There is no magic formula. The curves given by the duplexer manufacturers are "typical" for radios like Motorola and GE.
Duplexer isolation requirements have little if anything to do with "adjacent channel" receiver selectivity. That number only tells you how strong the neighboring frequency can be and not affect the one you want to hear. You are correct that receiver desense happens in the front end of the receiver and not in the IF stage. Receiver desense is a function of how much overload the first mixer can handle. In off frequency operation as in a duplex situation it is affected by the mixer capability itself, front end filter rejection capability and the preamp if one is used. Broadband noise is a function of how clean the transmitter is. Both of these specs are provided by the radio manufacturers of better radios. They will give that info at certain frequency separations. You can measure how much the receiver is affected by watching the limiter current in the receiver. Inject a low level signal which will produce a small amount of limiter current. If the current goes down when the transmitter is turned on it is the transmitter carrier causing desense in the front end. If the limiter current increases it is broad band noise causing the problem. Of course you could have both things going on at the same time. Each has to be sorted out separately by adding filters to either the transmitter or receiver. I see lots of guys worrying about turning down the transmitter power on a repeater to avoid desense. That should make little if any difference if things are set up right. Going from 100 watts to 25 watts is only 6 db of difference. Rarely are you that close with duplexer rejection capabilities. Transmitter noise may change when drive is reduced or it may not. Depending on how the power is controlled. Also many people think that "the more duplexer isolation you have the better." In fact any more than "enough" is a waste as it does nothing for you but drain your wallet. The charts that show antenna isolation with vertical separation are for antennas exactly in line with each other. Substituting a gain antenna for a dipole usually won't degrade isolation. A gain antenna will have a deeper null in the vertical direction in most cases. Isolation figures can vary with types of installation from stray coupling. Low band is a problem as it takes more separation to get the same isolation as vhf gives you. 73 Gary K4FMX _____ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Paul Plack Sent: Wednesday, October 31, 2007 10:31 PM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Re: Calculating required T/R isolation Guys, thanks for the suggestions. The duplexer manufacturer websites have scant info. TX-RX Systems has two graphs, one for VHF high band, one for UHF, but no info on the assumptions behind the math. I'm skeptical about the usefulness of receiver adjacent-channel rejection specs, for two reasons: (1) They include IF filtering, and desense happens due to compression in mixer stages ahead of the IF, and (2) some filters have poorer rejection out a couple hundred kHz than they do on the adjacent channels. I think what I'd need for a generic formula would be a way to account for the dynamic range of the receiver front end, which is often spec'd as the range limited by 1 dB compression in the first mixer, and I'd need accurate sideband noise numbers for the transmitter. I don't know what receiver or transmitter I'd use at this point. My former repeater was a converted Mastr II mobile on UHF, with a TX-RX Systems 4-can, and appeared to have plenty of reserve at -96+ dB on each side, 5 MHz spacing, and dialed back to 18 watts out (just above where it started getting "dirty" at QRP.) What I'm working on now is an antenna designed with minimum tower coupling as its first priority, preserving the deep null which naturally occurs between vertical colinear arrays sharing a common vertical axis. I've been bugged for decades by that ARRL graph showing dB isolation at given vertical separations. Like the TX-RX website graphs, there's no explanation of how it was derived. It assumes half-wave dipoles for both antennas, but indicates no correction factor is required for gain antennas, and that makes no sense. Now that we have software which can accurately model antennas on towers, I'm going to actually model my antenna idea on a virtual tower, and see what happens. The goal is to potentially allow 6M or even 10M single-site repeaters using separate antennas for transmit and receive. (I know, I know...the holy grail.) It's going to be lots of work to model and test it, so I'm trying to do calculations applicable to a number of end-user configurations, not just one specific TX / RX set I might personally use. I'd love to set up a spreadsheet that accounted for all the necessary factors, and contribute it to the web site. If I actually build the repeater itself, I have two 100-watt-class Johnson mobiles on low band / low split, retired from service in the broadcast RPU band. (They're crystalled and tuned for 26-point-something, where radio stations did linking for remote broadcasts before CB-ers discovered VFOs.) They're nice, clean old rockbound rigs. I also have some monster surplus heatsinks, a clean Astron RS35RM, and I still have the 7K. I may yet talk myself into building another repeater. For all the hassles, it sure was educational and fun! 73, Paul AE4KR ----- Original Message ----- From: Nate Duehr <mailto:[EMAIL PROTECTED]> To: Repeater-Builder@yahoogroups.com Sent: Wednesday, October 31, 2007 4:22 PM Subject: Re: [Repeater-Builder] Calculating required T/R isolation Paul Plack wrote: > Guys, sorry for this repeat - my first attempt at a post to the list > went out with an off-topic subject line related to Kenwood repeaters. > > Can anyone direct me to a good tutorial on how to calculate required > isolation in a duplexer or separate-antenna setup? I can convert > receiver sensitivity in uV and transmitter power in watts to dBm, and > insert things like transmitter sideband noise specs, but I keep coming > up with numbers like 150 dB to avoid desense. > > Since my last UHF repeater worked great with 96 dB measured > isolation thru the duplexer, I know I'm missing a step somewhere. > > Thanks! - 73, Paul AE4KR Did you see K5BP Bernie's reply under the original topic? He had it right -- you likely forgot to subtract out your particular receiver's off-channel selectivity numbers. How much does the receiver you're planning to use reject off-frequency signals by design? Subtract that from your duplexer numbers. You'll also probably want to factor in how a receive pre-amp and possibly additional receive-side bandpass filtering figure into the equation if you're using a highly selective/non-sensitive receiver if and if you're going for maximum receive performance. Nate WY0X
