At 6/30/2008 08:48, you wrote: > > >This is why it is best to put about 1-2 db 50 ohm pad > > between the duplexer > > >and the receiver, to help keep the receive side at 50 ohm. > > > > And degrade my system sensitivity by that same amount? No thanks. > >Maybe I misunderstood - I thought he was suggesting using pads while tuning, >not during long-term operation. > >Still, there are situations where pads in front of the receiver won't make a >change in usable sensitivity, but will improve intermod/overload >performance, such as at a site with an elevated noise floor. I remember >working on a 2m repeater on a cell tower that was under high-tension power >lines; a 10 dB pad ahead of the receiver wouldn't have changed the usable >sensitivity.
Around here a preamp in front of a stock Mastr II or MVP offers no sensitivity improvement due to the overall high antenna noise temperature at 2 meters. If you calculate the noise temperature of a 0.35 µV 12 dB SINAD RX, the antenna noise temperature must be at least 3000 K, or ~10 dB above the thermal noise of a 50 ohm load (much lower than UHF, where it's typically at or below 300 K). Rather than put a pad in front of a preamp, I just leave the preamp off the RX. This results in a very IMD-resistant setup, since there are no active devices ahead of the helical resonators. Bob NO6B
