𝐄𝐱𝐩𝐥𝐚𝐢𝐧 𝐭𝐡𝐞 𝐜𝐨𝐧𝐜𝐞𝐩𝐭 𝐨𝐟 𝐝𝐨𝐦𝐢𝐧𝐚𝐧𝐭 𝐦𝐨𝐝𝐞 𝐢𝐧 𝐰𝐚𝐯𝐞𝐠𝐮𝐢𝐝𝐞

1. 𝐃𝐞𝐟𝐢𝐧𝐢𝐭𝐢𝐨𝐧:
- 𝐓𝐡𝐞 𝐝𝐨𝐦𝐢𝐧𝐚𝐧𝐭 𝐦𝐨𝐝𝐞 𝐢𝐬 𝐭𝐡𝐞 𝐦𝐨𝐝𝐞 𝐨𝐟 𝐩𝐫𝐨𝐩𝐚𝐠𝐚𝐭𝐢𝐨𝐧 𝐢𝐧 𝐚
𝐰𝐚𝐯𝐞𝐠𝐮𝐢𝐝𝐞 𝐰𝐢𝐭𝐡 𝐭𝐡𝐞 𝐥𝐨𝐰𝐞𝐬𝐭 𝐜𝐮𝐭-𝐨𝐟𝐟 𝐟𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐲. 𝐈𝐭 𝐢𝐬 𝐭𝐡𝐞 𝐟𝐢𝐫𝐬𝐭
𝐦𝐨𝐝𝐞 𝐭𝐡𝐚𝐭 𝐜𝐚𝐧 𝐩𝐫𝐨𝐩𝐚𝐠𝐚𝐭𝐞 𝐰𝐡𝐞𝐧 𝐭𝐡𝐞 𝐟𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐲 𝐨𝐟 𝐭𝐡𝐞 𝐬𝐢𝐠𝐧𝐚𝐥
𝐞𝐱𝐜𝐞𝐞𝐝𝐬 𝐭𝐡𝐢𝐬 𝐟𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐲.

2. 𝐂𝐮𝐭-𝐎𝐟𝐟 𝐅𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐲:
- 𝐄𝐚𝐜𝐡 𝐦𝐨𝐝𝐞 𝐢𝐧 𝐚 𝐰𝐚𝐯𝐞𝐠𝐮𝐢𝐝𝐞 𝐡𝐚𝐬 𝐚 𝐬𝐩𝐞𝐜𝐢𝐟𝐢𝐜 𝐜𝐮𝐭-𝐨𝐟𝐟
𝐟𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐲. 𝐓𝐡𝐞 𝐝𝐨𝐦𝐢𝐧𝐚𝐧𝐭 𝐦𝐨𝐝𝐞 𝐡𝐚𝐬 𝐭𝐡𝐞 𝐥𝐨𝐰𝐞𝐬𝐭 𝐜𝐮𝐭-𝐨𝐟𝐟
𝐟𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐲, 𝐦𝐞𝐚𝐧𝐢𝐧𝐠 𝐢𝐭 𝐰𝐢𝐥𝐥 𝐩𝐫𝐨𝐩𝐚𝐠𝐚𝐭𝐞 𝐚𝐭 𝐭𝐡𝐞 𝐥𝐨𝐰𝐞𝐬𝐭
𝐟𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐲 𝐜𝐨𝐦𝐩𝐚𝐫𝐞𝐝 𝐭𝐨 𝐨𝐭𝐡𝐞𝐫 𝐦𝐨𝐝𝐞𝐬.

