waffler said:
The aerial probably only works when the signal is vertically polarised. The main transmitters are horizontally polarised and your aerial theoretically works by using the roof in some way. The only time yours will be OK is when there is a small local transmitter which are usually vertically polarised. We had the same problem and solved the problem by installing a Status directional. (I did a lot of research on this topic because we couldn't get a picture on a number of sites where we could see the transmitter but did with an old indoor aerial!,,)
More often than not failure of the TV tuning system to select a local transmitter is down to the (stupid one could argue) way that TVs self tune. They always start at the lowest channel and tune upwards so may find a weaker signal in lower channels than a stronger signal in higher channels. On up-to-date TVs it should select the strongest signal but as caravan aerials have a high gain amp on them the amp could be overloaded by the strong local signal enough that it is corrupted and the TV cannot decode it.
There are two solutions to this paradox: if you prefer to use auto tuning (or indeed your TV does not have manual tuning) then get a variable attenuator from Screwfix and tune with the attenuator at maximum and repeat using slightly less attenuation each time until your TV gets a 'valid' set of channels, then remove the attenuator. The preferred method is to do some homework before you leave home.
Go to www.wolfbane.com and enter the postcode of where you are going. This will show you the strongest transmitter serving that location, its direction, its polarity, and the channels that it uses. When you get to site first do an auto tune with no aerial connected to wipe the existing memory, then do a manual tune using the channels indicated by Wolfbane. Don't bother with the channels marked as PSB3 or Com 7 or Com 8 unless your TV has a Freeview HD tuner.
Incidently it does not follow that because you can see a tower with a white candle on the top it is a TV transmitter. Some FM radio transmitters use white candles as well!