waveguide vs transmission line

Waveguide vs transmission line

We now that we use trasmission line for propogation of electromagenic waves,in transmission line we use circuit models, but in waveguide for propogation  electromagnetic waves.So we here compare and discuss about them.

So i want to share a link that describe some difference between waveguide and transmission line:

For frequencies higher than 3 GHZ, the transmission of electromagnetic waves along transmission lines and cables is not possible because of high loss, here a metallic tube can be used to transmit electromagnetic waves.

A hollow metallic tube of uniform cross section which transmits electromagnetic waves by successive reflections from the inner walls of the tube is called waveguide.

Similarities between two wire transmission line and waveguide :-

1. As the wave propagates, they gets attenuated in both.

2. Irregularities or mismatches in the transmission line and waveguide causes reflection and thus standing waves.

3. Reflected wave can be eliminated by proper impedance match in both.

Disimilarities :-

1. Depending upon the shape and dimension of the waveguide, there is a cut off frequency fc, frequencies f > fc can only pass through it. Thus it acts as a high pass filter with cut off frequency fc.

2. Waveguide is a one conductor system, the whole body of the waveguide acts as the ground and the wave propagates through multiple reflections from the walls of the waveguide.

3. Transmission through waveguide is governed by field theory while in transmission line by circuit theory.

4. Velocity of propagation of wave inside a wave guide is different from that through free space due to reflections.

In below image you can see some  waveguides from this link :

Fig1: Electric field Ex component of the TE31 mode inside an x-band hollow metal waveguide.

Fig2: Example of waveguides and a diplexer in an air traffic control radar

Fig3:One of the most common types of transmission line, coaxial cable.

In this below table is good comparision,from this link.

Table1:Comparision waveguide and transmission line

Transmission Lines Waveguides
Supports TEM wave Cannot support TEM wave
All frequencies can pass through Only the frequencies that are greater than cut-off frequency can pass through
One conductor transmission Two conductor transmission
Reflections are less Wave travels through reflections from the walls of waveguide
It has characteristic impedance It has wave impedance
Propagation of waves is according to “Circuit theory” Propagation of waves is according to “Field theory”
It has a return conductor to earth Return conductor is not required as the body of the waveguide acts as earth
Bandwidth is not limited Bandwidth is limited
Waves do not disperse Waves get dispersed

 

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