[CQ-Contest] Paralleling band pass filters

[Tree]

N4ZR asks a contest question:

> 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 
> resistor?

I might have to have some coffee to totally understand this - but here 
are some results that might be applicable.  

If you are going to connect two filters in parallel (either inputs or
outputs) you need to understand what the impedance is for the off band
filter for the band of the other....

This is easier to think about if you identify two bands.

I have beverages that I use on 40 and 80.  I put the two filters in 
parallel coming off of my beverage box.  HOWEVER, the 40 meter filter
looks like a short to ground on 80 meters and visa versa.  Because of
this, I get no signal on either band.

I worked around this by putting a 1/4 wave coax stub on the input of
each filter.  This made it so the short to ground now looked like a 
high impedance on the opposite band.

However, this quickly gets rather complicated if you have more than 
two BPFs connected.

Tree N6TR