Support's Directional Couplers

Support for Directional Couplers.
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This release 4.0.0 is the first release of atlc that supports directional
couplers. Although at present the software only reports on the
odd and even mode impedance Zodd and Zeven, it is possible to calculate
the properties of a coupler with such mode impedances quite easily.

The coupler support is not complete, but it is there for vacuum
dielectrics and calculations against an exact model for thin stripline
couplers indicates an rms error of around 0.6 %.

Some software will be written later to produce plots of coupling vs.
impedances, but for now you will have to do it on external software such
as Spice, Puff or similar. See the documentation on couplers for more
information.

Dr. David Kirkby, 11/4/02.