Here passive device means a device whose characteristics can not be changed after the device is fabricated. The passive devices are given in the following. In all cases denotes the length of the device.
Directional coupler
The directional couples is a device which couples a small fraction of power from the main waveguide. This device can also be used for tapping of power from the main waveguide.
The power tapping can be realized in two ways:
(1)
Making use of dissimilar waveguides . Taking the length and , the coupled field is approximately, . In terms of dB the coupled power is
(2)
Identical Waveguides : In this case by changing the length of the device the coupled output can be changed. The coupled power in dB is
denotes the length of the device. 
and 
, the coupled field is approximately, 
. In terms of dB the coupled power is 
