MICROWAVE CHAPTER 10

Objective of the Lesson

Describe the various types and characteristics of waveguides and transmission lines used in microwave circuits.

Waveguides

• Are hollow metal conducting pipes designed to carry and constrain the electromagnetic waves of a microwave signal.

• Waveguides are made from copper, aluminum or brass.

• Often the insides of waveguides are plated with silver to reduce resistance and transmission losses.

6GHz

• Most microwave energy transmission above — is handled by this

Waveguides: Signal Injection and Extraction

• A microwave signal to be carried by a waveguide is introduced into one end of the waveguide with an antenna-like probe.

• The probe creates an electromagnetic wave that propagates through the waveguide.

• The electric and magnetic fields associated with the signal bounce off the inside walls back and forth as the signal progresses down the waveguide.

• The waveguide totally contains the signal so that none escapes by radiation.

Waveguides: Signal Injection and Extraction

• Probes and loops can be used to extract a signal from a waveguide.

• When the signal strikes a probe or a loop, a signal is induced which can then be fed to other circuitry through a short coaxial cable.

Injecting a sine wave into a waveguide and extracting a signal

Injecting a sine wave into a waveguide and extracting a signal.

Waveguides: Waveguide Size and Frequency

• The frequency of operation of a waveguide is determined by the inside width of the pipe (dimension (a) in the figure following).

• This dimension is usually made equal to one-half wavelength, a bit below the lowest frequency of operation. This frequency is known as the waveguide cutoff frequency.

• At its cutoff frequency and below, a waveguide will not transmit energy.

• Above the cutoff frequency, a waveguide will propagate electromagnetic energy.

Waveguides and Cavity Resonators

The dimensions of a waveguide determine its operating frequency range.

Angles of incidence and reflection

• The angles of incidence and reflection depend on the operating frequency.

• At high frequencies, the angle is large and the path between the opposite walls is relatively long.

• As the operating frequency decreases, the angle also decreases and the path between the sides shortens.

• When the operating frequency reaches the cutoff frequency of the waveguide, the signal bounces back and forth between the sidewalls of the waveguide. No energy is propagated.

Wave paths in a waveguide at various frequencies

(a) High frequency.
(b) Medium frequency.
(c) Low frequency.
(d) Cutoff frequency.

Operating mode

• When a microwave signal is launched into a waveguide by a probe or loop, electric and magnetic fields are created in various patterns depending upon the method of energy coupling, frequency of operation, and size of waveguide.

• The pattern of the electromagnetic fields within a waveguide takes many forms. Each form is called an operating mode.

Transverse electric (TE) field

In a waveguide, when the electric field is at a right angle to the direction of wave propagation, it is called a transverse electric (TE) field.

Transverse magnetic (TM) field

When the magnetic field is transverse to the direction of propagation, it is called a transverse magnetic (TM) field.

Waveguide Coupling Hardware and Accessories

• Waveguides have a variety of special parts, such as couplers, turns, joints, rotary connections, and terminations.

• Most waveguides and their fittings are precision-made so that the dimensions match perfectly.

• A choke joint is used to connect two sections of waveguide. It consists of two flanges connected to the waveguide at the center.

• A T section or T junction is used to split or combine two or more sources of microwave power.

Choke joint

A choke joint permits sections of waveguide to be interconnected with minimum loss and radiation.

Distributed Components

• The inductance in a transmission line comes about because a current is flowing in a metallic conductor

• The resistance is associated with the metallic conductor and current flow

• The capacitive reactance, which is a result of the line capacitance, decreases with an increase in frequency

• Conductance is the amount of leakage through the dielectric

Some terms associated with transmission lines

• VSWR

• Reflection coefficient

• Return loss.

Coaxial transmission lines

• A transmission line in which one conductor completely surrounds the other, the two being coaxial and separated by a continuous solid dielectric or by dielectric spacers.

  • Flexible

  • Semi-rigid

Parts of a basic flexible cable

• The center conductor can be either a solid wire or a series of wires in a stranded configuration.

• The outer conductor serves two functions. It is a ground reference for the signal on the center conductor and also is used as a shield.

• The braid construction may be single, double, or triaxial (two braids separated by an insulator). The single-braid construction consists of bare, tinned, or silverplated copper wires. The double braid consists of two single braids with no insulation between them. The triaxial consists of two single braids with a layer of insulation between them.

• The outer coating provides protection for the cable. Such protection is mainly environmental. It plays no part in the electrical performance of the cable.

BNC CONNECTOR

For low power RF signal below 3 MHz; 50 TO 75 ohms impedance

TNC CONNECTOR

It has a 50 Ω impedance and operates best in the 0–11 GHz

SMA- Sub Miniature Version A

From DC to 18 GHz; 50 ohms impedance

N CONNECTOR

• Carries RF signals up to 18 GHz; 50 to 75 ohms impedance

• 50 ohms (bottom)

• 75 ohms (top)

SEMI RIGID vs. FLEXIBLE

• Semirigid cables cost considerably more than flexible cables

• For testing applications, semirigid cables are not very practical. Most tests require many connect/disconnect operations, which can put strain on the cables.

• In some finished products, the cables must meander through the chassis to various locations. Semirigid cable would not fit those applications in many cases.

Strip transmission line (stripline)

It evolved from the circular coaxial device and still has all the original sections (center conductor, dielectric, outside shield, and electric fields) but now is in a form that will operate at much higher frequencies and be more efficient for RF and microwave applications.

Ground-Plane Spacing (GPS)

Ground-Plane Spacing (GPS) is the spacing between the ground planes, or copper on the circuit boards.

Microstrip

• Microstrip transmission line does away with the problem of inaccessibility that stripline poses.

• Microstrip transmission line is similar to stripline transmission line, except that there is no top on the transmission line. There is nothing but air on top of the circuitry and a dielectric material underneath.

Coplanar waveguide

• It is a modification of the microstrip circuitry

• In a coplanar waveguide, there is still a circuit trace on the top of the board that is a certain width and thickness, but there are also ground planes on both sides of the circuit trace and there is no ground plane on the bottom of the circuit board. A conductor surrounded by ground “guides” the electromagnetic wave down the transmission line.

rectangular.

• Most waveguides are

copper, aluminum or brass

Waveguides are made from .

silver

Often the insides of waveguides are plated with —- to reduce resistance and transmission losses.

probe

The — creates an electromagnetic wave that propagates through the waveguide.

Probes and loops

—-can be used to extract a signal from a waveguide.