In my instance here... Factory preamp puts you around -106 dBm sensitivity which at 44 dBm tx works out to needing about 91 dB of isolation at my 600 KHz split... You, however, have 50 dBm so you're looking at closer to 96 dB of isolation to sit at theorhetical zero desense. I'm sure someone on here can run the numbers through Commshop - stock VHF RX with 10 dB preamp is around .18 uV sensitivity.
I'm just coming off a 13 hour shift so my math may be (way) off lol :) -AJ, K6LOR On Sat, Jul 11, 2009 at 7:46 PM, NORM KNAPP <[email protected]> wrote: > > > I was checking the tuning on my duplexers for the 147.225 repeater after > noticing some desense on with the service monitor. I had something like 99 > watts going into the duplexers (sinclair Q-202g) and about 85 coming out. I > thought, WOW! This cannot be right. Sure enough they were aparently > mis-tuned. I switched the service monitor into the tracking gen mode and > began. The rx side was pretty close and when I got done I had a -85db or so > notch at 147.225mhz. All I had to do was move the ol' notches with the > little caps next to the connector. The pass at 147.825 looked good, about > -1.5db or so. Moved over to the TX side (low pass) and the notches were off. > A good bit off in fact. So I startes tuning them to where they needed to be > and all I could get was about -65db or so. What gives?!? Oh no, I thought. I > am gonna have to spin the loops! EGAD!!! I hate doing that. Oh well, here > goes nothing. I isolated the first jug and began spinning. I ended up > getting it (one jug) down to about -31db. The other side were in the -35db > or more area. Try as I may the -31db per can was about as good as I could > get. Whith both cans together I get a -79db notch on the 147.825 rx > frequency with about -1.4 db on the pass. Is this adequate for a MASTR II > with the factory preamp? The desense is pretty much gone now. I think if I > spent a little more time I could probably get the notch a little closer to > the -85db that I got on the other pair on the high pass rx side, but I am > not sure it is worth the effort. > What do you thing guys??? > > 73 de N5NPO > >
