Introduction To Microwave Engineering

The term microwave engineering is the term which deals with signals whose wavelength is in micrometers or less than that. Thus microwave engineering deals with signals having a range of frequencies 1GHz to 300 GHz.


    Microwave Frequencies

    IEEE radar band designationsIEEE radar band designations

    Electromagnetic waves with wavelength 1 to 10 mm are called millimeter waves. Different classification schemes are in use to designate frequency bands in the electromagnetic spectrum. The frequency band designation along with there are given in Tables elaborates application of various microwave frequencies.

    Applications of microwave frequencies
    Applications of microwave frequencies


    Limitations of Conventional Circuit Theory Concepts of Microwave Frequencies:


    Limitations of Conventional Circuit Theory Concepts of Microwave Frequencies

    Microwave engineering is the termed as the branch of engineering which deals with

    engineering and design of information handling systems in the frequency range from 1to 100 GHz corresponding to wavelengths as long as 30 cm and as short as 3 mm.


    The short wavelength of the Microwave signal has a marked difference from the conventional electronics engineering. Let's try to understand this with a simple example given below.

    Consider the circuit shown in above image.



    If we consider these two different signals having different wavelengths then it can be said that

    for signal having wavelength λ1=3 meters voltage drop across any two points can be measurable whereas if we consider λ2=3 millimeters the voltage across any two points of

    circuit cannot be easily measurable.


    Thus to KCL and KVL and various circuit theories which are applicable at low frequency

    are not valid at high frequencies.


    Thus we can conclude that at low frequencies where the wavelength is much larger than greatest dimensions of circuit or system, the circuit elements such as R, L or C or electron tubes or transistor can be used as basic building blocks for information transmission reception and processing.


    At this low frequency KVL, KCL and basic network and circuit theory is applicable. At

    this frequency, considerations of propagation effects are not required. Because of these

    frequencies time delay between cause and effect at different points in these circuits is so small compared with the period of applied signal as to be negligible.


    At microwave frequencies instead of applying KVL and KCL along with basic circuit theory,

    we have to consider propagation effects of electromagnetic wave.


    So instead of considering current through or voltage across circuit element, we have to consider field effects. Therefore at microwave frequencies, we consider a propagation of electromagnetic wave. When electromagnetic wave propagates a distance of one wavelength in a time interval equal to one period of a sinusoidal time varying applied signal. This time period is inverse of signal frequency whereas one wavelength equals to λ0=velocity of light/frequency.


    Therefore as frequency increases, we cannot neglect propagation effects. Thus instead of transmission of signal at microwave frequencies we are considering propagation of electron magnetic energy at given signal frequency.


    Limitations of Low Frequency Devices:

    Q. State the limitations of low frequency devices.


    - As we know that R,L and C components stores and dissipates energy. The behaviour of these components changes at microwave signal due to its short wavelength.


    - Now let's discuss the limitations of various types of devices available at low frequency which

    we want to use at microwave frequencies.


    - Low frequency electron tube is limited finite transit time to use it at microwave frequency band. This transit time is the time taken by electron beam to travel from cathode to control grid.


    - When transit time of transistor becomes comparable to time period becomes comparable with the period of the signal being amplified, behaviour of the tube changes.


    - To overcome problem associated with transit time at microwave frequency along with decrease in distance between cathode and control grid, velocity modulation of electron beam and beam interaction with slow electromagnetic waves are the principles which are used.


    - For high power microwave devices bulk semiconductor devices are preferred.


    We know that at high frequency, power loss due to radiation becomes more. Therefore while designing microwave signal care should be taken to avoid radiation losses. Thus one of the essential requirements in microwave circuit is the ability to transfer signal power from one point to another without radiation loss. Thus guiding structures for microwave signals should be designed in such a way that they should propagate and transfer energy without radiation loss. 


    Description of Typical Microwave System:

    Q. Describe typical microwave system.


    Microwave system consists of following two subsystems:


    1. Transmitter

    2. Receiver


    The transmitter consists of microwave source, waveguide, modulators, power amplifier and transmitting antenna. Whereas at the receiving end receiving antenna, demodulator and proper terminators to gain maximum power are present.

    Since microwave system is dealing with very high frequency, designing of microwave system is quite complicated.


    Advantages and Characteristics of Microwaves:

    Q. List out different characteristics of Microwave.

    Q. Enumerate and explain advantages of microwaves.


    Following are the advantages of microwaves over low frequencies:


    1.Increased bandwidth:


    The bandwidth offered by microwaves is very large which is from 1 GHz to

    300 GHz. Therefore large number of channels can be accommodated within

    microwave band.

    - It is current trend to use microwave more and more in various long distance communication application such as Telephone network, TV, space communication, Telemetry, Defense, Railways. 


    2. Sharp Beam:


    As frequency increases, directivity will increase which results in reduced beamwidth. Reduction in beamwidth gives sharper beam. This also gives reduced antenna dimensions. This feature is very useful for scanning purpose.


    3. Fading effect and Reliability :


    Fading effect due to variation in transmission medium is more effective at low frequency. With increase in frequency fading reduces. Hence microwave communication is more reliable.


    4. Power Requirement:


    Low power is available at microwave frequencies. Thus transmitter has low power requirements. 


    5. Reduction in System Size:


    With increase in frequency size of the component reduces. Therefore at microwave frequencies, total system size and weight reduces.


    Limitations of Microwave Frequencies:

    Q. State the limitations of microwave frequencies.


    1) Line-of-sight will be interrupted if any obstacle is in the path of communication. Microwave is mostly absorbed by the atmosphere. Thus atmospheric attenuation is more at microwave frequency. 


    2)Design of microwave trans-receiver system is expensive.


    3)Available power is less.


    Applications of Microwave :

    Q. Enumerate and explain applications of microwaves. 

    1.Telecommunication:

    For Telephone, T.V, Radio transmission microwave signals are used as carrier. With microwave as carrier audio and video signal are transmitted.


    2. Optical Communication:

    When infinite Bandwidth with infinite numbers of users is the main requirement, optical communication is used.


    3. Radars:

    This application includes detect aircrafts, track supersonic missiles, air defense, aircraft/ship guidance, air traffic control, burglar alarms, polic speed detector, weather, collision avoidance and imaging


    4.Biomedical Applications:

    Diathermy for localized superficial heating deep electromagnetic heating for treatment of cancer, cautery, selective heating, hyperthermia Heart beat monitoring, sterilization and imaging.


    5.Navigation:

    Microwave signal is used for marine, land and air navigation.


    (i) Microwave landing system which is used to guide the aircraft for safety landing.

    (ii) Global positioning system (GPS).


    6. Industrial Application:

    Mainly all industrial application of microwave frequency is based on its heating properly. Following are few examples:


    (i) Drying machines: Microwave frequency based drying machines are used in

    various industry for drying purpose such as Textile, food and paper industry.

    (ii) Food processing industry: Precooling cooking/ pasteurizing/ Sterility, heat

    frozen/ refrigerated precooled meats, roasting of food grains/ Beans.

    (iii) Fluid heating System.

    (iv) Automatic Toll Collection.

    (v)Highway traffic monitoring and control.

    (vi) Flow meter.

    (vii) Rubber industry/ plastics/ chemical and forest produce industry.


    7. Domestic Application:

    The microwave oven is the most famous application of microwave signal for domestic application.


    8. Surveillance:

    This application includes security systems, Electricwarfare. It also includes identifying object or personnel by non-contact method.











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