This routine designs a high-pass diplexer filter, that has a constant
input impedance of Zo at all frequencies, both in the pass-band and in
the stop-band. S11 is always good. S22 is poor in the stop-band.
When used at a mixer termination, make the cutoff frequency sqrt(IF*Image)
----------------C----------------------------------
                   |                          |
                   |                          |
                   |                          L
1 ->               |                          |        <- 2
                   -----------                |
                   |         |                |
                   R         C                |
                   |         |                |
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