The stock FT-817 is a MuRata CFK455K ceramic filter
having a bandwidth of about 2.7 kHz. This filter is used for both CW
& SSB unless an optional filter is installed. Without the
optional filter, CW reception is generally considered difficult when
several stations are nearby on the band. The optional CW filter is a
Collins Mechanical filter and is World renouned for its outstanding
performance and the improvement in CW reception is dramatic. The W4RT
One-Plug Filter contains a 7-pole Collins Mechanical Filter
essentially identical to what Yaesu uses in their optional YF-122C
500-Hz or YF-122CN 300-Hz CW filters. Filter termination has been
optimized for the FT-817. Connectors like those on the YF122C allow
you to install and remove this filter just as you do with the Yaesu
filter. Filter unit contains the coding for the FT-857 or FT-897 to
recognize it as either a 300 or 500-Hz CW filter.
A question frequently asked is why isn't there a crystal filter
option. The answer has two parts. The first it that crystal filters
are physically larger than mechanical filters and are too large to
fit into the small space available in the FT-817. The second part has
to do with their relative performance. Mechanical filters have lower
attenuation in the passband, less ripple, and (as noted above)
smaller size for a given performance. Mechanical filters are
complicated to manufacture and take 12-16 weeks complete! The
operational performance of these filters have been highly respected
for almost 50 years! They are very rugged compared to crystal
filters. Crystal filters typically have a somewhat better shape
factor (more selectivity), but suffer from greater in-band ripple.
Nevertheless, both the mechanical and crystal filters provide
outstanding performance. The ceramic filters are marginal by comparision.
Click
here to view a reprint of an article that appeared in QRP
Quarterly
reagrding both the CW and SSB filters.