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

Bob Wolbert k6xx at juno.com
Fri Nov 9 08:14:03 EST 2001


The transmitted phase noise will hurt you. Best results demand BPFs on
both rigs in a SO2R or multi- setup.

Using a '930, for example, transmitting on 15m and listening with another
rig through the filters on 20m, you will be inundated with the
transmitted phase noise. The higher frequency transmitter generates
broadband noise that passes freely through its output LPF. 

The opposite case, where you transmit on the lower band and receive on
the higher, is significantly better--but still not as good as when tight
filters or stubs are used on both rigs.

Significantly, there is little or no worry that equipment will be
damaged, if that is your main concern. But your lower frequency receiver
will not be very effective.

My (poor) results were with a TS-930. Perhaps other rigs, with better
transmitted phase noise characteristics, will provide much better
results. However, it doesn't take much energy to foul up a receiver.

73 de Bob, K6XX
k6xx at arrl.net

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? 
> 73, Pete N4ZR
> www.qsl.net/n4zr
> --
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73 de Bob, K6XX

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