[amsat-bb] Plagued with Mode J "noise" on TX

[Scott Townley <nx7u@xxxxxxxx>]

I know this is an old subject, but I can't seem to shake it...
I'm setting up a Mode JA mobile station, and when I TX on 2m (160W
RFConcepts brick to a 5/8wl whip) the "crud" on 435MHz RX (FT-100 using
dual-band 2m/440 whip) is horrendous.
Had the same problem in the "home" station using yagis, but never chased it
too seriously because I knew I'd be moving Mode J out to the truck.

I can do 145x3=435 so, since I don't have a 2m BPF on hand, I used (at
someone's suggestion) a 2m/440 duplexer (Comet CF-4160).  To be sure I
measured the LP/HP response of the duplexer on my network analyzer (doesn't
everyone have one?), and it was:
	VHF port (LPF)	-3dB thru loss at 215 MHz
				-49dB thru loss at 435 MHz
	UHF port (HPF)	-3dB thru loss at 264 MHz
				>-60dB thru loss at 145 MHz

Not a bad device!  So any transmitted third harmonic from my uplink should
be 60dB rejected, plus path loss and antenna mismatch.  But it STILL cruds
up the receiver so bad I can't hear my downlink!

What am I missing?  TX is known good (both on RS-12 and on service
monitor).  Duplexer was placed right at radio and unused UHF port was
terminated.  

Thanks!!
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Scott Townley		
nx7u@arrl.net
Gilbert, AZ  DM43
http://www.primenet.com/~nx7u     <-- Now containing details of my HF
Mobile Installation
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Collector of:
	Stoddart Aircraft EMI/RFI receivers and accessories
	Big Parts for that Big Linear Amp (someday I'll build one...right!)
	70's era RF test equipment HP/GR/Tek
	Radio-related technical reference material 1940+
------------
"When the Going gets Weird, the Weird turn Pro"
Raoul Duke, Sportswriter
	
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