Practical_Antenna_Handbook_0071639586

λ
o
= c / f
8
3×
10 m / s
=
9
C h a p t e r 2 0 : M i c r10 o w Hz a v e W a v e g u i d e s a n d A n t e n n a s
6.7 GHz ×
461
1GHz
×
= 3 10 8
m / s
9
6.7 × 10 Hz
= 0.0448 m
(b)
λ
g
=
=
λ
0
⎛ f ⎞
c
1– ⎜ ⎟
⎝ f ⎠
0.0448 m
⎛ 4.5 GHz ⎞
1– ⎜ ⎟
⎝6.7 GHz ⎠
= 0.0448 m
1– 0.67
= 0.0448
0.33
= 0.136 m
2
2
Transverse magnetic modes also propagate in waveguides, but the base TM 10 mode
is excluded by the boundary conditions. Thus, the TM 11 mode is the lowest magnetic
mode that will propagate.
Waveguide Impedance
All forms of transmission line, including the waveguide, exhibit a characteristic impedance,
although in the case of waveguide it is a little difficult to pin down conceptually.
The characteristic impedance of ordinary two-conductor transmission lines was developed
in Chap. 4. For a waveguide, the characteristic impedance is approximately equal
to the ratio of the electric and magnetic fields (E/H) and converges (as a function of
frequency) to the intrinsic impedance of the dielectric (Fig. 20.10). The impedance of the
waveguide is a function of waveguide characteristic impedance (Z 0 ) and the wavelength
in the waveguide:
Z Z 0lg =
(20.14)
l
Or, for rectangular waveguide, with constants taken into consideration:
0
π λ
Z = 120 g
(20.15)
λ
0