Practical_Antenna_Handbook_0071639586

C h a p t e r 2 0 : M i c r o w a v e W a v e g u i d e s a n d A n t e n n a s 453
A. High frequency
B. Medium frequency
C. Low frequency
(approaching cutoff)
Figure 20.7 Frequency effect on propagating wave.
= of incidence
= of reflection
Note that the angle of incidence with the waveguide wall increases as frequency drops.
The angle rises toward 90 degrees as the cutoff frequency is approached from above.
Below the cutoff frequency the angle is 90 degrees, so the wave bounces back and forth
between the walls without propagating.
Coordinate System and Dominant Mode in Waveguides
Figure 20.8 shows the coordinate system used to denote dimensions and directions in
microwave discussions. The a and b dimensions of the waveguide correspond to the x
and y axes of a cartesian coordinate system, and the z axis (into or out of the page) is the
direction of wave propagation.
0111057 FIG 18-07
Shorthand notation for describing the various modes of propagation is as follows:
Tx mn