WAVEGUIDE

A waveguide is an electromagnetic feed line used in microwave communications,

broadcasting, and radar installations. A waveguide consists of a rectangular or

cylindrical metal tube or pipe. The electromagnetic field propagates lengthwise.

Waveguides are most often used with horn antenna s and dish antenna s.

In electromagnetics and communications engineering, the term waveguide may

refer to any linear structure that conveys electromagnetic waves between its

endpoints. However, the original and most common meaning is a hollow metal pipe

used to carry radio waves. This type of waveguide is used as a transmission

line mostly at microwave frequencies, for such purposes as connecting

microwave transmitters and receivers to their antennas, in equipment such

as microwave ovens, radar sets, satellite communications, and microwave radio

links.

Waveguides are practical only for signals of extremely high frequency, where the

wavelength approaches the cross-sectional dimensions of the waveguide. Below

such frequencies, waveguides are useless as electrical transmission lines.

USES OF WAVEGUIDES

When functioning as transmission lines, though, waveguides are considerably

simpler than two-conductor cables—especially coaxial cables—in their

manufacture and maintenance. With only a single conductor (the waveguide’s

“shell”), there are no concerns with proper conductor-to-conductor spacing, or of

the consistency of the dielectric material, since the only dielectric in a waveguide

is air. Moisture is not as severe a problem in waveguides as it is within coaxial

cables, either, and so waveguides are often spared the necessity of gas “filling.”

Waveguides may be thought of as conduits for electromagnetic energy, the

waveguide itself acting as nothing more than a “director” of the energy rather

than as a signal conductor in the normal sense of the word. In a sense, all

transmission lines function as conduits of electromagnetic energy when

transporting pulses or high-frequency waves, directing the waves as the banks of

a river direct a tidal wave. However, because waveguides are single-conductor

elements, the propagation of electrical energy down a waveguide is of a very

different nature than the propagation of electrical energy down a two-conductor

transmission line.

These are the following uses of waveguides:

1. It is used where the transmission or reception is in the range of microwave frequencies.

2. It is also used for handling the high power of energy.3. It is mostly used in the airborne radar.4. In ground radar’s we also use the waveguide.5. The circular waveguide is mostly used in the ground radar to transmit or

receive the energy from antenna. Which revolves in 360˚ bearing continuously.

6. The waveguide is also used in communication system.7. In satellite communication the waveguide is mostly used.8. We also use the waveguide in the devices of navigation aids.9. In some cases the waveguide is used as attenuator where very high

frequencies are involved.10. The wave guides are also used with the cavity resonators to carry the

input and output signals.

Dominant Mode

This mode is one of the most efficient mode and all the waveguides are designed in such a way that only this mode will be used. To operate in the dominant mode, a waveguide must have an "a" (wide) dimension of at least one half-wavelength of the frequency to be propagated. For the possible modes of operation available for a given waveguide, the dominant mode has the lowest cutoff frequency. In practice, this dimension is usually 0.7 wavelength.

Propagation Modes in Waveguide

According to waveguide theory there are a number of different types of electromagnetic wave that can propagate within the waveguide. These different types of waves correspond to the different elements within an electromagnetic wave. In a waveguide a signal will propagate as an electromagnetic wave. Even in a transmission line the signal propagates as a wave because the current in motion down the line gives rise to the electric and magnetic fields that behaves as an electromagnetic field.

TE waves:   Transverse electric waves, also sometimes called H waves, are characterised by the fact that the electric vector (E) is always perpendicular to the direction of propagation.

TM waves:   Transverse magnetic waves, also called E waves are characterised by the fact that the magnetic vector (H vector) is always perpendicular to the direction of propagation.

TEM waves:   The Transverse electromagnetic wave is cannot be propagated within a waveguide, but is included for completeness. It is the mode that is commonly used within coaxial and open wire feeders. The TEM wave is characterised by the fact that both the electric vector (E vector) and

the magnetic vector (H vector) are perpendicular to the direction of propagation.

The transverse electromagnetic (TEM) field is the specific type of field found in transmission lines. We also know that the term “transverse” implies to things at right angles to each other, so the electric and magnetic fields are perpendicular to the direction of travel. These right angle waves are said to be “normal” or “orthogonal “to the direction of travel.

