You are of course correct, but most bandpass filter designs are just that...a
high impedance everywhere but in the passband and they will work fine in
parallel within reason. I'm not sure about tying all the inputs and most of
the outputs together as he's proposing. Filters that use stop-band or
band-reject/notch sections are inappropriate for this application.
I think for Pete's application it might be better to use a broadband splitter
(losing 3.5 db in both legs) or active high-impedance preamp tap (lose almost
nothing but risk preamp overload issues going to the QS1R) and then use
switched band-reject filters after that if you like.
What I do with my QS1R when I'm using the station is use preamp taps in the RX
ant loop for each radio and combine using a passive broadband combiner (I use
75 ohm cable TV stuff). I'm using some little single-board high-impedance
input preamp kits from Clifton Labs that I crammed into some ICE RX limiter
boxes, but if I did it again, I'd just add them outboard on a T connector. It
seems to work fairly well.
When I'm not using the station, I just combine my tribander stack output thru
an ARR preamp and K9AY beverage box output directly thru a CATV
combiner/splitter.
73, Ty K3MM
Dec 30, 2010 11:51:13 AM, tree@kkn.net wrote:
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
_______________________________________________
CQ-Contest mailing list
CQ-Contest@contesting.com
http://lists.contesting.com/mailman/listinfo/cq-contest
_______________________________________________
CQ-Contest mailing list
CQ-Contest@contesting.com
http://lists.contesting.com/mailman/listinfo/cq-contest
|