But Thai TV is broadcast on UBC !!

I even think you can get an access card for a few 1000 baht that does not have a monthly fee cant you ??? For Thai TV channels only. You of course will need a Sat receiver also. Doesnt help when the sat goes on the fritz tho..

Of course logically if your sat is down.. How would a sat dish help ??

Well it all depends on what type of dish the old dish is ?

How big is it ?

A big dish (1.8m and up) in Thailand usually means C band and the smaller red UBC dish is for Ku band.

There are a some FREE Channels on C band on the Thaicom sats.

If you have 2 small dishes you could of course point the second one at another satellite !

http://www.lyngsat.com/asia.html