The boundary conditions that apply to waveguides will not allow a TEM wave to propagate. However, the wave in the waveguide will propagate through air or inert gas dielectric in a manner similar to free space propagation, the phenomena is bounded by the walls of the waveguide and that implies certain conditions that must be met. The boundary conditions for waveguides are:

1. The electric field must be orthogonal to the conductor in order to exist at the surface of that conductor.

2. The magnetic field must not be orthogonal to the surface of the waveguide.

The waveguide has two different types of propagation modes to satisfy these boundary conditions:

1. TE – transverse electric (Ez = 0)2. TM – transverse magnetic (Hz = 0)

The transverse electric field requirement means that the E-field must be perpendicular to the conductor wall of the waveguide. This requirement can be met with proper coupling at the input end of the waveguide. A vertically polarized coupling radiator will provide the necessary transverse field.

One boundary condition will require the magnetic (H) field not to be orthogonal to the conductor surface. Since it is at right angles to the E-field, the requirement will be met. The planes that are formed by the H-field will be parallel to the direction of propagation and to the surface.

TYPES OF WAVEGUIDES

A. Rectangular Waveguide

Metal pipe waveguides are often used to guide electromagnetic waves. The most common waveguides have rectangular cross-sections and so are well suited for the exploration of electrodynamic fields that depend on three dimensions. Although we confine ourselves to a rectangular cross-section and hence Cartesian coordinates, the classification of waveguide modes and the general approach used here are equally applicable to other geometries, for example to waveguides of circular cross-section.

The parallel plate system considered in the previous three sections illustrates much of what can be expected in pipe waveguides. However, unlike the parallel plates, which can support TEM modes as well as higher-order TE modes and TM modes, the pipe cannot transmit a TEM mode. From the parallel plate system, we expect that a waveguide will support propagating modes only if the frequency is high enough to make the greater interior cross-sectional dimension of the pipe greater than a free space half-wavelength. Thus, we will find that a guide having a larger dimension greater than 5 cm would typically be used to guide energy having a frequency of 3 GHz.

B. Circular Waveguide

Circular waveguide is basically seems like the circular structure where it consist tubular circular conductor. As the frequency of transmitted signal changes the inner diameter of circular waveguide also changes. There are so many uses of Circular waveguides like in communication systems used in specific areas of radar and uses as rotating joints of the mechanical point of the antennas rotation. Generally the values of radius is very important for manufacturing of circular waveguides.

Modes of the circular waveguide shows that the Electric field is perpendicular to the length of waveguide and there is no any Electric lines parallel to the direction of propagation. The dominant mode of the circular waveguide is TE11 where m=1 and n=1.For complete one cycle ‘m’ and ‘n’ shows the no. of ½ cycle variations of the fields along the diameter. In this mode, waveguide shows the lowest cut off frequency required for operating. The cutoff wavelength of a circular guide is 1.71 times the diameter of the waveguide. In the circular waveguide, the E field is perpendicular to the length of the waveguide with no E lines parallel to the direction of propagation. Thus, it must be classified as operating in the TE mode.

WAVEGUIDE PROPERTIES

A. CUTOFF FREQUENCY

Waveguides will only carry or propagate signals above a certain frequency, known as the cut-off frequency. Below this the waveguide is not able to carry the signals. The cut-off frequency of the waveguide depends upon its dimensions. In view of the mechanical constraints this means that waveguides are only used for microwave frequencies. Although it is theoretically possible to build waveguides for lower frequencies the size would not make them viable to contain within normal dimensions and their cost would be prohibitive.

Signals can progress along a waveguide using a number of modes. However the dominant mode is the one that has the lowest cutoff frequency. For a rectangular waveguide, this is the TE10 mode.

The TE means transverse electric and indicates that the electric field is transverse to the direction of propagation.

The diagram shows the electric field across the cross section of the waveguide. The lowest frequency that can be propagated by a mode equates to that were the wave can "fit into" the waveguide.

As seen by the diagram, it is possible for a number of modes to be active and this can cause significant problems and issues. All the modes propagate in slightly different ways and therefore if a number of modes are active, signal issues occur.

It is therefore best to select the waveguide dimensions so that, for a given input signal, only the energy of the dominant mode can be transmitted by the waveguide. For example: for

a given frequency, the width of a rectangular guide may be too large: this would cause the TE20 mode to propagate.

