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: [email protected] >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: [email protected] [mailto:[EMAIL PROTECTED] On Behalf >Of John Barrett >Sent: Monday, October 22, 2007 7:47 PM >To: [email protected] >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) > >Iâm proposing a novel application of the circulator (an isolator without >the dummy load on one port)â¦. 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 âloadâ 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 âreversedâ 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 âwindowâ >for all the transmitters and receivers is less than 1mhz, so matching the >antenna shouldnât be a huge problem. > >From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf >Of Ron Wright >Sent: Monday, October 22, 2007 8:19 PM >To: [email protected] >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: [email protected] >>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.