3. 𝐏𝐫𝐨𝐩𝐚𝐠𝐚𝐭𝐢𝐨𝐧 𝐂𝐡𝐚𝐫𝐚𝐜𝐭𝐞𝐫𝐢𝐬𝐭𝐢𝐜𝐬:
- 𝐀𝐛𝐨𝐯𝐞 𝐢𝐭𝐬 𝐜𝐮𝐭-𝐨𝐟𝐟 𝐟𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐲, 𝐭𝐡𝐞 𝐝𝐨𝐦𝐢𝐧𝐚𝐧𝐭 𝐦𝐨𝐝𝐞 𝐢𝐬 𝐭𝐡𝐞
𝐦𝐨𝐬𝐭 𝐞𝐟𝐟𝐢𝐜𝐢𝐞𝐧𝐭 𝐟𝐨𝐫 𝐭𝐫𝐚𝐧𝐬𝐦𝐢𝐭𝐭𝐢𝐧𝐠 𝐞𝐥𝐞𝐜𝐭𝐫𝐨𝐦𝐚𝐠𝐧𝐞𝐭𝐢𝐜 𝐰𝐚𝐯𝐞𝐬
𝐭𝐡𝐫𝐨𝐮𝐠𝐡 𝐭𝐡𝐞 𝐰𝐚𝐯𝐞𝐠𝐮𝐢𝐝𝐞.
- 𝐈𝐭 𝐩𝐫𝐨𝐯𝐢𝐝𝐞𝐬 𝐭𝐡𝐞 𝐡𝐢𝐠𝐡𝐞𝐬𝐭 𝐟𝐢𝐞𝐥𝐝 𝐬𝐭𝐫𝐞𝐧𝐠𝐭𝐡 𝐚𝐧𝐝 𝐫𝐞𝐪𝐮𝐢𝐫𝐞𝐬 𝐭𝐡𝐞
𝐥𝐞𝐚𝐬𝐭 𝐩𝐨𝐰𝐞𝐫 𝐭𝐨 𝐩𝐫𝐨𝐩𝐚𝐠𝐚𝐭𝐞 𝐞𝐟𝐟𝐞𝐜𝐭𝐢𝐯𝐞𝐥𝐲.

4. 𝐌𝐨𝐝𝐞 𝐒𝐭𝐫𝐮𝐜𝐭𝐮𝐫𝐞:
- 𝐑𝐞𝐜𝐭𝐚𝐧𝐠𝐮𝐥𝐚𝐫 𝐖𝐚𝐯𝐞𝐠𝐮𝐢𝐝𝐞𝐬: 𝐓𝐡𝐞 𝐝𝐨𝐦𝐢𝐧𝐚𝐧𝐭 𝐦𝐨𝐝𝐞 𝐢𝐬 𝐭𝐲𝐩𝐢𝐜𝐚𝐥𝐥𝐲
𝐭𝐡𝐞 𝐓𝐄₁₀ 𝐦𝐨𝐝𝐞, 𝐰𝐡𝐢𝐜𝐡 𝐡𝐚𝐬 𝐨𝐧𝐞 𝐡𝐚𝐥𝐟-𝐰𝐚𝐯𝐞𝐥𝐞𝐧𝐠𝐭𝐡 𝐯𝐚𝐫𝐢𝐚𝐭𝐢𝐨𝐧
𝐚𝐥𝐨𝐧𝐠 𝐭𝐡𝐞 𝐰𝐢𝐝𝐭𝐡 𝐚𝐧𝐝 𝐧𝐨𝐧𝐞 𝐚𝐥𝐨𝐧𝐠 𝐭𝐡𝐞 𝐡𝐞𝐢𝐠𝐡𝐭.
- 𝐂𝐢𝐫𝐜𝐮𝐥𝐚𝐫 𝐖𝐚𝐯𝐞𝐠𝐮𝐢𝐝𝐞𝐬: 𝐓𝐡𝐞 𝐝𝐨𝐦𝐢𝐧𝐚𝐧𝐭 𝐦𝐨𝐝𝐞 𝐢𝐬 𝐮𝐬𝐮𝐚𝐥𝐥𝐲 𝐭𝐡𝐞
𝐓𝐄₀₁ 𝐦𝐨𝐝𝐞, 𝐰𝐡𝐢𝐜𝐡 𝐡𝐚𝐬 𝐨𝐧𝐞 𝐫𝐚𝐝𝐢𝐚𝐥 𝐯𝐚𝐫𝐢𝐚𝐭𝐢𝐨𝐧 𝐚𝐧𝐝 𝐧𝐨𝐧𝐞 𝐢𝐧 𝐭𝐡𝐞
𝐚𝐧𝐠𝐮𝐥𝐚𝐫 𝐝𝐢𝐫𝐞𝐜𝐭𝐢𝐨𝐧.

