WAVEGUIDES

Question 1

use electromagnetic waves that travel through
air, space, transmission lines, and waveguides. Understanding these systems requires knowledge of how electromagnetic waves propagate in these
mediums.

Answer 1

Radio communication systems

Question 2

become inefficient at high frequencies due to
increased losses, making waveguides a better alternative for microwave
signals.

Answer 2

Transmission lines

Question 3

are hollow, conductive tubes—rectangular, elliptical, or
circular in shape—that confine the waves and minimize losses.

Answer 3

Waveguides

Question 4

They act as
high-pass filters, only allowing frequencies above a certain cutoff to
propagate.

Answer 4

Waveguides

Question 5

They’re impractical at low frequencies but ideal in the
gigahertz range and above.

Answer 5

Waveguides

Question 6

Types of Waveguides

Answer 6

Rectangular
Circular
Elliptical

Question 7

are a number of ways in which electrical energy can propagate along a waveguide

Answer 7

Modes

Question 8

It requires a certain boundary condition, for instance:
there cannot be any electric field along the wall in ideal conductors for the waveguide.

Answer 8

Modes

Question 9

It requires a certain boundary condition, for instance:
Voltage gradient along the wall would have to be needed but is impossible since there cannot be any voltage across a short circuit.

Answer 9

Modes

Question 10

_______ in a waveguide can be understood by imagining waves as rays of light reflecting inside the guide

Answer 10

Modes

Question 11

Each mode corresponds to a different reflection angle. As the angle increases, the ray travels a longer path, which slows down the effective velocity along the guide, even though the wave still moves at the speed of light.

Question 12

Each mode in a waveguide has a cutoff
frequency below which it won’t propagate

Question 13

Single-mode operation, known as the ________, which uses the lowest cutoff frequency

Answer 13

Dominant mode

Question 14

operate between the cutoff of the dominant mode and the
next higher mode

Answer 14

Waveguides

Question 15

are classified as transverse electric (TE) or transverse
magnetic (TM) based on the field patterns inside the guide.

Answer 15

Modes

Question 16

occurs when multiple modes propagate through a waveguide at the same time. If a short pulse of microwave energy is sent into the waveguide, each mode travels at a slightly different speed, causing the pulse to arrive at the other end spread out over time

Answer 16

Dispersion

Question 17

This spreading _______ can lead to signal distortion, especially if another pulse follows closely behind, as the overlapping pulses may interfere with each other

Answer 17

Dispersion

Question 18

To avoid this, it’s best to operate the waveguide in single-mode, allowing only one mode to propagate and preserving the integrity of the signal.

Answer 18

Dispersion

Question 19

In an air-filled rectangular waveguide, electromagnetic waves reflect off the walls rather than traveling straight, causing the signal to move at a slower pace than the speed of light. This slower speed is called _________, and it’s significantly less than the speed of light due to the zigzag path the wave takes.

Answer 19

group velocity

Question 20

The variation in group velocity can be physically explained by the angle at which the wave reflects off the waveguide walls, which changes with frequency. Near the cutoff frequency, the wave bounces more frequently across the guide while covering the same distance along its length compared to higher frequencies.

Question 21

It is often necessary to calculate the wavelength of a signal in a waveguide. For instance, it may be required for impedance matching. It might seem that the wavelength along the guide could be found using the group velocity, in much the same way that the velocity factor of a transmission line is used. However, this common-sense approach does not work because what is really important in impedance-matching calculations is the change in phase angle along the line.

Question 22

refers to the rate at which the phase of a wave appears to move along a waveguide, and it can exceed the speed of light because it doesn’t represent the movement of any physical object or energy

Answer 22

Phase velocity

Question 23

This is similar to how a water wave crest can appear to travel along the shore faster than the actual wave motion—it’s just an effect of the wave’s angle and not a true high-speed motion.

Answer 23

Phase velocity

Question 24

Like any transmission line, a waveguide has a characteristic
impedance. Unlike wire lines, the _______ is a
function of frequency. You might expect that the impedance of a waveguide with an air dielectric would have some relationship to the impedance of free space, which is 377 Ω, and that is true.

Answer 24

waveguide impedance

Question 25

Techniques for matching impedances using waveguides also differ from those used with conventional transmission lines. Shorted stubs of adjustable length can be used, but a simpler method is to add capacitance or inductance by inserting a tuning screw into the guide. As the screw is inserted farther into the guide, the effect is first capacitive, then series-resonant, and finally inductive.

Question 26

The three basic ways to launch a wave down a guide

Answer 26

- Probe - Loop - Hole

Question 27

To launch a wave in one direction within a waveguide, a ______ is placed at an electric-field maximum— typically at the center of the wide dimension for the TE₁₀ mode—and positioned a quarter guidewavelength from the shorted end. This setup ensures that the reflected wave, after two 180° phase shifts, adds constructively to the forward wave.

Answer 27

probe

Question 28

A _______ couples to the magnetic field in a waveguide and is placed where the magnetic field is strongest— near the end wall for the TE₁₀ mode. This is similar to how current peaks at the shorted end of a conventional transmission line, with magnetic and electric fields in the guide analogous to current and voltage, respectively.

