Why use waveguides

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. 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:. Dielectric Waveguides consists of dielectrics and the reflection from dielectric interfaces helps in the propagation of electromagnetic waves along the Waveguide. The two types of Wave-guide Modes that is necessary for propagation of Electromagnetic waves in the Waveguides are:. 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.

They do not operate under transverse electromagnetic modes TEM as they are built with single conductor. 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. There are also waveguides which do the same for audio energy, such as the Bose Wave Radio.

PC board microstrip and stripline structures are also EM-energy waveguides. For EM energy, there are two types of waveguides: the coaxial cable and the older, classic waveguide which is still in use. A: Yes, it has a solid center conductor surrounded fully by an enveloping shield. Together, they function to support the propagation of RF energy while confining it to the desired path.

But when RF engineers use the term waveguides, they generally do not mean coaxial cable, as a coaxial cable is more like a transmission line.

A: The classic waveguide is a metal tube, usually with a rectangular cross-section, which can range in length from a few centimeters to many meters Figure 1. While waveguides are usually conductive metal enclosures, it is possible to build waveguides using dielectric surfaces to confine the RF wave energy, but they are rarely used now for various reasons.

They often did the basic sheet-metal work of cutting and soldering the needed waveguides, as standard ones did not exist or would take time to order, fabricate, and deliver! Q: Why do you even need a waveguide when you have convenient coaxial cable which can carry RF?

However, they have increasing power losses as frequency increases. If the pipe is straight, we can see through it! So certainly electromagnetic waves go through a pipe. But we also know that it is not possible to transmit low-frequency waves power or telephone through the inside of a single metal pipe. So it must be that electromagnetic waves will go through if their wavelength is short enough. Therefore we want to discuss the limiting case of the longest wavelength or the lowest frequency that can get through a pipe of a given size.

Since the pipe is then being used to carry waves, it is called a waveguide. We will begin with a rectangular pipe, because it is the simplest case to analyze. We will first give a mathematical treatment and come back later to look at the problem in a much more elementary way.

The more elementary approach, however, can be applied easily only to a rectangular guide. The basic phenomena are the same for a general guide of arbitrary shape, so the mathematical argument is fundamentally more sound.

Our problem, then, is to find what kind of waves can exist inside a rectangular pipe. Perhaps it is the Bessel function we found for a cavity? No, because the Bessel function has to do with cylindrical geometries. First, the electric field should have no tangential components at the conductors. Our field satisfies this requirement; it is perpendicular to the top and bottom faces and is zero at the two side faces.

Equation Besides the electric fields there are magnetic fields that will travel with the wave, but we will not bother to work out an expression for them right now. In solving Eq. Naturally, it should be possible for waves to go in either direction.

Since both types of waves can be present at the same time, there will be the possibility of standing-wave solutions. But now notice that if we go toward low frequencies, something strange happens. Or do we? What if it does come out imaginary? Our field equations are still satisfied.

Looking at Eq. The fields penetrate very little distance from the source. For waves , however, an imaginary wave number does mean something. The wave equation is still satisfied; it only means that the solution gives exponentially decreasing fields instead of propagating waves. The wave velocity we have used above is the phase velocity, which is the speed of a node of the wave; it is a function of frequency. If we combine Eqs. We have already seen in Chapter 48 of Vol.

I that phase velocities greater than light are possible, because it is just the nodes of the wave which are moving and not energy or information. In order to know how fast signals will travel, we have to calculate the speed of pulses or modulations made by the interference of a wave of one frequency with one or more waves of slightly different frequencies see Chapter 48 , Vol.

The group velocity of the waves is also the speed at which energy is transported along the guide. If we want to find the energy flow down the guide, we can get it from the energy density times the group velocity. There is also some energy associated with the magnetic field.

In Fig. The driving stub can be connected to a signal generator via a coaxial cable, and the pickup probe can be connected by a similar cable to a detector. It is usually convenient to insert the pickup probe via a long thin slot in the guide, as shown in Fig. Then the probe can be moved back and forth along the guide to sample the fields at various positions. These will be the only waves present if the guide is infinitely long, which can effectively be arranged by terminating the guide with a carefully designed absorber in such a way that there are no reflections from the far end.

This is mainly because of the factors like radiation leakage and conduction resistance. To solve this problem waveguides are widely used.

Waveguides can direct the power where it is required. It can at the same time handle large amount of the power.

Principle of Operation Waveguides acts as high pass filters. Types of Waveguides The types of waveguides are: Metal Waveguides Dielectric Waveguides Metal Waveguides Metal Waveguides are generally of two types; rectangular and circular waveguides. Dielectric Waveguides Dielectric waveguides consists of the dielectrics and reflections from the dielectric interfaces which propagate the EM waves along waveguide. Ideal Waveguide Consider the waves propagating in the z direction in a waveguide.

Taking the derivatives of Es and Hs w. Mode Classification in a Waveguide Based on the Maxwells equations, electric and magnetic fields inside the waveguide will have a particular form or shape known as modes. Comparison between Transmission Lines and Waveguides Waveguides Transmission Lines Metal waveguides are conductors enclosed with an insulating medium. Use TE and TM mode. Use TEM mode. Lower attenuation at high frequencies.

Higher attenuation at high frequencies.