Tony, I once chatted with the developer of CommShop for Windows, and learned that his formulae are based upon a number of assumptions- the most important being that the repeater is of commercial quality. This translates to GE Mastr II and Exec II, and Motorola Micor and Mitrek, for example. He gained a great deal of information from the GE Duplex Curves, all of which are available for download from the RBTIP.
It is important to realize that the results provided by the CommShop program- or any similar software package- are ballpark estimates. A transmitter with a crystal oscillator or a PLL exciter will probably require less isolation than a synthesized unit, due to improved spectral purity. A receiver with poor selectivity or a "barn door front end" will probably require more isolation. Nevertheless, I have found CommShop to be a very useful program, since it includes much more than just duplexer isolation calculations. More info is here: <www.dcico.com/dcilmr.htm> Basic calculations of suggested duplexer isolation require only four factors: TX power in watts, RX sensitivity in microvolts at 12 dB SINAD, TX frequency, and RX frequency. The antenna gain, mounting height, and length of feedline are irrelevant, since they have nothing to do with how the duplexer performs with a 50 ohm dummy load connected. As always, YMMV... 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Tony KT9AC Sent: Saturday, August 22, 2009 10:07 AM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] isolation Other than ordering the software, is there an "old-school" formula that can be used for this? I been using the "decibel-wheel" to convert from microvolts to dbm. NORM KNAPP wrote: > > > Hmmmm > How about a mastr ii pll Vhf 147.225/147.825 with db224a @ 270' with > 300' ldf7-50a and 110watts. I am running 45 watts right now. > Thanks > De N5NPO > Norm > > ----- Original Message ----- > From: Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com> > <Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com>> > To: Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com> > <Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com>> > Sent: Sat Aug 22 10:22:36 2009 > Subject: RE: [Repeater-Builder] isolation > > > > 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 > > > -----Original Message----- > From: Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com> > [mailto:Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com> ] On Behalf Of kj4si > Sent: Saturday, August 22, 2009 6:55 AM > To: Repeater-Builder@yahoogroups.com <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com> > <mailto:Repeater-Builder%40yahoogroups.com> > Subject: [Repeater-Builder] isolation > > 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