As a result, for low aspect ratio rectangular waveguides the TE20 mode is the next higher order mode and it is harmonically related to the cutoff frequency of the TE10 mode. This relationship and attenuation and propagation characteristics that determine the normal operating frequency range of rectangular waveguide.

Rectangular waveguide cutoff frequency

Although waveguides can support many modes of transmission, the one that is used, virtually exclusively is the TE10 mode. If this assumption is made, then the calculation for the lower cutoff point becomes very simple:

Fc= c2a

Where: fc = rectangular waveguide cutoff frequency in Hz c = speed of light within the waveguide in metres per second a = the large internal dimension of the waveguide in metres

It is worth noting that the cutoff frequency is independent of the other dimension of the waveguide. This is because the major dimension governs the lowest frequency at which the waveguide can propagate a signal.

Circular waveguide cutoff frequency

the equation for a circular waveguide is a little more complicated (but not a lot).

Fc=1.8412c2Π

Where:    fc = circular waveguide cutoff frequency in Hz     c = speed of light within the waveguide in metres per second     a = the internal radius for the circular waveguide in metres

Although it is possible to provide more generic waveguide cutoff frequency formulae, these ones are simple, easy to use and accommodate, by far the majority of calculations needed.

B. Cut off Wavelength

The cut off wavelength is defined as the maximum wavelength of the waves to be transmitted through the waveguide. All the wavelengths greater than λc are attenuated and those which is less than λc are allowed to propagate inside the waveguide. The cut off wavelength is denoted as:

In circular waveguide the cut off wavelength is maximum when power is minimum in its different modes of TE and TM. But in rectangular waveguide the cut off wavelength rises for some point and then gets saturated at certain level of wavelength being unaware of power in the waveguide.

C. GUIDE WAVELENGTH

The guide wavelength is defined as the distance between two equal phase planes along the waveguide. It is defined as the distance travelled by the wave in order to undergo a phase shift of 2 π. It is given as:

D. PHASE VELOCITY

Phase Velocity is defined as the rate at which the wave changes its phase in terms of the guide wavelength. The wave propagates in the waveguide when guide wavelength λg is greater than the free space wavelength λo. Since the velocity of propagation is the product of λ and f, it follows that in a waveguide, Vp =λxf where Vp is the phase velocity. But Vp is greater than the speed of light since λg>λo. This is contradicting since no signal travels faster than speed of light. However the wavelength in the guide is the length of the cycle and Vp represents the velocity of the phase. The phase velocity is given as;

E. POWER IN WAVEGUIDES

Electromagnetic energy transmitted into space consists of electric and magnetic fields that are at right angles (90 degrees) to each other and at right angles to the direction of propagation. To establish the relationship this paper use “Poynting vector” theorem. It indicates that a screw (right-hand thread) with its axis perpendicular to the electric and magnetic fields will advance in the direction of propagation if the E field is rotated to the right (toward the H field).

F. ATTENUATION

Waveguides can support travelling waves at frequencies higher than their cutoff frequencies. However, the travelling waves are attenuated by losses in the dielectric medium that fills a waveguide and by losses in the conducting sidewalls. Radiation and Induction Losses are similar in that both are caused by the fields surrounding the conductors. Losses occur because some magnetic lines of force about a conductor do not return to the conductor when the cycle alternates. These lines of force are projected into space as radiation and this result in power losses. In waveguide, current flows only along the inside surface of the waveguide, but remember that waveguides have four solid walls to carry the current. So the current-carrying area is large in waveguide but it is small in wire. The current carrying area in waveguide has some little amount of resistance, so due to skin effect losses are very low.

At the frequencies below the cut off frequency (f<fc), the propagation constant ‘λ’ will have only the attenuation term ‘α’ that is the phase constant will be the imaginary part. The cut off attenuation is

For f>fc, the waveguide exhibits very low loss and for f<fc, the attenuation is high and results in full reflection of the wave. Similar to rectangular

waveguides it is possible to determine the attenuation in circular waveguide for TE &TM modes. The attenuation in an airfilled circular waveguide is due to an infinite conductivity of the guide walls and is given by;

Waveguide Properties Note: formulas only apply for dominant mode 

Where:

Pamantasan ng Lungsod ng Maynila

(University of the City of Manila)

Intramuros, Manila

Principles of Communication

WAVEGUIDES

Submitted by:

YAMSON, Eirry Rose Anne R.

BS CpE

2012-10707

Submitted to:

Engr. Nidea

August 29, 2015