[CQ-Contest] band-pass filtering -- a new approach?

David O Hachadorian k6ll at juno.com
Fri Nov 9 09:31:26 EST 2001

I think the wideband noise issue is a show-stopper. You can test it
by putting one of the radios into transmit mode, without actually
keying down. You will hear the wideband noise on the other radio,
and can decide if it's acceptable. It will sound like white FM
interchannel hiss.

You should be able to reduce the transmitted noise to an acceptable
level by putting switchable stubs on the tx. Here's an interesting
little trick. A 23' RG-213 stub has the following pass/reject

shorted - pass 40, 15; reject 20, 10
open - pass 20, 10; reject 40, 15
You can use a relay or switch at the end of the stub to short/unshort
the stub. The characteristics of the 23' stub allow you to operate
on any two adjacent bands at once, which is usually what is desired.

You can make two of these stubs and use one on the rx rig as well.
Add another one for 80/160.

You may not even need the bpf's if the antennas are far enough

Dave Hachadorian, K6LL
Yuma, AZ
K6LL at juno.com

On Fri, 09 Nov 2001 10:12:12 -0500 Pete Smith <n4zr at contesting.com>
> I've been researching band-pass filtering for SO2R, and it has 
> occurred to
> me to try a different approach if the peer review sounds favorable.
> What I'm planning is to build a box for each of my transceivers, 
> containing
> a receiver protection circuit (diodes and a resistor), receive-level
> bandpass filters and a relay switching matrix to select the filters 
> bypass them on transmit.  This would seem to offer several 
> advantages --
> I can use W3LPL's 3-pole receive filter circuit, which uses standard 
> cap
> values and delivers performance that is probably superior to any 
> commercial
> alternative except for W3NQN's expensive transmit filters.
> I avoid issues with burning out the filter components from transmit 
> power,
> particularly with a messy amplifier input SWR on some bands.
> I avoid the reduction in drive to my SB-220, which really needs its 
> 120 watts.
> I can be fairly relaxed about the physical layout, since SWR is not 
> the
> issue it is on the transmit side.
> I realize that having a bandpass filter on the transmit side offers
> advantages too, in particular reducing harmonic output and wideband 
> noise.
> But on balance, this seems like a good idea, plus it'll give me a 
> project
> for the winter.  Comments? 

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