Coupling Coefficient K For a Resonator Filter

K = FindK(filename)

This program uses the SBOX
(S-parameter toolbox) by "Tudor Dima".
Specifically (SXPParse.m and Phrase2Word.m)

The toolbox is an EXCELLENT interface for manipulation of Scattering
Parameter files in Matlab.

This function will read in a Touchstone file (.snp) and calculate
a quantity K. The function finds the two highest peaks in |S21|.
The frequency of the first peak (lower frequency) is called F_low.
The frequency of the second peak (high frequency) is called F_high.

Then
K =
(F_high^2-F_low^2)/(F_high^2+F_low^2)

This formula is from the Daniel G. Swanson Jr. article entitled
"Narrow-Band Microwave Filter Design. The article was found in the October 2007 IEEE Microwave Magazine.
The formula is on page 108.

This program was written in response to a question posted by member
"junz" on the Sonnet Community Forum (www.sonnetlist.com).