The speed of microwaves within a measuring tube is apparently slower when compared to the velocity in
free space. The degree to which the running time slows down depends on the diameter of the tube and the
wavelength of the signal.
There are different modes of propagation of microwaves within a wave guide. However, an important
value is the minimum diameter of pipe (d ) that will allow microwave propagation. c
The value of the minimum or critical diameter (d ) depends upon the wavelength (ë) of the microwaves. c
The higher the frequency of the microwaves, the smaller the minimum diameter of measuring tube that
can be used.