Waveguides

WAVEGUIDES

CALDERON

DE GUZMAN

QUITEVIS

RF Waveguides

• Form of RF feeder used for microwave applications

• Used in a variety of applications to carry radio frequency energy from one point to another

• System of material that is designed to confine electromagnetic waves in a direction defined by its physical boundaries

Waveguides

• Only carry or propagate signals above a certain frequency, known as the cut-off frequency.

RF Waveguide

TYPES

Rectangular waveguide

• This is the most commonly used form of waveguide and has a rectangular cross section.

Circular waveguide• Circular waveguide is less common

than rectangular waveguide. They have many similarities in their basic approach, although signals often use a different mode of propagation.

Circuit board stripline

• This form of waveguide is used on printed circuit boards as a transmission line for microwave signals. It typically consists of a line of a given thickness above an earth plane. Its thickness defines the impedance.

Waveguides

Theory of Propagation

• 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.

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

• 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.

Waveguide Theory

Rules of Thumb

• For rectangular waveguides, the TE10 mode of propagation is the lowest mode that is supported.

• For rectangular waveguides, the waveguide width, i.e. the widest internal dimension of the cross section, determines the lower cut-off frequency and is equal to 1/2 wavelength of the lower cut-off frequency.

• For rectangular waveguides, the TE01 mode occurs when the height equals 1/2 wavelength of the cut-off frequency.

• For rectangular waveguides, the TE20, occurs when the width equals one wavelength of the lower cut-off frequency.

Waveguide Impedance

• Where Zo = 120(pi) or 377Ω for air filled waveguide

Circular/Cylindrical Waveguides

• Waveguide having a circular cross-section, used whenever a rotating element (radar antenna) should be attached to the transmitter/receiver.

• Differences versus rectangular waveguides:

• Where – R = waveguide radius– Bmn = Bessel function solution for a particular

m,n mode being propagated•

=1.84 for the dominant mode of operation

All TEmn and TMmn modes are supported since m and n subscripts are defined directly.

Dominant mode is TE11.

Advantages• High power-

handling capacity• Lower

attenuation for a given cutoff wavelength

Disadvantages• Polarization

may rotate