Keith: A very good point - and multiplied since there will be 3 receivers involved :-) Perhaps a good reason to stick with 3 cans per receiver, instead of 2, since that will give me the same protection any standard duplexer would, and even more isolation when things are working right, at the cost of a little more insertion loss. Depending on the insertion loss of the cavities in the receive chain - I'm looking at 6 to 9db of loss between the splitter and the cavity filters.
It makes putting a preamp before the receive splitter an interesting possibility, to make up some of the losses further down the chain, at the cost of intermod in the preamp, which might be covered with a single band pass filter right before the preamp (1mhz wide, centered on my receive window). though that wont stop intermod with my own transmit signals. Still, if the antenna were to go bad, its more likely the preamp would smoke before enough got through to smoke the radios :-) _____ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Keith McQueen Sent: Tuesday, October 23, 2007 12:11 AM To: Repeater-Builder@yahoogroups.com Subject: RE: RE: RE: [Repeater-Builder] Re: duplexer isolation and reciever noise budget The danger I see with this is when your antenna goes bad (and they all do eventually), your receiver will be hit with the full reflected power of the PA almost certainly turning it into a smoldering doorstop. Keith McQueen 801-224-9460 [EMAIL PROTECTED] -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of John Barrett Sent: Monday, October 22, 2007 7:47 PM To: Repeater-Builder@yahoogroups.com Subject: RE: RE: RE: [Repeater-Builder] Re: duplexer isolation and reciever noise budget An isolator wont cause intermod, but it may cause harmonics. Commercial installations usually use either a harmonic filter and 3db hybrid coupler, or a special type of band pass cavity to couple the output from the isolator to the feed line. (This info from an RX TX application note on transmitter combiners) I'm proposing a novel application of the circulator (an isolator without the dummy load on one port).. Instead of the dummy load, the 3rd port feeds the receiver chain. the transmit chain will still use more or less conventional combining techniques to merge the signals from the 3 transmitters. the output from the transmitter combiner goes to the input of an additional circulator, the circulator output goes to the antenna as you would normally expect for an isolator, and the "load" port goes to the receive chain instead of a dummy load. Since the path from the transmit chain port to the receive chain port is "reversed" compared to the normal signal flow in a circulator, it will incur 20-30db of loss, depending on the circulator specs. So long as the antenna is well matched, there will be minimum reflected power fed back into the receive chain. My "window" for all the transmitters and receivers is less than 1mhz, so matching the antenna shouldn't be a huge problem. _____ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Ron Wright Sent: Monday, October 22, 2007 8:19 PM To: Repeater-Builder@yahoogroups.com Subject: Re: RE: RE: [Repeater-Builder] Re: duplexer isolation and reciever noise budget I am not sure why an isolator would cause intermod. Usually there are not active or non-linear components in them and they are often used to prevent intermod by preventing outside signals from coming in thru the feedline into the transmitter. In the past commerical sites would often require an isolator for this reason with strong transmitters close by. In better repeater equipment an isolator was built in. 73, ron, n9ee/r >From: Jeff DePolo <[EMAIL PROTECTED] <mailto:jd0%40broadsci.com> com> >Date: 2007/10/22 Mon PM 07:27:09 CDT >To: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com >Subject: RE: RE: [Repeater-Builder] Re: duplexer isolation and reciever noise budget > >> This isn't guessing - its called RESEARCH > >When I said guessing, I was talking about quantifying the performance of >your radios rather than guessing how much isolation you need. In other >words, make measurements to actually determine how much noise supression and >carrier attenuation you need using the actual frequencies involved. Once >you know how much isolation you truly need, then you can work backwards from >there to determine the filtering requirements. > >I still think using an isolator is going to cause you new problems with >respect to IM into your receivers unless you have adequate filtering between >the isolator and antenna, which I believe you have no way of acheiving if >I'm understanding your layout right (i.e. isolator is connected directly to >the antenna with nothing in between save for a harmonic filter). > > --- Jeff > > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
