A bit of background to my question - my original "flying saucer" aerial has given poor signal on the last couple of sites where I've tried to use it - I don't know if it's developed a fault or if I've just been on two sites with poor signal. It could even be that the caravan's developed a fault with the internal aerial wiring, and there's no problem with the aerial at all.
So I bought a budget directional aerial to try.
Last weekend (Setthorns in the New Forest) the flying saucer gave a dodgy signal and the directional gave no signal at all. The app reckons I should have been getting a good signal from Rowridge on the Isle of Wight.
The trouble is, from where I have to stand to adjust the direction of the aerial, I can't see the TV inside the van, and if the TV is left without signal for more than about half a minute it just displays a "no signal" message and needs to be retuned.
So now I don't know if the problem with the directional was wrong direction, or a problem with the cable attached to the directional, or a problem with the external aerial socket on the 'van (fitted by me many years ago and has worked OK in the past) - or as said above, the wiring inside the van between the distribution amp and the TV.
I thought with a signal strength meter I could at least check if there was a signal from the new aerial to the plug that I normally plug into the caravan's external socket. Then once I've got signal there, I could check I was still getting one where the cable from the external socket appears inside the van - and so on down the chain until I determined where the signal wasn't getting through so I could rectify it. And I could do all those checks at the storage site where there's no mains to plug the telly in.
I should add that I'm fairly experienced with electronics and antenna theory (although it's not how I've actually earned my living for years) so I don't THINK I'm missing anything obvious, but again, a meter would let me check.