Norm, If the Q202-G duplexer is tuned exactly to spec, you should see about 1.5 dB insertion loss on each side. That translates to about 29% loss, so with 99 watts going in, you should see about 70 watts coming out. Since you have measured 85 watts coming out, it means that the loops are mis-adjusted on the TX side, and desense is almost guaranteed. When properly tuned, you should see about 1.5 dB insertion loss on both RX and TX sides, and at least 80 dB isolation on each side. I'm curious: What test equipment are you using to tune this duplexer?
73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of NORM KNAPP Sent: Saturday, July 11, 2009 6:46 PM To: [email protected] Subject: [Repeater-Builder] Notch on duplexers... How low should they go??? 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
