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Re: Another interesting preamp story



Yes, the diplexer does pass 1.3-170 MHz and 350-540 MHz but to separate 
ports.  The two-meter rig is connected to the 1.3-170 MHz filter and hence 
does not see or pass 432 energy but receives and transmitts the 144 MHz 
energy with little attenuation.  Hence, the 144 MHz third harmonic is 
blocked at the source.  Similarly, the 432 rig is connected to the 350-540 
MHz port and is not subjected to the 144 MHz transmitter output.  Remember 
the 144 MHz third harmonic was blocked by the 1.3-170 MHz band pass filter.

John  WA4WDL


----- Original Message ----- 
From: "wa6fwf@amsat.org" <kevinschuchmann@sbcglobal.net>
To: "AMSAT-BB" <amsat-bb@amsat.org>
Sent: Wednesday, March 29, 2006 8:22 PM
Subject: Re: [amsat-bb] Another interesting preamp story


> 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.
>
>   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
> problem.
>
> 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...
>
> http://www.its.bldrdoc.gov/fs-1037/
> http://www.microwaves101.com/encyclopedia/diplexers.cfm
> http://www.datacomm.ch/hb9abx/duplexer.html
> http://en.wikipedia.org/wiki/Duplexer
> http://en.wikipedia.org/wiki/Diplexer
>
> 73
> Kevin WA6FWF
> Amsat-UK #6505
> ----
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----
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