Active reflector transmitters.

[colly0410]

I remember in the old analog days reading about active reflector TV relay TX's, (Bethesda being one) they'd transmit on the same frequency they received on. Do they still use the same thing for digital TV? Are they used for digital single frequency networks?

[Murphyv310]

Yes I've heard about these. No idea if they would use them on digital.
What I have tried which works for me..... I can receive Northern Ireland on digital perfectly with a good aerial which I have on my shed. You can also get 3 RTE channels from the minimux that is 1kw from Divis. If I run the signal from the aerial through an old high power distribution amplifier to another uhf aerial and point it at the loft where the house TV aerial is I can tune some of the NI channels and RTE in, it's a bit hit and miss but it does prove that there is no reason it shouldn't work with the right equipment

[colly0410]

Hmm interesting Murphy v310. If you lived somewhere that received the main (analog) station & the active reflector then I presume you'd have a very big ghosting & co-channel problems. I understand (I think) that ghosting & co-channel interference can't happen on digital TV & that's why digital can use single frequency networks..

[ppppenguin]

It's more complicated than that regarding SFNs. You can engineer SFNs using the multicarrier techniques used for terrestrial DTV and DAB but there are reasons why you might not want to. I'd need to do a bit of work to give a decent explanation but one reason is to with the guard intervals that give resistance to multipath. Another is to do with regional differences in required channels.

Certainly UK digital trerrestrial is engineered to give very good multipath resistance so an active reflector might be very workable. The original channl would just seem like a ghost image.

[colly0410]

Ahh yes I'd forgotten about regional differences for local news & adverts. I know the BBC use a single frequency network for it's national programs on DAB as there are no regional opt outs to worry about AFAIK..

[colly0410]

Did they ever use active reflector TX's at VHF? I'm thinking 'no' because of feedback problems, it would be hard to impossible to isolate the receiver from the TX, unless others know different that is..

[Terry]

If the receiving and rebroadcast aerials were on opposite sides of the obstruction, I don't see any real problem but, as I understand it, permission to install such setups was only granted after the move to 3 (later 4) channel UHF on the basis that they might be community self-help projects in isolated sheltered areas that are too small to justify a relay transmitter.