Eric, The primary reason for a isolator is to prevent intermod, intermod inside the PA...keeping out outside RF from entering the PA, mixing, being amplified and then being re-amplified by the PA and transmitted. As you know.
A site owner don't give a rats butt about your PA smoking...he cares about what RF damage one will do to his other on site customers...so he requires, often, a isolator. Now from my stand point it does give my PA some protection as well. I am just trying to determine what is non-linear about an isolator...there aint no active components inside. It is just some transmission lines coupling and phasing different ports. 73, ron, n9ee/r >From: Eric Lemmon <[EMAIL PROTECTED]> >Date: 2007/10/22 Mon PM 08:46:30 CDT >To: [email protected] >Subject: RE: RE: RE: [Repeater-Builder] Re: duplexer isolation and receiver >noise budget > >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. > > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
