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 469
TE 10 Mode
Figure 20.16 Representative practical
rotating joint.
Choke
joint
TM 01
Mode
A
Rectangular
waveguide
Rotating
Stationary
TE 10 Mode
/ 4
0111057 FIG 18-16
C
B
D
Large diameter
probe
Low power
Large diameter
small diameter
High power
Broadband probes
wavelength in a fixed-frequency system.
The probe is analogous to the
vertical monopoles employed at
lower frequencies. A characteristic of
this type of radiator is that the E-field
is parallel to the waveguide top and
bottom surfaces. This arrangement
satisfies the first boundary condition
for the dominant TE 10 mode.
The radiator is placed at a point
that is a quarter-wavelength from the
rear wall (Fig. 20.17B). By traversing
the quarter-wave distance (90-degree
phase shift), reflecting off the rear
wall (180-degree phase shift), and then
retraversing the quarter-Âwavelength
distance (another 90-Âdegree phase
shift), the wave undergoes a total
phase shift of one complete cycle,
or 360 degrees. Thus, the reflected
wave arrives back at the radiator in
phase to reinforce the outgoing wave,
and none of the excitation energy is
lost.
Some waveguides have an adjustable
end cap (Fig. 20.17C) to accommodate
multiple frequencies. The end
cap position is varied as required for
signals of different wavelengths.
Figure 20.17D shows high- and
low-power broadband probes that are
typically not a quarter-wavelength
except at one particular frequency.
Broadbanding is accomplished by attention
to the diameter-to-length ratio.
The degree of coupling can be varied in
any of several ways: the length of the
probe can be varied, the position of the
probe in the E-field can be changed, or
shielding can be used to partially shade
the radiator element.
Figure 20.17 Probe (capacitive) coupling.