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Corrections to Webpage article, hopefully my final comment



Several people, including Emily, have emailed me stating they see no errors 
in the Mode J filter article. So...I've oulined some of the worst here.

First off, the device is properly called a diplexer, not a duplexer. Not a 
big deal as even the manufacturers get it wrong sometimes. Duplexers are 
used for in band signals like on a repeater, diplexers are used between 
bands.

Harmonics are not really even the issue. The issue is the overload of the 
receiver or preamp by the 2m signal. If it was the harmonic that was the 
problem, the diplexer acting as a high pass filter passing 70cm would do 
nothing to attenuate the problem signal, right? If the problem WAS the 3rd 
harmonic, putting the diplexer as a low pass filter on the 2m transmitter 
may or may not help.

Another error is the statement that the 3rd harmonic is only 9 db down from 
the fundamental. Quote follows:
"Since we are working with the 3rd harmonic (down 9db from the fundamental), 
radiating 100 watts (probably 50W at the antenna but still assuming 100 
watts for the sake of the article) would be the equivalent of radiating 12.5 
watts minus the free space loss between the antennas and the inefficiency of 
the 70cm antenna for receiving 2m." End quote.
 Your 3rd harmonic should never only be 9 db down. FCC rules part 97.307 
requires that all harmonics and spurs be 60db down from the fundamental. But 
what's 51db among friends? The math is pointless from here out. One more 
example though.
Quote:  "What about transmitting on 70cm? Doesn't this still cause a 
problem?  It shouldn't. The 60db isolation of the duplexer should limit the 
amount of power into the load to approximately 143mw."
This is horribly wrong. To get 143mw on the 2m port with 60db of isolation, 
you'd have to be feeding 150,000 watts into the 70cm side. Punch this into 
any log calculator to see for yourself. I don't run that much power, do you?

So, there are several of the errors in that article in a nutshell. Fix it or 
leave it as a big dunce cap on the website, at this point I really don't 
give a damn anymore.

73, Drew KO4MA 
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