Ron, An isolator is, by definition, a non-linear device- that's how it works. And that is why it should always be followed by either a low-pass filter or a notch filter, to remove the second harmonic that is sometimes generated by its nonlinearity.
The primary purpose of an isolator- which is simply a circulator with a dummy load attached to the open port- is to prevent intermod interference. It does this by causing RF power that is heading the "wrong way" toward the PA to be diverted to a dummy load, instead of back into the PA itself. This also protects the PA from its own output, should the antenna be damaged or removed; the PA "sees" the 50-ohm dummy load instead of an open. Your last statement is correct, in that isolators were added to many stations (especially UHF) because the PAs were burning up with even minor impedance mismatches. Most high-power PAs have an isolator/circulator built-in as protection for the PA- not to prevent IM interference. More often than not, it is necessary to augment them with additional circulators or IM panels. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Ron Wright Sent: Monday, October 22, 2007 6:19 PM To: [email protected] Subject: Re: RE: RE: [Repeater-Builder] Re: duplexer isolation and receiver 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 commercial 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> > >Date: 2007/10/22 Mon PM 07:27:09 CDT >To: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> >Subject: RE: RE: [Repeater-Builder] Re: duplexer isolation and receiver 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 suppression 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 achieving 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.