Answer 28

loop

Question 29

A _______ can be made in the waveguide to allow electromagnetic energy to enter or exit from the surrounding space. Since waveguides are reciprocal devices—like transmission lines—the same methods used to couple power into a waveguide can also be used to extract it

Answer 29

hole

Question 30

use variations like two holes spaced a quarter-wavelength apart to ensure wave propagation in only one direction. This setup causes signals in the undesired direction to cancel due to a 180° phase difference, while allowing constructive interference in the desired direction. Additional holes can be used to enhance coupling, and resistive material absorbs unwanted signals at the end of the secondary guide.

Answer 30

Directional couplers

Question 31

are characterized by their insertion loss, coupling, and directivity.

Answer 31

Directional couplers

Question 32

All three are normally specified in decibels

Answer 32

insertion loss, coupling, and directivity

Question 33

is the amount by which a signal in the main guide will be attenuated

Answer 33

insertion loss

Question 34

gives the amount by which the signal in the main guide is greater than that coupled to the secondary waveguide

Answer 34

coupling specification

Question 35

refers to the ratio between the power coupled to the secondary guide, for signals travelling in the two possible directions along the main guide

Answer 35

directivity

Question 36

The use of waveguides requires redesign of some of the ordinary components that are used with feedlines. The lowly tee connector is an example. In addition, other components, such as resonant cavities, are too large to be practical at lower frequencies

Question 37

Waveguide components

Answer 37

- Bends - TEES - Cavity Resonators

Question 38

Changes in a waveguide’ s shape or size affect its internal electric and magnetic fields, potentially altering its characteristic impedance if the change is significant. However, gradual bends or twists have minimal impact.

Answer 38

BENDS

Question 39

are classified as E-plane or H-plane, depending on whether they affect the direction of the electric or magnetic field, respectively, with reference to the TE₁₀ mode

Answer 39

BENDS

Question 40

with its carefully designed gradual bends, resembles plumbing and is just as tricky to install

Answer 40

Rigid waveguide

Question 41

is used for awkward installations

Answer 41

Flexible waveguide

Question 42

like their transmission line counterparts, split a signal into two paths and come in E-plane and H-plane types

Answer 42

Waveguide tees

Question 43

produces in-phase outputs

Answer 43

H-plane tee (shunt tee)

Question 44

gives out-of-phase signals

Answer 44

E-plane tee (series tee)

Question 45

combines both types, offering features like signal isolation—e.g., an input at one port results in outputs at two ports but none at a fourth, depending on the input configuration

Answer 45

hybrid or "magic" tee

Question 46

is a short section where waves reflect back and forth. If its length is exactly half a wavelength, the reflections reinforce each other, creating resonance and a buildup of energy. This resonant behavior, like in other systems, is characterized by a high quality factor (Q), often in the thousands.

Answer 46

Waveguide Cavity or Cavity Resonators

Question 47

can be tuned by adjusting their size, and various shapes exist beyond the rectangular type

Answer 47

Cavities

Question 48

are widely used in microwave systems and even in VHF applications where high Q justifies their larger size

Answer 48

Resonant Cavities

Question 49

At lower frequencies, loads and attenuators use resistors, but at microwave frequencies, traditional resistors become impractical due to unwanted inductance and capacitance.

Question 50

use resistive materials like carbon to absorb energy

Answer 50

Waveguide Attenuators

Question 51

Inserts a carbon vane into the guide

Answer 51

Flap Attenuator

Question 52

varies loss based on the vane’s orientation to the electric field

Answer 52

Rotating Vane Attenuator

Question 53

use carbon inserts to absorb energy without reflections

Answer 53

Terminating loads

Question 54

also known as a dummy load

Answer 54

Waveguide Termination

Question 55

is a component used in waveguide systems to absorb RF energy and prevent reflections. It's a resistive load matched to the waveguide's characteristic impedance, ensuring the waveguide operates effectively and prevents standing waves. This is crucial for maintaining signal integrity and overall system performance.

Answer 55

Waveguide Termination

Question 56

are useful microwave components that generally use ferrites in their operation

Answer 56

Isolators and circulators

Question 57

is a device that allows a signal to pass in only one direction. In the other direction, it is greatly attenuated

Answer 57

ISOLATOR

Question 58

can be used to shield a source from a mismatched load. Energy will still be reflected from the load, but instead of reaching the source, the reflected power is dissipated in this component

Answer 58

ISOLATOR

Question 59

is a multi-port device that directs signals from one port to the next in a counterclockwise direction, allowing signal separation. In a common three-port setup, a signal from port 1 goes only to port 2, and so on

Answer 59

CIRCULATOR

Question 60

A typical use of this is as a transmit-receive switch, where it routes the transmitter output to the antenna and the received signal to the receiver, while preventing the strong transmit signal from damaging the receiver

Answer 60

CIRCULATOR

Question 61

Isolators and Circulators Applications

Answer 61

- Optical Fiber - Waveguide Components for Medical Linear Accelerators - Photonic Integrated Circuits - Optical interferometers