Advanced Communication : Microwave | Electronic Engineering

INTRODUCTION

Electromagnetic Spectrum consists of entire range of electromagnetic radiation. Radiation is the energy that travels and spreads out as it propagates. The types of electromagnetic radiation that makes the electromagnetic spectrum is depicted in the figure

Properties of Microwaves

Following are the main properties of Microwaves.

• Microwaves are the waves that radiate electromagnetic energy with shorter wavelength.
• Microwaves are not reflected by Ionosphere.
• Microwaves travel in a straight line and are reflected by the conducting surfaces.
• Microwaves are easily attenuated within shorter distances.
• Microwave currents can flow through a thin layer of a cable.

Advantages of Microwaves

There are many advantages of Microwaves such as the following:

• Supports larger bandwidth and hence more information is transmitted. For this reason, microwaves are used for point-to-point communications.
• More antenna gain is possible.
• Higher data rates are transmitted as the bandwidth is more.
• Antenna size gets reduced, as the frequencies are higher.
• Low power consumption as the signals are of higher frequencies.
• Effect of fading gets reduced by using line of sight propagation.
• Provides effective reflection area in the radar systems.
• Satellite and terrestrial communications with high capacities are possible.
• Low-cost miniature microwave components can be developed.
• Effective spectrum usage with wide variety of applications in all available frequency ranges of operation.

  Disadvantages of Microwaves

There are a few disadvantages of Microwaves such as the following:

• Cost of equipment or installation cost is high.
• They are hefty and occupy more space.
• Electromagnetic interference may occur.
• Variations in dielectric properties with temperatures may occur.
• Inherent inefficiency of electric power.

Applications of Microwaves

Microwaves are radio waves radio waves with wave lengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz.

This broad definition includes both UHF and EHF (millimeter waves), and various sources use different boundaries. In all cases, microwave includes the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum, with RF engineering often putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (3 mm). The prefix “micro-” in “microwave” is not meant to suggest a wavelength in the micrometer range. It indicates that microwaves are “small” compared to waves used in typical radio broadcasting, in that they have shorter wavelengths. The boundaries between far infrared light, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study.

Microwave technology has wide range of application areas. Traditionally it has been used for telecommunication/communication purposes, but it is also used for different kinds of sensing and imaging applications. Heating of different substance such as food is another area. The application areas are many can be categories in different ways.

• Telecom
• Point-to-point communication, Satellite, Cellular access technologies
• Space
• Sensing/Spectroscopy, Communication, Radio astronomy
• MedTech
• Diagnostics, imaging, and treatment applications.
• Defense
• Radar, Communication
• Security
• Car avoidance radar, Traffic surveillance, Air traffic security “cameras”
• Navigation, Positioning & Measurement
• GPS
• Food
• Heating & detection of foreign bodies in food New and novel application areas are constantly being added.

MICROWAVE DEVICES

Just like other systems, the Microwave systems consists of many Microwave components, mainly with source at one end and load at the other, which are all connected with waveguides or coaxial cable or transmission line systems.

Following are the properties of waveguides.

• High SNR
• Low attenuation
• Lower insertion loss

MICROWAVES ─ CAVITY KLYSTRON

For the generation and amplification of Microwaves, there is a need of some special tubes called as Microwave tubes. Of them all, Klystron is an important one.

The essential elements of Klystron are electron beams and cavity resonators. Electron beams are produced from a source and the cavity klystrons are employed to amplify the signals. A collector is present at the end to collect the electrons. The whole set up is as shown in the following figure.

The electrons emitted by the cathode are accelerated towards the first resonator. The collector at the end is at the same potential as the resonator. Hence, usually the electrons have a constant speed in the gap between the cavity resonators.

Initially, the first cavity resonator is supplied with a weak high frequency signal, which has to be amplified. The signal will initiate an electromagnetic field inside the cavity. This signal is passed through a coaxial cable.

Due to this field, the electrons that pass through the cavity resonator are modulated. On arriving at the second resonator, the electrons are induced with another EMF at the same frequency. This field is strong enough to extract a large signal from the second cavity.

MICROWAVES ─ TRAVELLING WAVE TUBE

Travelling wave tubes are broadband microwave devices which have no cavity resonators like Klystrons. Amplification is done through the prolonged interaction between an electron beam and Radio Frequency RFRF field.

Construction of Travelling Wave Tube

Travelling wave tube is a cylindrical structure which contains an electron gun from a cathode tube. It has anode plates, helix and a collector. RF input is sent to one end of the helix and the output is drawn from the other end of the helix.

An electron gun focusses an electron beam with the velocity of light. A magnetic field guides the beam to focus, without scattering. The RF field also propagates with the velocity of light which is retarded by a helix. Helix acts as a slow wave structure. Applied RF field propagated in helix, produces an electric field at the center of the helix.

The resultant electric field due to applied RF signal, travels with the velocity of light multiplied by the ratio of helix pitch to helix circumference. The velocity of electron beam, travelling through the helix, induces energy to the RF waves on the helix.

The following figure explains the constructional features of a travelling wave tube.

Operation of Travelling Wave Tube

The anode plates, when at zero potential, which means when the axial electric field is at a node, the electron beam velocity remains unaffected. When the wave on the axial electric field is at positive antinode, the electron from the electron beam moves in the opposite direction. This electron being accelerated, tries to catch up with the late electron, which encounters the node of the RF axial field.

At the point, where the RF axial field is at negative antinode, the electron referred earlier, tries to overtake due to the negative field effect. The electrons receive modulated velocity. As a cumulative result, a second wave is induced in the helix. The output becomes larger than the input and results in amplification.

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