5. 𝐎𝐩𝐞𝐫𝐚𝐭𝐢𝐨𝐧𝐚𝐥 𝐄𝐟𝐟𝐢𝐜𝐢𝐞𝐧𝐜𝐲:
- 𝐎𝐩𝐞𝐫𝐚𝐭𝐢𝐧𝐠 𝐚 𝐰𝐚𝐯𝐞𝐠𝐮𝐢𝐝𝐞 𝐢𝐧 𝐢𝐭𝐬 𝐝𝐨𝐦𝐢𝐧𝐚𝐧𝐭 𝐦𝐨𝐝𝐞 𝐞𝐧𝐬𝐮𝐫𝐞𝐬
𝐨𝐩𝐭𝐢𝐦𝐚𝐥 𝐩𝐞𝐫𝐟𝐨𝐫𝐦𝐚𝐧𝐜𝐞, 𝐦𝐢𝐧𝐢𝐦𝐢𝐳𝐢𝐧𝐠 𝐬𝐢𝐠𝐧𝐚𝐥 𝐥𝐨𝐬𝐬 𝐚𝐧𝐝
𝐦𝐚𝐱𝐢𝐦𝐢𝐳𝐢𝐧𝐠 𝐞𝐟𝐟𝐢𝐜𝐢𝐞𝐧𝐜𝐲
 - 𝐇𝐢𝐠𝐡𝐞𝐫-𝐨𝐫𝐝𝐞𝐫 𝐦𝐨𝐝𝐞𝐬 𝐜𝐚𝐧 𝐚𝐥𝐬𝐨 𝐩𝐫𝐨𝐩𝐚𝐠𝐚𝐭𝐞 𝐛𝐮𝐭 𝐫𝐞𝐪𝐮𝐢𝐫𝐞 𝐡𝐢𝐠𝐡𝐞𝐫
𝐟𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐢𝐞𝐬 𝐚𝐧𝐝 𝐚𝐫𝐞 𝐥𝐞𝐬𝐬 𝐞𝐟𝐟𝐢𝐜𝐢𝐞𝐧𝐭.

6. 𝐈𝐦𝐩𝐨𝐫𝐭𝐚𝐧𝐜𝐞: - 𝐓𝐡𝐞 𝐝𝐨𝐦𝐢𝐧𝐚𝐧𝐭 𝐦𝐨𝐝𝐞 𝐢𝐬 𝐜𝐫𝐮𝐜𝐢𝐚𝐥 𝐟𝐨𝐫 𝐞𝐟𝐟𝐢𝐜𝐢𝐞𝐧𝐭

𝐰𝐚𝐯𝐞𝐠𝐮𝐢𝐝𝐞 𝐨𝐩𝐞𝐫𝐚𝐭𝐢𝐨𝐧. 𝐈𝐭 𝐝𝐞𝐭𝐞𝐫𝐦𝐢𝐧𝐞𝐬 𝐭𝐡𝐞 𝐥𝐨𝐰𝐞𝐬𝐭 𝐟𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐲 𝐚𝐭
𝐰𝐡𝐢𝐜𝐡 𝐭𝐡𝐞 𝐰𝐚𝐯𝐞𝐠𝐮𝐢𝐝𝐞 𝐜𝐚𝐧 𝐞𝐟𝐟𝐞𝐜𝐭𝐢𝐯𝐞𝐥𝐲 𝐭𝐫𝐚𝐧𝐬𝐦𝐢𝐭 𝐬𝐢𝐠𝐧𝐚𝐥𝐬 𝐚𝐧𝐝 𝐢𝐬
𝐮𝐬𝐞𝐝 𝐭𝐨 𝐞𝐧𝐬𝐮𝐫𝐞 𝐫𝐞𝐥𝐢𝐚𝐛𝐥𝐞 𝐚𝐧𝐝 𝐨𝐩𝐭𝐢𝐦𝐚𝐥 𝐩𝐞𝐫𝐟𝐨𝐫𝐦𝐚𝐧𝐜𝐞.

