Dan Kagabine, the chief engineer at TX-RX systems use to always say that "once you have enough isolation to overcome any desense, then any more is a waste of money as it does nothing for you". "If you only need 70 db then a 100 db duplexer does nothing more for you than a 70 db duplexer".
73 Gary K4FMX _____ From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Kevin Custer Sent: Saturday, August 22, 2009 5:38 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] isolation Eric, You may have missed the point. While your program calculated a necessary isolation amount of 99.65 dB, using a GE M2 PLL exciter would dictate 77.65 dB of necessary isolation - which is easily obtainable with a quality 4 cavity (okay, 4 large cavity) duplexer. While I certainly wouldn't recommend a duplexer using four 5" cavities, a four cavity duplexer utilizing 8 inch cavities would provide more than adequate isolation (90+ dB of isolation) for this gentleman's arrangement. If he were using a multiplier exciter (which the 'program' assumed), then one can certainly understand your recommendation - but - he did say PLL exciter and M2 equipment. I'm not sure I understand your statement "Nothing about duplexers is for certain". All of the duplexers I have ever tuned came out to factory specifications or better. If not, something was physically wrong - lightning damage - cabling problems - loop problems, etc. I don't believe that if this person were to utilize a quality four cavity duplexer that we'd be setting him up for failure. Engineering is on our side, and he can benefit from not needing to spend extra money for something that isn't really necessary. BTW: It is possible to duplex a PLL exciter (200 mW) and M2 receiver (no preamp) at 600 kHz with nothing more than a tee connector. You do have to skew the helical tuning a bit so the skirt is sharp on the side of the transmitter; which reduces receive sensitivity to less than factory specification. I won't say there will be zero desense, but you won't even get close with a multiplier exciter in the same test. It's fun - tastes great - less filling! Kevin Kevin, Nothing about duplexers is "for certain." While I agree that a PLL exciter is inherently less noisy than its multiplier counterpart, I never assume that it's okay to plan ahead for less than optimum isolation. Some duplexer designs are known to have better performance than physically identical designs from other manufacturers- the silver-plated copper cans from Decibel Products are one example. I feel that it's better to have a duplexer that is perfectly tuned and has absolutely zero desense, than a lower-performance duplexer that has only a little desense. In an ideal world, KJ4SI should be able to buy a four-cavity BpBr duplexer and try it out for 30 days to see if it had zero desense- with the option to purchase two more cans and the appropriate jumpers at a discount for upgrading it to a six-cavity duplexer. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Kevin Custer Sent: Saturday, August 22, 2009 12:52 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] isolation Eric, Are you sure about your six-cavity recommendation? The MASTR II PLL exciter has 22 dB less side-band noise than a typical multiplier exciter - using 600 kHz TX to RX separation. Assuming his preamp isn't driven into a non-linear region (it shouldn't be), a good 4 cavity duplexer, like a WACOM WP-641, should give plenty of isolation... Kevin Custer My CommShop calculates 99.65 dB is required. I'd definitely be looking at a six-cavity BpBr duplexer for this station. 73, Eric Lemmon WB6FLY ---------------------------------------------------------- Hope someone may have a program,commshop? What I need to know is what amount of isolation with duplexers that is required for a GE m2 receiver with .1...@12db and a m2 pll exciter,100 watt PA on vhf,600kc split?1/2in helix,with 4pole db224 antenna at 70 ft. thanks kj4si
