Top 50 Microwave Engineering Interview Questions You Must Prepare 02.Mar.2024

  1. Wider bandwidth due to higher frequency
  2. Better resolution due to smaller wavelength
  3. Higher speed of operation
  4. Higher antenna gain (size reducing)
  5. As the production of  frequency in microwave range by natural resources are less , it is more available and less crowed frequency spectrum. 
  6. Lower interference due to lower signal crowding.

Microwave are unidirectional. This me that when an antenna trmits microwaves, they travel in one direction. That is why microwave propagation is line of sight (LOS) propagation. They are not reflected by ionosphere. High frequency microwaves can not penetrate an object like wall.

An H-plane tee is a waveguide tee in which the axis of its side arm is "shunting" the E field or parallel to the H field of the main guide.

An £-plane tee is a waveguide tee in which the axis of its side arm is parallel to the E field of the main guide.

When the tristor junction is joined by two similar materials such as silicon- to silicon or germanium-to-germanium, it is called a homo junction tristor.

Maximum velocity of charge carriers in a semiconductor is called saturation drift velocity.

Quality factor Q which is a measure of the frequency selectivity of a cavity.

An ideal isolator completely absorbs the power for propagation in one direction and provides lossless trmission in the opposite direction. Thus the isolator is usually called uniline.

A trmission line terminated in its characteristic impedance Zo is called a properly terminated line. Otherwise it is called an improperly terminated line.

The reflection loss is a measure of power loss during trmission due to the reflection of the signal as a result of impedance mismatch.

Following are the applications of microwave engineering:

  1. Antenna gain is proportional to the electrical size of the antenna. At higher frequencies, more antenna gain is therefore possible for a given physical antenna size, which has important consequences for implementing miniaturized microwave systems.
  2. More bandwidth can be realized at higher frequencies. Bandwidth is critically important because available frequency bands in the electromagnetic spectrum are being rapidly depleted.
  3. Microwave signals travel by line of sight are not bent by the ionosphere as are lower frequency signals and thus satellite and terrestrial communication links with very high capacities are possible.

They are used in several applications. Some of these are:

  1. Communication (astronomy, satellite, WLAN, mobile,cellular, radar, TV etc)
  2. Remote sensing and navigation (prediction/monitoring/guidence- traffic control, weather, missile, aircraft etc)
  3. Medical applications (cautery imaging, heart stimulation, therapy etc)

When the tristor junction is joined by two different materials such as Ge to GaAs, then it is called a homo junction tristor.

Following are the two examples of two port junctions:

  1. The junction of two rectangular guides of unequal height
  2. A symmetrical junction consisting of two similar rectangular guides joined by an Intermediate guide of greater width.

It is defined as the ratio of power delivered to the load to that of the power from the source into an amplifier.

A microwave circuit consists of several microwave devices connected in some way to achieve the desired trmission of MW signal. The interconnection of two or more microwave may be regarded as MW junction.

E.g.: TEE, HYBRID RING

Diffusion and ion implantation are the two processes used in controlling amounts of dopants in semiconductor fabrications.

Matrix is used in MW analysis to overcome the problem which occurs when H,Y & Z parameter are used in high frequencies.

A magic tee is a combination of the £-plane tee and H -plane tee.  The magic tee is commonly used for mixing, duplexing, and impedance measurements.

The electromagnetic wave being propagated in free space near the surface of the earth is divided into two parts: 

  1. Ground wave: Classified as direct wave, earth-reflected wave and surface wave
  2. Sky wave or ionosphere wave. 

In a microwave junction there is an interaction of three or more components.There will be an output port, in addition there may be reflection from the junction of other ports. Totally there may be many combination, these are represented easily using a matrix called S matrix.

Properties of s- matrix:

  1. it possess symmetric properties sij=sji
  2. it possess unitary property
  3. [s][s]*=[i]

The devices which are having the properties that the forward characteristics are not equal to the reverse characteristics are called non-reciprocal devices.

Due to above properties  microwaves have following advantages and disadvantages:

  1. Due to line of sight propagation, a pair of antennas can be  aligned without interfering with another pair of aligned antennas.
  2. Due to wide frequency bandwidth, high data trfer rate is possible.
  3. Due to line of sight propagation, it can be trmitted long distance. But a problem is faced due to its penetration properties.It can not be trfer inside a bound area. Also its trformation has obstacles like hills trees, mountains, earth curvature etc. That is why we need repeaters or tall towers for long distance trmission.
  4. As they are not reflected by ionosphere, they can be used for space trmission.

Electromagnetic waves having frequencies between 1 GHz to 300 GHz (it may be 300 MHz to 300 GHz) are called microwaves.

