Wow, lots of great info there. I do have the PLL exciter. It is working better after I adjusted the cans yesterday, but I am wondering if I need to spend the time to tweak it more. Thanks for your input... EVEREYBODY! De N5NPO Norm
----- Original Message ----- From: Repeater-Builder@yahoogroups.com <Repeater-Builder@yahoogroups.com> To: Repeater-Builder@yahoogroups.com <Repeater-Builder@yahoogroups.com> Sent: Sun Jul 12 09:50:54 2009 Subject: [possible spam] RE: [Repeater-Builder] Notch on duplexers... How low should they go??? With a preamp, if you're only getting -106 dBm sensitivity, something is wrong. If memory serves, the spec for 12 dB SINAD without the preamp for a highband Mastr II is 0.35 uV (-116 dBm), and with the UHS preamp the sensitivity was 6 dB better (-122 dBm). Looking at the GE isolation curves, for a 100 watt highband transmitter with a phase-modulated exciter you need a minimum of 81 dB of noise supression (Tx cavity notch depth) and 69 dB of carrier supression (Rx cavity notch depth). For the PLL exciter, you only need 59 dB of noise supression. GE's curves are based on the assumption that 1 dB degradation (desense) is acceptable. If you add the UHS preamp, GE recommends adding another 6 dB to both isolation requirements. Personally I think that's cutting it rather close. The preamp has about 10 dB gain, but its noise figure isn't very good, hence the usable improvement in sensitivity is only about 6 dB. But that 10 dB of gain is still there, so you want at least 10 dB more carrier supression to prevent receiver overload (at least that's the way I look at it). On top of the calculated minimums, I always want to see AT LEAST 10 dB more isolation than the minimum required, more if I can get it. So, assuming you're using a regular PM exciter, 100 watt PA, and UHS preamp, if I were building the box I'd be targeting 100+ dB of noise supression and 90 dB of carrier rejection at a minimum. --- Jeff WN3A > -----Original Message----- > From: Repeater-Builder@yahoogroups.com > <mailto:Repeater-Builder%40yahoogroups.com> > [mailto:Repeater-Builder@yahoogroups.com > <mailto:Repeater-Builder%40yahoogroups.com> ] On Behalf Of AJ > Sent: Saturday, July 11, 2009 10:00 PM > To: Repeater-Builder@yahoogroups.com > <mailto:Repeater-Builder%40yahoogroups.com> > Subject: Re: [Repeater-Builder] Notch on duplexers... How low > should they go??? > > > > 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 > <nkn...@twowayradio.net <mailto:nknapp%40twowayradio.net> > <mailto:nkn...@twowayradio.net <mailto:nknapp%40twowayradio.net> > > 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 > > > > > > Checked by AVG - www.avg.com > Version: 8.5.375 / Virus Database: 270.13.12/2233 - Release > Date: 07/12/09 08:20:00 > > >
