Microwave
Electromagnetic waves having frequencies between 1GHz to 300GHz (it may be 300MHz to 300GHz) are called microwaves.
Properties of Microwaves:
Microwave are unidirectional. This means that when an antenna transmits 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.
Why? Microwave aren't reflected by Ionosphere:
The Ionosphere is that region of the earth's atmosphere in which the constituent gases are ionized by solar radiation. This region extends from about 50 km above the earth sea label and has different layers designated as C, D, E and F layers in order of height. The electron-density distribution of each layer varies with the time of day, season, year, and the geographical location. During the day the electron density N is approximately 1012 electrons per cubic meter at an altitude between 90 and 1000 km. The E and F layers have a permanent existence, but the D layer is present only during the day. The electron density determines the reflection and refraction of Electromagnetic waves. For vertical incidence, the critical frequency is given by-
This means that a Electromagnetic waves of frequency Fcr will be reflected back to the earth if the electron density is equal to or higher than the required maximum electron density Nmax (electrons per cubic meter).
As density of ionosphere varies in a range, that is why it is observed that a wave with wavelength shorter than about 4 m will not return to the earth from the ionosphere. ( velocity / frequency = wavelength ) That is why radio wave (RF) reflects back and most of microwaves don't.
Merits and Demerits of Microwaves:
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 transfer rate is possible.
3. Due to line of sight propagation, it can be transmitted long distance. But a problem is faced due to its penetration properties.It can not be transfer inside a bound area. Also its transformation has obstacles like hills trees, mountains, earth curvature etc. That is why we need repeaters or tall towers for long distance transmission.
4. As they are not reflected by ionosphere, they can be used for space transmission.
Applications of Microwave:
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.)
Reasons for using Microwave in Communication:
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.
Velocity of Microwave:
All electromagnetic wave has velocity of light but vary according to medium characteristics (Light, microwave, radio wave, infrared ray all are electromagnetic wave).
Why do we need Microwave Engineering?
Microwave System:
A microwave system normally consists of a transmitter subsystem, including a microwave oscillator, waveguides, and a transmitting antenna, and a receiver subsystem that includes a receiving antenna, transmission line or waveguide, a microwave amplifier, and a receiver. Figure 0-1 shows a typical microwave system.
Pointing Theorem:
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 transmitted across the surface enclosed by the volume.
Boundary Conditions:
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.
Why Uniform wave is a TEM wave?
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 transmitted by the two fields in the direction of propagation. Thus a uniform plane wave is a transverse electromagnetic wave or a TEM wave.
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.
Wave Propagation In Free Space:
The electromagnetic wave being propagated in free space near the surface of the earth is divided into two parts:
i) Ground wave: Classified as direct wave, earth-reflected wave and surface wave
ii) Sky wave or ionosphere wave.
Figure shows the wave components of electromagnetic wave from a non-directional antenna to a receiving station.
Properties of TEM modes in a lossless medium are as follows:
1. Its cutoff frequency is zero.
2. Its transmission 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.
Terminated line
A transmission line terminated in its characteristic impedance Zo is called a properly terminated line. Otherwise it is called an improperly terminated line.
The mode having the lowest resonant frequency
Microwave junction
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.
Tee junction
In microwave circuits a waveguide or coaxial-line junction with three independent ports is commonly referred to as a tee junction.
E-plane Tee (series Tee)
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.
H-plane Tee (shunt Tee)
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.
Magic Tees (Hybrid Tees)
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.
Waveguide Corners, Bends, and Twists
These are normally used to change the direction of the waveguide through an arbitrary angle.
Waveguide twists are used to change the plane of polarization of a propagating wave.
Directional Couplers
A directional coupler is a four-port waveguide junction. Several types of directional couplers exist, such as a two-hole directional coupler, four-hole directional coupler, reverse-coupling directional coupler (Schwinger coupler), and Bethe-hole directional coupler.
Hybrid Couplers
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.
Microwave Isolators
An isolator is a nonreciprocal transmission device that is used to isolate one component from reflections of other components in the transmission line.
Why Isolator is uniline?
An ideal isolator completely absorbs the power for propagation in one direction and provides lossless transmission in the opposite direction. Thus the isolator is usually called uniline.
How does Isolator increase frequency stability ?
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.
Non Reciprocal Devices
Non reciprocal devices are defined as devices having different forward and reverse propagating characteristics.
Cavity Resonator
It is a metallic enclosure that confines the electromagnetic energy. Some cavity resonators: rectangular-cavity resonator, circular-cavity resonator, and reentrant-cavity resonator etc.
What is the need of Quality factor Q?
Quality factor Q which is a measure of the frequency selectivity of a cavity.
A circulator is a multiport junction in which the wave can travel from one port to the next immediate port in one direction only. They are useful in parametric amplifiers, tunnel diode, amplifiers and duplexer in radar.
It is a two port device that has a relative phase difference of 180˚ for transmission from port1 to port2 and no phase shift for transmission from port2 to port1.
What is the function of input and output matching networks?
Input and output matching networks are needed to reduce undesired reflections and improve the power flow capabilities.
When the electric field is varied from zero to threshold value, the carrier drift velocity is increased from zero to maximum, when the electric field is beyond the threshold value of 3000 V/ cm , the drift velocity is decreased and the diode exhibits negative resistance.
Transferred Electron Effect
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 transferred into the higher energy band. This behavior is called transferred electron effect.
When the doping level is increased, the depletion region reduces. Due to thin depletion region, even for very small forward bias, many carriers penetrate through the junction and appear at the other side. This phenomenon of penetration of carriers through the depletion region is known tunneling.
It is a power sensor whose resistance changes with temperature as it absorbs microwave power. Examples: Barretter, Thermistor.
The microwave power meter consists of a power sensor, which converts the microwave power into energy. The corresponding temperature rise provides a change in the electrical parameters resulting in an output current in low frequency circuitry and indicates the power.
Slotted line is a fundamental tool for microwave measurements. Slotted line consists of a section of waveguide or coaxial line with a longitudinal slot. The slot is roughly 1 mm wide and allows an electric field probe to enter the waveguide for measurement of the relative magnitude of field at location of the probe.
Maximum velocity of charge carriers in a semiconductor is called saturation drift velocity.
Homo- Junction Transistor
When the transistor junction is joined by two similar materials such as silicon- to silicon or germanium-to-germanium, it is called a homo junction transistor.
Hetero junction transistor
When the transistor junction is joined by two different materials such as Ge to GaAs, then it is called a homo junction transistor.
What is the need of diffusion and ion implantation?
Diffusion and ion implantation are the two processes used in controlling amounts of dopants in semiconductor fabrications.
It is a measure of the loss of the energy in transmission 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.
Noise figure F is defined as the ratio of the input SNR to the output SNR
Available power gain is defined as the power available from the microwave network to that of the product from the source.
Power gain of an amplifier
It is defined as the ratio of power delivered to the load to that of the power from the source into an amplifier.
The reflection loss is a measure of power loss during transmission due to the reflection of the signal as a result of impedance mismatch.
Zero property of S matrix
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”.
The return loss is a measure of the power reflected by a line or network through a line.
When feedback effect of the amplifier is neglected ( S12 = 0), the amplifier power gain is known as unilateral power gain.
Scattering matrix is a square matrix which gives all the combinations of the power relationships between the various input and output ports of a microwave junction.
In lossless passive network, the power entering the circuit is always equal to power leaving network which leads to the conservation of power.
