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www.amsat.org/amsat-new/echo/ModeJ.php



>Besides Domenico's reply, you might check out:
>
>www.amsat.org/amsat-new/echo/ModeJ.php
>
>GL
>Mike
>K9QHO


That page, in particular lacks accuracy. Dont ask where those numbers
come from...
Desense is a very common problem in mode V/U, Lots has been written about it. The problembmanifests itself in distortion or blocking of the receive signal.

A 100w transmitter's output measured on a spectrum analyzer typically
has almost all the signal located on the frequency of interest, the
rest of the energy is in harmonics and noise around the center frequency (caused by noise in the pll or LO's). Requirements for spectral purity call for 80 to 85db down on the harmonics and trash.

So if the tx signal is 100w (+50dbm) and the rest of the signals are
80db down that means that -30dbm is the most that is getting to the 
reciever. Depending on the receiver this may be just fine but in some
cases, I.E. satellite receivers, they are very sensitive, capable of
receiving the noise floor which may be less then -110 db.

The receivers front end filters attenuate out of band signals quite a bit
but you might need a diplexer or duplexer if it does not do it completely.
A diplexer usually consists of a high pass and a low pass filter together in one box. They are rated by their power handling, insertion loss and
isolation. Depending on the quality of the front end of your receiver you
may only need a diplexer to attenuate the signal sufficiently (the extra 80 db attenuation we need) to hear the satellite well. Some preamps have tuned
filters in the front end and between the preamp and the receiver may do the job.

Going by our figures earlier if the tx signal is -30db and we are capable of hearing the noise floor at -110db then our diplexer and our receiver front
end isolation should equal at least 80db. In this case a cheap diplexer would suffice. Remember a diplexer also has insertion loss,  which means if it is
the first thing in your system you are raising your noise figure and decreasing your MDS (minimum discernable signal).

If ya got all that then you can see also see that turning your power down , ,seperating your antennas and making sure your coax dont leak helps a lot.
I hope this helps out.


Keith N6ORS
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