[CQ-Contest] Bandpass filtering

Pete Smith n4zr at contesting.com
Wed Dec 29 14:04:16 PST 2010

This feels a little like 1995, when CQ Contest was the only ham 
reflector out there and you had to make your message relevant to 
contesting.  Here goes...

I have a YCCC receiving bandpass filter which I want to modify to feed a 
QS1R broadband SDR, to monitor CONTEST activity on 5 of the bands at 
once, while I operate on the 6th.  The problem is that this box has all 
6 BPFs in *series*, bypassing the 5 that aren't in use on a given band, 
while I need to feed 5 and disable the 6th (the band I'm on).

 From what W3LPL has told me, I am pretty confident that I can simply 
parallel the output of the 6 filters, but can I also parallel the 
inputs?  I'm hoping that the stop-band impedance of all six filters will 
be high enough so that signals on one band will not "see" the BPFs for 
the other bands, so that paralleling the inputs won't be lossy.  Frank's 
traveling right now, or I'd ask him, but is this reasonable to assume?  
If it is, can I just use the relay on the BPF for my transmit band to 
open its output, so that in-band energy doesn't pass through to the 
receiver, or would it be best to ground the output through a 50-ohm 

73, Pete N4ZR

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