[R-390] Clevite Filters

[Tisha Hayes]

Clevite filters are an interesting footnote in the history of the R-390A. At
the time of their conception they were more expensive than the Collins
mechanical filter.

The downsides; The input/output impedances and capacitances were completely
different than the mechanical filters. Insertion losses were higher, all of
this resulted in circuit changes to interconnect to the Clevite filter very
differently than the mechanical filter. The shape factor was not as good as
the Collins mechanical filter so IMD was worse.

Because of the differences in electrical specifications the Clevite filter
required a different impedance matching network. As a result, you cannot mix
and match Clevite's with mechanical filters in the same radio.

The upside; The Clevite filter did not have as much phase noise as a
mechanical filter. This made it very useful for installations where
direction finding was important as you could get a much better bearing on
the transmitting site.

What that means now: The Clevite filter was a very early crystal filter.
Over time, ceramic filters became much better with better shape factors
approaching those of mechanical filters. Insertion losses decreased and the
ceramic filter is now available in more appropriate circuit impedances.

Today you can find multi-pole ceramic filters with better shape factors than
the Collins mechanical filter. Their ultimate rejection is not as good
because of crosstalk between the input and output connections. Notice how
the R-390A mechanical filters have a mounting flange between the top and the
bottom. This was to improve filter action by isolating the leads with a
ground plane (the chassis and the mounting flange).

If the R-390A was a new design today (with tubes), it would use 9 to 11 pole
ceramic filters with the electro/mechanical seperation between the inputs
and outputs for better isolation. For DF applications, more advanced
techniques like doppler shift , interferometery and advanced phase analysis
would make for a much better DF platform.

That is my opinion,

Ms. Tisha Hayes
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