(a) Define the term w.r.t. waveguide (i) cut-off frequency

(ii) cut-off wavelength (iii) Guide Wavelength
(i) Cut-off Frequency
 The cut-off frequency is the minimum frequency at which a particular
mode of electromagnetic wave can propagate in a waveguide.
 Below this frequency, the mode cannot transmit energy and becomes
evanescent.
 It depends on the waveguide's dimensions and the mode of
propagation.
 For a rectangular waveguide, it's given by:

(ii) Cut-off Wavelength

The cut-off wavelength corresponds to the cut-off frequency and is the

longest wavelength (lowest frequency) at which a specific mode can
propagate.
Beyond this wavelength, the mode becomes non-propagating.
It is inversely related to the cut-off frequency:

(iii) Guide Wavelength

  The guide wavelength is the wavelength of the electromagnetic wave
as it travels through a waveguide.
 It is typically longer than the wavelength in free space for frequencies
above the cut-off.
 It is given by

Waveguide and Its Operation (for 4 Marks)

What is a Waveguide?
- Definition: A waveguide is a structure that directs the transmission of
electromagnetic waves from one point to another, commonly used for high-
frequency signals such as microwaves and radio waves.
- Material and Shape: Typically made from conductive materials, waveguides
come in various shapes like rectangular, circular, or elliptical.
- Purpose It confines and guides waves, ensuring minimal signal loss and
efficient transmission.

Operation of a Waveguide
1. Wave Confinement:
- Electromagnetic waves are confined within the waveguide walls, which
are usually reflective, causing the waves to bounce back and forth as they
travel along the guide.

2. Modes of Propagation:
- Waves propagate in specific patterns known as modes (e.g., Transverse
Electric (TE) and Transverse Magnetic (TM)).
- Each mode has a corresponding cut-off frequency, below which the mode
will not propagate.

3. Cut-off Frequency:
 - The cut-off frequency is the lowest frequency at which a mode can
propagate through the waveguide. Below this frequency, the wave is
attenuated and does not propagate.

4. Signal Transmission:
- Above the cut-off frequency, the signal travels through the waveguide by
reflecting off the walls, maintaining the signal's strength over long distances.

Neat Sketch of a Rectangular Waveguide

Sketch Description
- Rectangular Waveguide: A rectangular waveguide with width \(a\) and
height \(b\) is shown.
- Electromagnetic Wave: A wave is depicted propagating along the length of
the waveguide, with arrows indicating the direction of travel.
- Reflection Points: Points where the wave reflects off the top and bottom
walls of the waveguide are highlighted, illustrating the confinement.

This sketch would include the wave's path within the waveguide,
emphasizing the internal reflections and the wave confinement mechanism.

TE and TM Modes in Rectangular Waveguides QPS22 Q.3) B

1. TE (Transverse Electric) Modes
- Definition: In TE (Transverse Electric) modes, the electric field is entirely
transverse to the direction of propagation (along the waveguide's length).
This means that the electric field has no component in the direction of wave
propagation.
- Characteristics:
- The magnetic field has both transverse and longitudinal components.
- TE modes are denoted as {TE}_{mn} , where:
- ( m ) indicates the number of half-wavelength variations across the
waveguide's width.
- ( n ) indicates the number of half-wavelength variations across the
waveguide's height.
o Key Point: The dominant mode in rectangular waveguides (where
a>b) is TE10 , which has the lowest cut-off frequency.

2. TM (Transverse Magnetic) Modes

- Definition: In TM (Transverse Magnetic) modes, the magnetic field is
entirely transverse to the direction of propagation. This means that the
magnetic field has no component in the direction of wave propagation.
- Characteristics:
- The electric field has both transverse and longitudinal components.
- TM modes are denoted as TM{mn}, where:
- \( m \) and \( n \) have similar meanings as in TE modes, representing
variations across the waveguide's dimensions.
 Key Point: There is no TM00 mode because both mmm and nnn must
be non-zero for TM modes to exist.

