[CQ-Contest] Bandpass filters for M/S or M/M environment

[W0UN -- John Brosnahan]

I always use coaxial stub filters on the outputs of the amplifiers
in addition to the bandpass filters between the radio and the amp.
The coaxial stubs can provide an additional 30-40 dB attenuation
to the "other" band signals.  I guess it is POSSIBLE to couple
100 watts into a very close antenna (stack) but normally any
feedpoint matching network (gamma, hairpin, etc) would make
the input highly reactive on the "other" bands.  But if you assume
that you CAN couple 100 watts into another, nearby antenna, the
coaxial stubs will reduce this to a negligible level.  Well within
what a 200 watt filter can handle out-of-band.  Even within
the ratings of a receive-only filter.

You REALLY want coaxial stubs on the amps to reduce their
harmonic content.  The benefit is that they can greatly reduce
other (incoming) signals as well--although this will often require
combinations of shorted and OPEN stubs (to reduce the signals
from LOWER bands).

--John  W0UN


>My challenge is what I expected to see someone would challenge my
>earlier message:
>
>
>The filters can take 200W with good SWR or 100W with poorer SWR
>on the fundamental pass-frequency.
>
>The power handling capability is poorer at the disturbing signal.
>This is 40m filter can take 100W easily on 40 but much less if
>you feed 20m, 80m or any other band signal into it.
>
>You can blow a filter very easily if you transmit into it
>-directly on wrong band
>-indirectly via poor isolation somewhere, like between antennas
>
>
>Power levels to cause for this filter breaking are in order of a
>couple of tens of watts at most.
>I have not seen any specification on the subject, but would like
>to see some test results if anyone has buts to start to break
>the units.
>
>Measuring power is difficult as the impedance is not even close to
>match with 50 ohms.
>Maybe voltage would be the right measurable.
>
>
>To summarize:
>the decision if a filter is good or bad or 'good enough', should
>also depend on the stop band power handling capability.
>
>
>For the great majority of us, the performance of
>post amplifier stubs + ICE BPFs should be enough.
>
>
>73,
>Jukka OH6LI
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>
>
>
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