WAVEGUIDESA Report by:

Geron AdvinculaJoel Cristales Jr.

J.Kenneth JalandoniMehfrell Javellana

Lorenz Angelo SumagaysayRalph Malvin Uy

Definition

As a review, transmission lines are the means for carrying elecromagnetic energy from one place to another.

Definition

Most microwave energy transmission is handled by waveguides.

WAVEGUIDES- a hollow metal tube designed to carry microwave energy from one place to another. They may be used to carry energy between pieces of equipment or over longer distances to carry transmitter power to an antenna or microwave signals from an antenna to a receiver.

Construction

Waveguides are made from copper, aluminum or brass. These metals are extruded into long rectangular, circular or elliptical pipes. Often the insides of these waveguides are plated with silver to reduce their resistance to a very low level. The dimensions of the cross section are selected such that electromagnetic waves can propagate within the interior of the guide .

Operation

A waveguide does not conduct current in the true sense, but rather serves as a boundary that confines electromagnetic energy. The walls of the waveguides are conductors and, therefore, reflect electromagnetic energy from their surface. If the wall of the waveguide is a good conductor and very thin, little current flows in the interior walls and, consequently, very little power is dissipated.

In a waveguide, conduction of energy does not occur in the walls of the waveguide, but rather through the dielectric within the waveguide, which is usually dehydrated air or inert gas. In essence, a waveguide is analogous to a metallic wire conductor with its interior removed. Electromagnetic energy propagates down a waveguide by reflecting back and forth in a zigzag pattern. The cross-sectional area of a waveguide must be on the same order as the wavelength of the signal it is propagating. Therefore, waveguides are generally restricted to frequencies above 1 GHz.

Types of Waveguides

• RECTANGULAR WAVEGUIDERectangular waveguides, as opposed to circular and elliptical waveguides, are by far the dominant configuration for the installed base of waveguides for compact systems like radar and inside equipment shelters. That is probably due to the generally greater rigidity of rectangular structures because the wall thickness can be easily made thicker than with circular. It is also easier to route and mount in close quarters, and attaching penetrating objects like probes and switches is much simpler.

Velocity and Frequency Relation

In waveguides, the velocity varies with frequency. Group and phase velocities have the same value in free space and in parallel wire TL. However, if these two velocities are measured at the same frequency in a waveguide, it will be found that, in general, the two velocities are not the same. At some frequencies they will be nearly equal and at other frequencies they can be considerably different.

In addition, it is necessary to distinguish between two different kinds of velocity:

• Phase Velocity is the apparent velocity of a particular phase of the wave. It is the velocity with which a wave changes phase in a direction parallel to a conducting surface, such as the walls of a waveguide.

It is determined by measuring the wavelength of a particular frequency wave and substituting it into the formula:

Vph = λf

Where: Vph is the phase velocity (metres per second) f is frequency in hertz λ is wavelength in meters per cycle

• Phase Velocity in a waveguide is greater than its velocity in free space, the wavelength for a given frequency will be greater in the waveguide than in free space and it is given with the relationship:

Vg= λo(Vph/ c)

where: λo is the free space wavelength in m per cycle

• Group Velocity is the velocity of a group of waves. It is the velocity at which information signals of any kind are propagated and at which energy is propagated. It can be measured by determining the time it takes for a pulse to propagate at a given length of waveguide.

VgVph=c2

Where: Vg is the group velocity in m per sec Vph is the phase velocity c is the speed of light

Cutoff Frequency and Wavelength

• Cutoff frequency is the minimum frequency of operation of a waveguide. It is the absolute limiting frequency( frequencies below the cutoff frequency will not be propagated by the waveguide).

• Cutoff wavelength is the minimum wavelength that the waveguide can propagate. It is defined as smallest free-space wavelength that is just unable to propagate in the waveguide. Only frequencies with wavelengths less than the cutoff wavelength can propagate down the waveguide.

• The mathematical relationship between the guide wavelength at a particular frequency and the cutoff frequency is:

λg=c/√(f2-fc2)

where: λg= guide wavelength in m per cycle fc = cutoff frequency in hertz

f = frequency of operation in hertz

Electromagnetic Wave Propagation

CHARACTERISTIC IMPEDANCE

• Waveguides have a characteristic impedance that is analogous to the characteristic impedance of parallel- wire transmission lines and closely related to the characteristic impedance of free space.

Zo =

Other Types• Circular Waveguides are used in radar and microwave applications

when it is necessary or advantageous to propagate both vertically and horizontally polarized waves in the same waveguide.

• Ridged Waveguide is more expensive to manufacture than a standard rectangular waveguide but it also allows operation at lower frequencies for a given size. It has more loss per unit length. It is useful for specialized applications.

• Flexible Waveguide consists of spiral- wound ribbon of brass and copper. The outside covered with a soft dielectric coating to keep the waveguide air- and watertight. It is used extensively in microwave test equipment.