The applet encompasses the latest published design rules for accurately designing microstrip broadside coupled-line, directional couplers in the frequency range 1 to 20 GHz.
The geometrical and electrical structures are shown in the figures below the main plot area. The plots show the frequency response of signal voltages appearing at the input, the through, the coupled, and the isolated coupler ports for the coupler parameters indicated by the data scrollbars.
The data values given beside the figures are:
1) The coupler center frequency set by the effective electrical length of the coupled region.
2) The matched coupling value, calculated from the odd and even mode impedances at the centre frequency.
3) The microstrip line effective characteristic impedance; to which the source and load impedances should be adjusted to achieve the matched condition.
Displayed curves are: red, input power; blue, through path (Z2); green, coupled port (Z4); magenta, isolated port (Z3).
The orange plot indicates the phase difference between the through and coupled ports, nominally 90 degrees.
In general, the scrollbar values can be adjusted to achieve the required design performance and minor adjustments used to assess the design tolerances of the various parameters.
Linear voltage or logarithmic dB plot scales are selected by clicking the radio buttons.

User Notes:
The design accuracy is dependent upon the choice of sensible parameters, for example, impractical, very thick substrates will cause the model to break down. Also, very narrow line spacings needed to obtain high coupling values may not be physically realizeable.
The isolation degradation with frequency is caused by substrate dispersion.
Note the effect of lossy material on the through/coupled port phase difference.