Mode Patterns:

 TE Modes: Characterized by the presence of the magnetic field component Hz, essential
for energy transmission.
 TM Modes: Characterized by the presence of the electric field component Ez , necessary
for wave propagation.
Dominant Mode:

 In rectangular waveguides, the TE10 mode is the most common and dominant, with
higher modes decaying quickly near sources or discontinuities

Sketch of Field Pattern for TE{2,0} Mode

TE{2,0} Mode Explanation:
- Mode Designation:
- In TE{2,0} mode, ( m = 2 ) and { n = 0 ). This means:
- There are two half-wavelength variations across the waveguide's width
(along the ( x )-axis).
- There is no variation in the waveguide's height (along the ( y )-axis).

Field Pattern:
- Electric Field (E-field:
- The electric field is horizontal (along the ( x )-axis).
- It has two distinct regions of opposite polarity across the width, with the
field lines going from one side to the other.

- Magnetic Field (H-field:

- The magnetic field lines circulate around the electric field lines.
- Since ( n = 0), there is no variation in the field pattern along the height
(along the ( y )-axis), so the magnetic field lines will be uniform in this
direction..
UNIT 2 MICROWAVE PASSIVE COMPONENTS

Explain the working principle of E-plane Tee with constructional sketch.

1. Introduction to E-plane Tee:
 An E-plane tee is a type of waveguide tee where the axis of its side
arm (port 3) is parallel to the electric field (E-field) of the main
waveguide.
 It is often referred to as a series tee because of its configuration and
how it interacts with the electric field.
2. Construction:
 The E-plane tee has three ports:
o Port 1 and Port 2: These are the collinear arms that align with
the main waveguide.
o Port 3: The side arm, which is connected perpendicular to the
collinear arms.
 The side arm is positioned in the plane of the electric field of the main
guide, hence the name "E-plane" tee.
3. Working Principle:
 Case 1: Input at Port 3 (Side Arm)
o When a signal is fed into Port 3, the waves split and appear at
Port 1 and Port 2.
o The waves at Port 1 and Port 2 will have equal magnitude but
opposite phase.
 o This behavior results in the cancellation of any reflection back
into Port 3 if the tee is perfectly matched.
 Case 2: Input at Port 1 and Port 2 (Collinear Arms)
o When signals of equal magnitude and opposite phase are
fed into Port 1 and Port 2, the waves combine constructively at
Port 3, producing a strong output.
o If the signals at Port 1 and Port 2 are in-phase and of equal
magnitude, they cancel out at Port 3, resulting in no output at
Port 3.
4. Scattering Matrix (S-Matrix):
 The S-matrix for an E-plane tee is a 3x3 matrix that characterizes the
relationships between the input and output signals at the three ports.
 The matrix elements relate the power transmitted and reflected at
each port.
 If the tee is perfectly matched, the diagonal elements S11,S22 and S33
will be zero, indicating no reflection.
 The off-diagonal elements S13, S31, S23, and S32 describe the
transmission characteristics, with S13=S31 and S23=−S32 indicating
the phase relationships between the ports.

Explain the working principle of H-plane Tee with constructional sketch.

