Hi Mike, That is a nice article. Someone else suggested that the reference to "old curves" should have been to the older tx-rx isolation curves that were published by GE. Those are also valuable as a starting point in figuring total isolation needed. I agree that those are very tx-rx type dependent. You might want to incorporate those into the article or that part might best be left to another article (maybe one already exists?).
It could be noted that those curves represent an approximate amount of isolation needed but with a caveat that all tx and rx are not created equal. The graphs of antenna separation are typical of isolation provided by antenna separation distance. If additional isolation is needed it would be necessary to add cavities to the transmitter and or receiver in order to obtain the total isolation needed as indicated by the tx-rx isolation curves with a note that those curves may or may not be adequate depending on transmitter and receiver quality. If sticking to antenna separation, then I would reword that part to deal only with isolation available with antenna separation. As another note on the tx-rx isolation curves, a synthesized transmitter is not always worse than a crystal controlled transmitter as far as broad band noise output. If I remember right There are somewhere some curves showing that a certain model GE synthesized transmitter has less broad band noise than a typical crystal controlled transmitter therefore requiring less isolation in a duplex operation. Isolation curves used to be available for many tx-rx combinations. Very handy when selecting a duplexer for a particular setup. This leads to another subject, duplexer isolation. Lots of people are under the impression that if a duplexer that is adequate with 70 db of isolation and provides no desense then using one with 100 db of isolation will be much better. In this case the 100 db duplexer is only better at emptying your wallet. Once you have reached the needed isolation, more does nothing for you. RECEIVER FRONT ENDS Some of the older radios obtained their intermod specs by using very narrow front end filters. Some of the newer radios have much wider front ends and provide the same intermod performance. This is because the newer radios have much better dynamic range in the mixer. It is all about how much total power a mixer can handle before being overloaded. The narrower the front end filter the less total signal power reaches the mixer. All signals received add together in the mixer and are a determining factor in overload not just the strongest signals. SPECTRUM ANALYZER Good point about using the spectrum analyzer to look for/ tune for minimum spurs in the local oscillator. One thing to watch out for when using a spectrum analyzer and watching for low level noise or spurs along with a large signal (either on a receiver or a transmitter) is overload on the analyzer. I have seen many people fooled by analyzer overload and false apparent spurs etc. created within the analyzer itself. This is particularly true of analyzers that are incorporated within a service monitor. Most suffer badly from lack of dynamic range. Using a sharp notch tuned to the strong signal will often allow you to look much further down to reliably see other noise and spurs. This got longer than expected. 73 Gary K4FMX > -----Original Message----- > From: Mike Morris WA6ILQ [mailto:[EMAIL PROTECTED] > >I notice in the below referenced article on antenna separation that > >it is stated that the "graphs are misleading and no where near > >accurate for modern day solid state equipment". > >http://www.repeater-builder.com/antenna/separation.html> > > > >That is not correct as the graphs for antenna isolation show > >approximately how much isolation can be expected with a given amount > >of separation either vertically or horizontally. This has nothing to > >do with the amount of isolation NEEDED for proper duplex operation. > > Very true. I should have pointed that out. > > >The amount of isolation needed is a function of the transmitter and > >receiver being used. One type of tx-rx combination may need a > >different amount of isolation than another type used with the same > >antenna isolation. > > > >Maybe someone would want to correct the above? > > I wrote that article, and you have pointed out something that needs > to be changed. THANK YOU. > And if you have a better way of saying it, I'd be glad to hear your > suggested rewording.
