Laryn, That's a very good question! When we speak of "notch duplexers" we usually mean a mobile duplexer, which usually comprises six helical resonators that are about 1" square. The coupling loops or probes are usually fixed at the factory, and the longitudinal tuning screws simply move the notch. The pass insertion loss is more or less fixed by the design of the duplexer, and is extremely broad. Most such duplexers are intended for splits of 5 MHz or greater.
The design of a BpBr duplexer, especially one for 2m application, sacrifices a sharp bandpass response so that the notch can be deep enough at a narrow split- 600 kHz at 2m. At a solitary site, the typical BpBr duplexer will probably work just fine. It's when there are other transmitters nearby that problems occur. The "modest" bandpass response of a 2m BpBr duplexer may not be tight enough to shut out nearby transmitters that cause desense. That's exactly the situation where additional bandpass-only filtering is necessary. I have one 40 watt MTR2000 2m repeater at a site at which the only other emitter is a 10,000 watt FM broadcast station. The duplexer is a Sinclair Q-202G unit that I tuned on a network analyzer. The repeater had significant desense until I put one 8" bandpass cavity set for 1.0 dB IL on the receive side, between the duplexer RX output and the receiver. With no preamp, the desense went away, and the repeater has phenomenal sensitivity and range. Your comment that the additional pass cavity loss is nil, might be overly optimistic. Truth be told, two pass cavities set for 0.5 dB IL are significantly better than one cavity at 1.0 dB. Lower IL means a much broader response. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Laryn Lohman Sent: Thursday, February 09, 2006 7:17 AM To: [email protected] Subject: [Repeater-Builder] Re: Duplexer questions --- In [email protected], "Eric Lemmon" <[EMAIL PROTECTED]> wrote: > > The names are somewhat misleading, because the bandpass effect is relatively > modest, although the notch is quite sharp. It is a good idea to have a pure > bandpass cavity or two between the duplexer and the receiver, especially if > you have a preamplifier. Eric, I agree with that. Since the bandpass effect is so modest, and since an additional bandpass cavity or two will probably be needed anyway, why are BpBr duplexers considered to be so important at a busy site, compared to say- a notch duplexer??? It seems notch duplexers are automatically considered to be almost a useless item when the subject of duplexers is discussed here, and should (almost) never be used. The additional loss of that pass cavity is nil. Comments Eric or anyone? Laryn K8TVZ Yahoo! Groups Links Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
