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

[k6xx at juno.com]

Several years ago, I bought a DS-600 BPF and had access to an (also brand
new) HP4396B network/spectrum analyzer. The resulting measurements are
posted to

http://www.k6xx.com/radio/dunestar/ds600.pdf

and

http://www.k6xx.com/radio/dunestar/ds600.html

(I recommend the PDF version for enhanced resolution).

Similar curves on the older DS-504 BPF is also available.

http://www.k6xx.com/radio/dunestar/ds504.html (and .pdf).

In actual use, I find both provide acceptable attenuation for SO2R at the
kW level. There is some blow-by, especially on the second harmonic, when
running a C4XL (40-10m, separate feed for 40m) on both 40m and 20m. Of
course, the elements are interleaved in this situation, so it's a fairly
difficult test. Given reasonable separation, the filtering is excellent.

I also have an ICE419A BPF, but no longer have access to the network
analyzer. Performance is similar to the DuneStars--given reasonable
antenna separation, there is no interstation interference.

Someone commented that one need only use the filters on receive, not
transmit. This is NOT true with many (most?) solid-state transceivers.
Their output low-pass filtering passes lower frequency broadband hash
(phase noise, etc.) right to the antenna. Sure, its low-level, but this
low level is still strong QRM to a co-located receiver. You must use the
filters on the transmitter output to remove  the hash generated in the
transmitter--a receive-only filter cannot discriminate between in-band
hash and in-band signals. This condition affects the situation where a
higher-band unfiltered transmitter is run with a lower-band filtered
receiver, but not the opposite.

73 de Bob, K6XX