John: There are two things that you must protect you repeater receiver from. One is the on frequency signal, which will come from the repeater transmitter noise output and the second one is the blocking signal which prevents the repeater receiver from receiving the desired signal, which will come from the repeater transmitter main carrier output. The first has to be eliminated at the transmitter output signal, while the second is eliminated at the receiver input source. Both areas usually require between 70 and 80 db rejection each!
Fred W5VAY ----- Original Message ----- From: John Barrett To: Repeater-Builder@yahoogroups.com Sent: Tuesday, October 23, 2007 11:39 AM Subject: RE: RE: RE: RE: [Repeater-Builder] Re: duplexer isolation and reciever noise bud I’m trying to understand your figures, and I got lost some place. From the transmitters… I’m max 50w (+47dbm) into the isolator -1db from the isolator -3db from the combiner -30db from the antenna circulator (assuming the antenna is matched) -3db from the receive splitter -60 to -90db from the receiver cavities Total isolation = 97 to 127db (as good or better than most stock duplexer setups) Putting my transmit signals at -50 to -80 dbm The receiver has 0.15uV sensitivity, which is -123dbm, which puts the transmitters well above the receivers “floor” However, the receiver specifies 75db spurious/image/intermod rejection which I take to mean that any off channel signal -48dbm (-123dbm sensitivity + 75db rejection) or less should be completely ignored by the receiver, as it will be attenuated below the receiver floor in the IF. Anything stronger than that will start to cause de-sense, swamping any on-channel signals at the same power or less. That’s what I need confirmed – if the receivers stated sensitivity + spurious/image/intermod rejection = maximum adjacent channel signal before de-sense kicks in. Is there an RF Engineer in the house ?? ------------------------------------------------------------------------------ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Ron Wright Sent: Tuesday, October 23, 2007 6:31 AM To: Repeater-Builder@yahoogroups.com Subject: Re: RE: RE: RE: [Repeater-Builder] Re: duplexer isolation and reciever noise bud There will be some reflected power from the antenna. And this reflected power will be wide tx band noise also which will affect the receiver. If you are putting power into the isolator tx port there will be reflected noise and 30 db will not be enough. With the TX -80 db down and 30 db from isoloator that is only 110 db. It will swamp the receiver. Not sure why isolator cause harmonics for it has no non-linear components. It might cause tx to generate harmonics. Isolators are on the output of many repeater transmitters including my UHF Micor and it is built to work directly into an antenna although most applications use a duplexer which will give some harmonic suppression. Using the dummy load port for the receiver might good idea. One way of getting TR relay. 73, ron, n9ee/r >From: Keith McQueen <[EMAIL PROTECTED]> >Date: 2007/10/23 Tue AM 12:10:47 CDT >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 [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]> >>Date: 2007/10/22 Mon PM 07:27:09 CDT >>To: Repeater-Builder@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. > > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
