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Re: Another interesting preamp story



At 05:22 PM 3/29/2006 -0800, wa6fwf@amsat.org wrote:

>Hi All,
>
>Snip!
> > 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
>Snip!
>
>      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.

Hi Kevin,

You are correct.

The device is actually a diplexor (ANSI: a device that separates
signals by frequency) not a duplexor (ANSI: a device to allow a radar
receiver to share an antenna with a radar transmitter) and the
problem that it solves is not third harmonic
but fundamental overload. Somewhere in the really distant
past, someone incorrectly used the term duplexor to mean
a diplexor (probably in QST! but I do not remember) but the
term stuck. Even the manufacturers use the term now and that
is what everyone calls it. Some battles are not worth fighting :)

And I personally recall (but only faintly) that some ARRL
publication from ancient times said that the problem was
the 3rd harmonic but this was also wrong. Unfortunately,
old myths die hard!

For the right description, see the AMSAT web page:
http://www.amsat.org/amsat-new/articles/Mode-J

73,
Tony AA2TX
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