1. Introduction to H-plane Tee:
 An H-plane tee is a type of waveguide tee where the axis of its side
arm (Port 3) is parallel to the magnetic field (H-field) of the main
waveguide.
 It is also known as a shunt tee because the side arm is "shunting" or
connected across the main waveguide.
2. Construction:
  The H-plane tee consists of three ports:
o Port 1 and Port 2: These are the collinear arms that align with
the main waveguide.
o Port 3: The side arm, which is perpendicular to the collinear
arms but parallel to the H-field of the main guide.
 The orientation of the side arm relative to the magnetic field of the
main guide gives it the name "H-plane" tee.
3. Working Principle:
 Case 1: Input at Port 3 (Side Arm)
o When a signal is fed into Port 3, the waves split and appear at
Port 1 and Port 2.
o The waves at Port 1 and Port 2 will have equal magnitude and
the same phase.
o This phase relationship is due to the side arm being parallel to
the H-field, resulting in no phase difference between the outputs
at Port 1 and Port 2.
 Case 2: Input at Port 1 and Port 2 (Collinear Arms)
o When signals of equal magnitude and opposite phase are
fed into Port 1 and Port 2, the waves cancel each other out at
Port 3, resulting in no output.
o This is because the out-of-phase signals destructively interfere at
Port 3.
 Case 3: Input at Port 1 and Port 2 (Collinear Arms) with In-
phase Signals
o When signals of equal magnitude and in-phase are fed into
Port 1 and Port 2, the waves combine constructively at Port 3,
producing a strong output.
4. Scattering Matrix (S-Matrix):
 The S-matrix for an H-plane tee is also a 3x3 matrix, similar to the E-
plane tee, describing the relationship between the signals at each port.
 The matrix elements indicate the phase and amplitude relationships
between the ports:
 o S13=S31 and S23=S32, with the phase relationships determining
whether the signals at Port 3 add or cancel out.
o For a perfectly matched H-plane tee, the diagonal elements ( S11
,S22,S33) are zero, indicating no reflection at any port.

5. Key Points:
 The H-plane tee is often used to split or combine signals in microwave
circuits, with its behavior depending on the phase and magnitude of
the input signals.
 It effectively separates or combines signals based on their phase
relationships, making it useful in various applications where phase
control is important.

Explain magic tee Draw neat sketches of Magic Tee and label it.
1. Introduction to Magic Tee:
 A Magic Tee, also known as an E-H Plane Tee, is a special type of
waveguide junction.
 It combines features of both an E-plane tee and an H-plane tee.
 The unique way it handles signals gives it the name "Magic Tee."
2. Structure:
 Port 1 and Port 2: These are the collinear arms (main arms).
 Port 3 (E arm): Aligned with the electric field (E-field), like an E-plane
tee.
 Port 4 (H arm): Aligned with the magnetic field (H-field), like an H-
plane tee.
3. Operation:
1. Equal Magnitude, Same Phase (Input at Port 1 and Port 2):
o The signal at Port 3 is zero (canceled out).
o The signal at Port 4 is doubled (added together).
2. Equal Magnitude, Opposite Phase (Input at Port 1 and Port 2):
 o The signal at Port 3 is doubled (added together).
o The signal at Port 4 is zero (canceled out).
3. Input at Port 4 (H arm):
o The signal splits equally between Port 1 and Port 2.
o No signal appears at Port 3.
4. Input at Port 3 (E arm):
o The signal splits equally between Port 1 and Port 2, but they are
opposite in phase.
o No signal appears at Port 4.
5. Input at Port 1 or Port 2:
o The signal does not show up at the other collinear port due to
phase shifts caused by the E and H arms.
4. Why It's Called Magic Tee:
 The "magic" comes from how the signals interact in surprising ways:
o Signals cancel out or add up perfectly depending on where they
enter.
o It’s used in various microwave circuits for this unique ability to
control signal phase and magnitude.

Describe RAT RACE RING (HYBRID RING )

1. Introduction:

 The Rat-Race Ring, also known as a Hybrid Ring, is a four-port junction used in microwave
systems.

 It looks like a ring, with four ports spaced at specific distances to control how signals travel
inside it.

2. Structure:

 Ports 1, 2, 3, and 4 are positioned around the ring.

  The ring's mean circumference (total length) is 1.5 times the wavelength of the signal inside
the waveguide.

 Spacing between ports: The distance between adjacent ports is a quarter of the wavelength,
except between Ports 1 and 4, which is three-quarters of the wavelength.

3. Operation:

1. Input at Port 1:

o The signal splits equally and travels in both clockwise and counterclockwise directions.

o At Ports 2 and 4: The signal combines in phase, meaning it adds up, so you get an
output.

o At Port 3: The signal cancels out due to a 180° phase difference, so no output appears.

