Probe and loop coupling mechanism:
The field intensities of the desired mode in a waveguide can be established by means of a probe or loop. coupling device.
The probe may be called a monopole antenna, the coupling loop is the loop antenna.
This is the point at which the E field is maximum in the dominant mode. Therefore, energy transfer (coupling) is maximum at this point.
Note that the quarter-wavelength spacing is at the the frequency required to propagate the dominant mode.
In many applications a lesser degree of energy transfer, called loose coupling, is desirable.
The amount of energy, the transfer can be reduced by decreasing the length of the probe, moving it out of the center of the E field, or by shielding it.
Where the degree of coupling must be varied frequently, the probe is made retractable so the length can be easily changed.