Hi John, my personal view is that they're useful for overcoming the losses incurred by using long lengths co-ax downlead, but they do need to be as close to antenna as possible (ie mast-head preferred), for RB and SBS-1 (IMO) you need one designed and built for 1090MHz, not a JIM M-75 for example, which is better suited to wide band range use - eg scanner . For my money, it would come down to the Elad or Kuhne (IMO the RR version!). All a pre-amp does, is amplify the wanted signal (and also the unwanted noise). A pre-amp will also input its own noise and losses.
If you've already got a really good signal, a pre-amp could overload or swamp the front-end of the RB/SBS and lose info.. Pre-amps only amplify what is already being received!
I nealy put a comment on Dean's post with his Polar Diagrams, if he's happy with the apparent increase of aircraft being received (although a 100% increase is very unlikely IMO) then that's great, but the PDs need to be built up over days or weeks, not just a snapshot of a couple of hours, or a single day even, routing is affected by weather and other changing conditions, times of day and days of the week,and most of all the heights of the aircraft that were received. On GlynH's AN supplied standard antenna, which I've weather-proofed and improved electrical connections, I am regularly seeing aircraft at 250nm if they're above FL330. I hasten to add, I'm not criticising Dean's post in any way, but Im extremely sceptical about a 100% increase - it could be poor contact in a plug and socket has improved by disturbing the connection. I think pre-amps are seen as a cure-all, but are more reliant on the placebo effect unless they're bsing used to counteract losses in the co-ax.
Placing a pre-amp at the RB/SBS end of the feeder is a total waste of time and money.