2. Input at Port 3:

o Similar to Port 1, the signal splits, and you get outputs at Ports 2 and 4.

o No signal appears at Port 1 due to phase cancellation.

3. Combining or Splitting Signals:

o If two different signals are fed into Port 1, the outputs at Ports 2 and 4 will reflect their
sum, while Port 3 will show the difference between the two signals.

o This feature allows the Rat-Race Ring to combine or divide signals effectively.

4. Applications:

 In Radar Systems: Used as a duplexer.

o During Transmission: The ring directs energy from the transmitter (Port 1) to the
antenna (Port 2) and prevents it from reaching the receiver (Port 3).

o During Reception: The ring directs the signal from the antenna (Port 2) to the receiver
(Port 3), while keeping the transmitter (Port 1) isolated.

Explain the working principle of isolator with neat sketch and state it’s two applications

1. What is an Isolator?

 An isolator is a two-port microwave device that allows signals to pass in one direction (from
Port 1 to Port 2) with very little loss but blocks signals in the reverse direction (from Port 2 to
Port 1) with high attenuation.

2. Why Use an Isolator?

  Purpose: To protect microwave generators (like amplifiers or oscillators) from reflected waves
caused by mismatched loads. This prevents fluctuations in the generator's output power and
frequency.

 Problem Without Isolator: Reflected waves from the load can destabilize the microwave
generator, causing it to malfunction.

 Solution: Placing an isolator between the generator and the load absorbs any reflected waves,
ensuring the generator operates smoothly.

3. Construction:

 Components:

o Circular Waveguide: Supports the TE11 mode.

o Rectangular Waveguide Transitions: At both ends of the circular waveguide,

supporting the TE10 mode.

o Ferrite Rod: A pencil-shaped ferrite located inside the circular waveguide, supported
by polyfoam.

o Permanent Magnet: Surrounds the waveguide and generates a magnetic field in the
ferrite rod.

o Resistive Plates: Positioned at both ends of the rectangular waveguide.

 Function of the Ferrite Rod:

o The magnetic field from the permanent magnet rotates the plane of polarization of
the wave by 45° as it passes through the ferrite rod. This rotation depends on the
magnetic field strength and the ferrite rod's dimensions.

4. Operation:

 Forward Direction (Port 1 to Port 2):

1. A TE10 wave enters from Port 1, perpendicular to the resistive plate.

2. The wave passes through the ferrite rod with a 45° twist in the anti-clockwise
direction.

3. Another 45° twist occurs in the clockwise direction due to the ferrite rod.

4. The wave exits at Port 2 without any attenuation because the polarization of the wave
at Port 2 matches that at Port 1.

 Reverse Direction (Port 2 to Port 1):

1. A TE10 wave enters from Port 2 and undergoes a 45° clockwise twist due to the ferrite
rod.

2. The wave then rotates another 45° in the same direction because of the twist.
 3. When the wave reaches the resistive plate near Port 1, its polarization is parallel to
the plate, causing the wave to be absorbed.

4. Thus, no signal reaches Port 1, effectively isolating it.

5. Applications:

 1. Protecting Microwave Generators: Ensures that reflected waves from mismatched loads do
not affect the generator's stability.

 2. Signal Routing: Used in microwave systems where unidirectional signal flow is critical, like
in radar and communication systems to prevent interference and ensure signal integrity.

State the working principle of circulator

1. What is a Circulator?

 A circulator is a multi-port microwave device that routes signals in a specific sequence. In a

four-port circulator, the signal entering one port exits through the next port in a clockwise or
counterclockwise direction, depending on the design.

2. How Does a Circulator Work?

 Basic Operation:

o Port 1 to Port 2: When a signal enters Port 1, it transitions from the TE1,0 mode
(rectangular) to the TE1,1 mode (circular) as it passes through a gradual transition
from a rectangular to a circular waveguide.

o Ferrite Rotation: The signal passes through a ferrite material, which causes a 45°
rotation of the wave's plane of polarization.

o Port 3 to Port 4: The signal continues through Port 3 unaffected and is rotated another
45° in the ferrite. This 90° rotation aligns the signal with Port 2, where it exits.

