Practical_Antenna_Handbook_0071639586
454 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s a b Y Z 90 90 X TE M,N TM M,N Figure 20.8 Rectangular waveguide coordinate system. where x = E for transverse electric mode and M for transverse magnetic mode m = number of half-wavelengths along x axis (a dimension) n = number of half-wavelengths along y axis (b dimension) The TE 10 mode is called the dominant mode and is the best mode for low-attenuation propagation along the z axis. The nomenclature TE 10 indicates that there is one half-Â 0111057 FIG 18-08 wavelength in the a dimension and zero half-wavelengths in the b dimension. The dominant mode exists at the lowest frequency at which the waveguide is a half-wavelength. Velocity and Wavelength in Waveguides Figures 20.9A and 20.9B show the geometry for two wave components, simplified for the sake of illustration. There are three different wave velocities to consider with respect to waveguides:
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 455 Antenna a b c Figure 20.9A Antenna radiator in capped waveguide. • Free-space velocity c • Group velocity v g • Phase velocity v p 0111057 FIG 18-09A Of course, c is the velocity of propagation in unbounded free space, 3 × 10 8 m/s. The group velocity is the straight-line velocity of propagation of the wave down the centerline (z axis) of the waveguides. The value of V g is always less than c, because the actual path length taken, as the wave bounces back and forth, is longer than the straight- B / 4 a g/ 4 0 A a V g C Figure 20.9B Wave propagation in waveguide.
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454 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s<br />
a<br />
b<br />
Y<br />
Z<br />
90<br />
90<br />
X<br />
TE M,N<br />
TM M,N<br />
Figure 20.8 Rectangular waveguide coordinate system.<br />
where x = E for transverse electric mode and M for transverse magnetic mode<br />
m = number of half-wavelengths along x axis (a dimension)<br />
n = number of half-wavelengths along y axis (b dimension)<br />
The TE 10 mode is called the dominant mode and is the best mode for low-attenuation<br />
propagation along the z axis. The nomenclature TE 10 indicates that there is one half-Â<br />
0111057 FIG 18-08<br />
wavelength in the a dimension and zero half-wavelengths in the b dimension. The dominant<br />
mode exists at the lowest frequency at which the waveguide is a half-wavelength.<br />
Velocity and Wavelength in Waveguides<br />
Figures 20.9A and 20.9B show the geometry for two wave components, simplified for<br />
the sake of illustration. There are three different wave velocities to consider with respect<br />
to waveguides:
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