Waveguide helps in the propagation of high frequency Electromagnetic waves like Microwaves, Radio waves, and Infrared Rays. George C Southworth of the AT&T Company demonstrated the first Waveguide in the early nineteenth century. This post provides a detailed description about Waveguide, different Waveguide modes, its classification, working principle, applications, advantages and disadvantages.
Who Invented Waveguide
The Waveguide was invented by George C Southworth. It played an important role in Radar Systems during World War II. To honor his work in Wave-guide, he was bestowed with the Morris N. Liebmann award of the IRE in 1938 and the Stuart Ballantine Medal of the Franklin Institute, in 1947. He also received the Louis Levy Medal of the Franklin Institute for his work on Microwave Radiation from the sun. His contributions to the field of Radio Physics were commendable.
Fig. 1 – Dr. George. C. Southworth and First Waveguide
What is Waveguide
Waveguides are used to direct and propagate Electromagnetic waves from one point to another. They are generally used to transmit high frequency waves such as Microwaves, Radio waves, Infrared waves etc. For low frequency waves which are less than 1 MHz, parallel transmission lines or co-axial cables are used.
Wave-guide is represented by its dispersion characteristics that has a certain cut-off frequency. The signals having frequencies above this cut-off frequency are allowed to propagate through the Wave-guide and the signals having frequencies below this frequency will face a high reflection. A Waveguide acts like a high pass filter due to this characteristics. The dispersion characteristics can be altered by loading the Wave-guide with metal or di-electric medium.
The most common type of Waveguide is a hollow conductive metal pipe which carries high frequency Radio Waves. They also exist in the form of wires, coaxial cables, parallel plates, or optical fibers.
Fig. 2 – An image of Waveguide
Classification of Waveguide
Waveguides are classified into two types:
- Metal Waveguides
- Dielectric Waveguides
Metal Waveguides consists of an enclosed conducting metal pipe and the wave guiding principle works on the total internal reflection from the conducting walls. They are of two types:
- Rectangular Waveguides
- Circular Waveguides
Dielectric Waveguides consists of dielectrics and the reflection from dielectric interfaces helps in the propagation of electromagnetic waves along the Waveguide. They are of two types:
- Dielectric Slab Waveguides
- Optical Fiber
Fig. 4 –Dielectric Waveguides
The two types of Wave-guide Modes that is necessary for propagation of Electromagnetic waves in the Waveguides are:
- TE (Transverse Electric) Mode
- TM (Transverse Magnetic) Mode
TE (Transverse Electric) Mode
In TE (Transverse Electric) Mode, Electric Field (E) vector is transverse or perpendicular to the Waveguide’s axis.
TM (Transverse Magnetic) Mode
In TM (Transverse Magnetic) Mode, Magnetic Field (E) vector is transverse or perpendicular to the Waveguide’s axis.
TE and TM Modes in Ideal Waveguide must satisfy Maxwell’s equations and are based on certain assumptions:
- The Wave-guide is considered to be infinitely long, oriented along the z-axis, and uniform along its length.
- The Wave-guide is constructed from a perfectly conducting pipe (PEC) and is filled with a perfect insulator (lossless dielectric).
- The Electric and Magnetic fields are time-harmonic
Fig. 5 – TE and TM Modes for Wave Propagation
How does Waveguide Work
Waveguide is defined as a geometrical structure which propagates electromagnetic energy in a preferred direction in space from one point to another within a certain frequency range. Waveguide’s operating bandwidth is an important factor as the lower operating frequency is determined by the electrical property of the Wave-guide structure. They do not operate under transverse electromagnetic modes (TEM) as they are built with single conductor.
The fundamental Waveguide’s Mode is the mode with the lowest cut-off frequency. The propagation of a wave in a Wave-guide (TE or TM waves) has very different characteristics than the propagation of a wave on a transmission line (TEM waves).This is because when a wave is transmitted at one end of the Wave-guide, it gets reflected from the sides of the Wave-guide.
These reflected waves interact with each other and an infinite number of discrete characteristic patterns called modes are generated. These modes entirely depends on the size and shape of the Waveguides, medium in the Waveguides and the operating frequency.
For propagating a wave through Waveguides for a specific mode, the source should operate at a frequency higher than the cut-off frequency. If a Waveguide’s source is made to operate at a frequency less than the cut-off frequency of its mode, then the wave is attenuated.
To have a clear picture, let us consider a Rectangular Waveguide which consists of a hollow pipe of infinite length and rectangular cross section of dimension a×b . The propagation of the wave is along the z direction i.e;
All the field quantities can be expressed in terms of the derivatives of and and
The propagation of the wave which is along the z direction undergoes a phase shift of 2π radians. This is nothing but the Phase Velocity of a Sine wave which undergoes modulation. This modulated wave travels through the Waveguides at a certain velocity called as Group Velocity. Generally, the velocity of modulated signal is slower than the carrier signal.
The Group Velocity of the wave which is less than the speed of light is represented by the equation:
The Phase Velocity is obtained using the equation:
By applying boundary conditions in Helmholtz equation, we obtain Ez and Hz
Fig. 6 – Cross- Section of Rectangular Waveguide
Applications of Waveguide
The applications of Waveguides are:
- Waveguides are used in Optical fiber communication.
- They are used Photonic integrated circuits.
- They are extensively used in Microwave ovens.
- Waveguides are used for broadcasting and radar installations.
- They are used in space crafts.
Advantages of Waveguide
The Advantages of Waveguides are:
- High power handling capability.
- Low insertion loss.
- They provide wide bandwidth.
- Due to the dispersion characteristics, they are used in designing Waveguide filters.
Disadvantages of Waveguide
The Disadvantages of Waveguides are:
- Waveguides are difficult to install and special couplings are required.
- Physical size becomes a limitation.
- Cost is high.
- Weak thermal stability.
- 3-D Waveguides implementation is difficult.