  1. Its cutoff frequency is zero.
  2. Its trmission line is a two-conductor system.
  3. Its wave impedance is the impedance in an unbounded dielectric.
  4. Its propagation constant is the constant in an unbounded dielectric.
  5. Its phase velocity is the velocity of light in an unbounded dielectric.

Input and output matching networks are needed to reduce undesired reflections and improve the power flow capabilities.

A uniform plane wave is a wave whose magnitude and phase are both constant. Electromagnetic waves in free space are typical uniform plane waves. The electric and magnetic fields are mutually perpendicular to each other and to the direction of propagation of the waves. The phases of the two fields are always in time phase and their magnitudes are always constant. The stored energies are equally divided between the two fields, and the energy flow is trmitted by the two fields in the direction of propagation. Thus a uniform plane wave is a trverse electromagnetic wave or a TEM wave.

It states that, “ for a passive lossless N- port network, the sum of the products of each term of any row or any column multiplied by the complex conjugate of the corresponding terms of any row or column is zero”.

There are four basic rules for boundary conditions at the surface between two different materials:

  1. The tangential components of electric field intensity are continuous across the boundary.
  2. The normal components of electric flux density are discontinuous at the boundary by an amount equal to the surface-charge density on the boundary.
  3. The tangential components of magnetic field intensity are discontinuous at the boundary by an amount equal to the surface-current density on the boundary.
  4. The normal components of magnetic flux density are continuous across the boundary.

A non-uniform plane wave is a wave whose amplitude (not phase) may vary within a plane normal to the direction of propagation. Consequently, the electric and magnetic fields are no longer in time phase.

Hybrid couplers are interdigitated microstrip couplers consisting of four parallel striplines with alternate lines tied together.  Hybrid couplers are frequently used as components in microwave systems or subsystems such as attenuators, balanced amplifiers, balanced mixers, modulators, discriminators, and phase shifters.

The point of interconnection of two or more microwave devices is called a junction. Commonly used microwave junctions include such waveguide tees as the E-plane tee, H -plane tee, magic tee, hybrid ring (rat-race circuit), directional coupler and the circulator.

Slow wave structures are special circuits that are used in microwave tubes to reduce wave velocity in a certain direction so that the electron beam and the signal wave can interact. In TWT, since the beam can be accelerated only to velocities that are about a fraction of the velocity of light, slow wave structures are used.

It is a measure of the loss of the energy in trmission through a line or device compared to the direct delivery of energy without the line or device.

It refers to the situation where amplifier remains stable for any passive source and load at the selected frequencies and bias conditions.

All electromagnetic wave has velocity of light. (Light,microwave,radio wave, infrared ray all are electromagnetic wave).

It states that the total complex power fed into a volume is equal to the algebraic sum of the active power dissipated as heat, plus the reactive power proportional to the difference between time-average magnetic and electric energies stored in the volume, plus the complex power trmitted across the surface enclosed by the volume.

When GAAs is biased above a threshold value of the electric field, it exhibits a negative differential mobility. The electrons in the lower energy band will be trferred into the higher energy band. This behavior is called trferred electron effect.

The mode having the lowest resonant frequency.

An isolator is a nonreciprocal trmission device that is used to isolate one component from reflections of other components in the trmission line.

The main applications of a reflex klystron are as follows:

  1. Signal source in MW generator
  2. Local oscillators in receivers
  3. It is used in FM oscillator in low power MW links.
  4. In parametric amplifier as pump source.

Available power gain is defined as the power available from the microwave network to that of the product from the source.

Microwave engineering is the study and design of microwave circuits , components , and systems. Fundamental principles are applied to analysis , design and measurement techniques in this field.The short wavelengths involved distinguish this discipline from electronic engineering . This is because there are different interactions with circuits, trmissions and propagation characteristics at microwave frequencies.

In microwave circuits a waveguide or coaxial-line junction with three independent ports is commonly referred to as a tee junction.

The advantages of ABCD matrix are as follows-

  1. They are used in power trmission lines
  2. They are helpful in case of cascade networks.

It is a metallic enclosure that confines the electromagnetic energy. Some cavity resonators:  rectangular-cavity resonator, circular-cavity resonator, and reentrant-cavity resonator etc.

Isolators are generally used to improve the frequency stability of microwave generators, such as klystrons and magnetrons, in which the reflection from the load affects the generating frequency. In such cases, the isolator placed between the generator and load prevents the reflected power from the unmatched load from returning to the generator. As a result, the isolator maintains the frequency stability of the generator.

A microwave system normally consists of a trmitter subsystem, including a microwave oscillator, waveguides, and a trmitting antenna, and a receiver subsystem that includes a receiving antenna, trmission line or waveguide, a microwave amplifier, and a receiver.

Non reciprocal devices are defined as devices having different forward and reverse propagating characteristics.

These are normally used to change the direction of the waveguide through an arbitrary angle.