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Re: Another interesting preamp story

Hi All,

> As you suggested the use of a Duplexer as a filter helps to suppress the third
> harmonic of VHF getting into the passband of the UHF preamplifier

     I am not singling anyone out on this statement, it has been made by many but
I firmly believe that it is wrong,I think as a technical group we should strive to be
correct on technical matters,  If  I'm wrong and the above statement is actually
correct then I would like for someone to please explain why it is correct.

   Here is why I think this statement is wrong....

take  a diplexer/duplexer ( I will get to this later)  a comet cf-416 for example, its
  LPF range is  1.3-170 MHz that it will pass, its HPF range is 350-540 MHz that it will pass.

the 3rd harmonic of 144 MHz is 432 MHz the 3rd harmonic of 145 MHz is 435 MHz

as you can see the 3rd harmonic of either 2m signal falls inside of what the diplexer will
pass through on the high side filter, it can not tell the difference between the signal you want
and the signal you do not want.

but right now your saying "But It Works!!!!"  I have seen it work!!! ,  I put to you that
you are not seeing the 3rd harmonic but fundamental overload, the  huge  2m signal is
getting past the filtering if any on your preamp and is saturating the front end with RF,
when you install the diplexer it does do a excellent job of attenuating the 2m signal while
allowing the 70cm signal to pass.

Also as someone else pointed out to me, it may solve a harmonic problem where
the 2nd harmonic of the 2m signal is mixing with something else to create a 70cm signal,
in this case since the 2nd harmonic is below the passband on the HPF side of the diplexer
 it will get filtered out and will never get in to do any intermod mixing .

So a diplexer may help  with  several different types of problems  but never with a 3rd harmonic

 Again if I'm missing something here Please tell me...

and last but not least its a diplexer not a duplexer, I know everyone has been using duplexer
including manufactures'  but its wrong, if were going to get sloppy on terms then I move that
we start calling it a resistor since it resists desense... :-)

some links to back up my statement...


Kevin WA6FWF
Amsat-UK #6505
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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