 Signal Path:

o Port 1 to Port 2: The signal entering Port 1 exits at Port 2.

o Port 2 to Port 3: The signal entering Port 2 exits at Port 3, following a similar 90°
rotation and alignment process.

o Port 3 to Port 4: The signal entering Port 3 exits at Port 4.

o Port 4 to Port 1: The signal entering Port 4 exits at Port 1.

  Directionality:

o The circulator ensures that each signal flows only in one direction, i.e., a signal
entering any port flows to the next port in sequence and does not backtrack or move
to non-adjacent ports.

What is a Gyrator?

A gyrator is a two-port device that changes the phase of a signal passing through it. If a signal enters
from one side (port 1), it comes out on the other side (port 2) with its phase shifted by 180°. However,
if the signal enters from the opposite side (port 2), it comes out on port 1 without any phase change.

Construction

 Waveguide Structure: The gyrator has a circular waveguide that connects to rectangular
waveguides at both ends. Inside the circular waveguide, there is a ferrite rod, which is a
special magnetic material.

 Magnetic Field: A permanent magnet surrounds the waveguide to create a magnetic field,
which is crucial for the device to work.

 Twist in Waveguide: The rectangular waveguide at port 1 is twisted by 90°, which also affects
the signal passing through it.

How Does It Work?

1. Signal from Port 1 to Port 2:

o When a signal enters at port 1, it first experiences a 90° twist due to the waveguide's
design.

o Then, as it passes through the ferrite rod inside the circular waveguide, it experiences
another 90° rotation.

o By the time it exits at port 2, the total phase shift is 180°.

2. Signal from Port 2 to Port 1:

o When the signal enters at port 2, it undergoes a 90° rotation due to the ferrite rod.

o Then, it is twisted back by another 90° as it exits at port 1.

o The net effect is no change in phase; the signal comes out with the same phase it had
when it entered.

Key Points to Remember

  180° Phase Shift: The signal phase changes by 180° when it moves from port 1 to port 2.

 0° Phase Shift: The signal phase remains unchanged when it moves from port 2 to port 1.

 Non-reciprocal Device: This means the device behaves differently depending on the direction
of the signal, which is why it’s useful in various microwave circuits.

What is a Two-Hole Directional Coupler?

A two-hole directional coupler is a device used in microwave engineering to couple a specific amount
of power from one waveguide (the main guide) into another (the auxiliary guide) while maintaining
the directionality of the power flow. It is particularly useful for measuring and controlling power in
microwave systems.

Construction

 Main Waveguide: This is where the primary signal is transmitted.

 Auxiliary (Secondary) Waveguide: This waveguide is coupled to the main waveguide through
two small holes.

 Two-Holes: These are placed between the main and auxiliary waveguides at a precise distance
apart, typically a quarter of the guide wavelength (λg/4\lambda_g / 4λg/4).

Working Principle

1. Power Coupling:

o When a signal enters the main waveguide, part of the signal leaks through the two
holes into the auxiliary waveguide.

o The position of the holes is critical: they are spaced by λg/4\lambda_g / 4λg/4, which
ensures that the signals from the two holes combine in-phase at one port (forward
coupled port) and out-of-phase at another (backward port).

2. Forward Coupled Port (Port 4):

o The signals leaking from holes A and B travel equal distances to reach this port, so
they arrive in phase.

o Since the signals add up, power is transmitted to this port.

3. Backward Port (Port 3):

o For this port, the signals from holes A and B travel different distances. The signal from
hole B has to travel an extra distance of λg/2\lambda_g / 2λg/2, which introduces a
180° phase shift.
 o Because the signals are out of phase, they cancel each other out, ideally resulting in no
power at this port.

4. Power Control:

o The amount of power coupled into the auxiliary waveguide depends on the size of the
holes. By adjusting the hole dimensions, the coupling level can be controlled.

5. Frequency Sensitivity:

o The directivity and coupling are highly dependent on the frequency because the hole
spacing is a function of the wavelength. Achieving high directivity over a broad
frequency range is challenging