Waveguides (single conductor lines)
- At frequecies higher than 3 GHz, there are more losses in Transmission lines.
- A hollow metallic tube used to transmit EM waves by successive refelections from inner walls of tube is called waveguide.
Difference with transmission line:
- In waveguide, only wa ves having frequencies grea ter than cut off frequency fc will be propagated that is why waveguide act as a HPF.
- Waveguide is one conductor transmission line.
- TM11 is dominant mode for TM waves.
- TE10 is dominant mode for TE wave and it is also overall dominant mode (when a >b).
In this TM10 or TM01 does not exist.
Cut of frequency :
Group velocity :
In waveguide: vp > c>vg
i.e. vpvg =c2
In vacuum : vp = c = vg Intrinsic wave impedance:
For TE Mode:
For TM mode:
Guided wavelength (lg):
Where lo = free space wavelength lc = Cutoff wavelength lg > lc >l0
Power handling capacity of rectangular waveguide
For standard rectangular waveguide : a = 2b
- Dominat mode: it is the mode with lowest cutoff frequency ((longest cutoff wavelength).
Circular /Cylindrical waveguide:
- TM01 is dominant mode for TM waves.
- TE11 is dominant node for TE waves and it is overall dominant mode.
- In circular waveguide second suffix of TE or TM modes cannot be zero (as TMmo or TEmo can not exist)
Cut off wavelength: For
r= ra diu s o f c ircula r waveguide
Power handling capacity of circular waveguide:
d= Intenal diameter
- When both the ends of the waveguide are terminated, signal bounces back and forth between two shorting plates.
- This results in resonance in cavity that is why it is called cavity resonator.
- Cacity resonators act as BPF and it is used for energy storage.
- TE101 is the dominant mode of cavity resonator.
- TM110 is lowest order of cavity resonator.
Quality factor of cavity resonator
W ® Maximum stored energy
P ® average power loss Integer m ® no. of half cycle variations in X direction.
Integer n ® no. of half cycle variations in Y direction.