The applet demonstrates the operation of lossy and lossless coupled-line directional couplers, with a variety of mid-band coupling values, under a range of conditions of port match.
Air dielectric is assumed together with zero dispersion and identical propagation factors for both TEM modes.
The display offers the options of linear voltage plots or logarithmic (dB) power plots.
Displayed curves are: red, input power; blue, through path (Z2); green, coupled port (Z4); magenta, isolated port (Z3); gray, input impedance-to-line impedance ratio; orange, through-to-coupled path phase difference.
The scrollbars allow independent adjustment of port source and load impedances, mid-band coupling and equivalent medium loss in dB/cm.
Data readout values follow the mouse x-axis position and design data is saved to the data console.

User Notes:
The operating bandwidth is defined by the acceptable power and phase deviation from ideal. Practical devices are sometimes slightly over-coupled to extend the useful bandwidth to around an octave for example.
Mismatching the through and coupled ports diverts power to the isolated port. Equal impedance mismatched loads are preferred.
Pairs of 3-dB couplers are often used in power amplifier combining and it is interesting to model the effect of minor match unbalance in this situation.
Note the effect of lossy material on the through/coupled port phase difference.