Student: Khusanboev B.S. Teacher: Arabboev M.M. Plan - Introduction
- Limits and Constraints
- How to form images
Introduction - The electromagnetic (EM) spectrum is the range of all types of EM radiation. Radiation is energy that travels and spreads out as it goes – the visible light that comes from a lamp in your house and the radio waves that comes from a radio station are two types of electromagnetic radiation. The other types of EM radiation that make up the electromagnetic spectrum are microwaves, infrared light, ultraviolet light, X-rays and gamma rays.
- Here we talk about how to imaging using electromagnetic (EM) spectrum.
Limits and Constraints - Microwaves are a form of electromagnetic radiation with wavelengths ranging from one meter to one millimetre, with frequencies between 300 MHz (100 cm) and 300 GHz (0.1 cm).
- In radio engineering the range of a microwave is between 1 and 100 GHz (300 and 3 mm).
- In all cases. Microwaves include the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum.
- Since microwave is an electro-magnetic wave (EM wave) it consists of all the characteristics of a EM wave.
- Microwaves occupy a place in the electromagnetic spectrum between ordinary radio waves and infrared light.
- It carries out photon energy of 1.24meV – 1.24microeV
- Microwave too has a frequency band containing different areas with different frequencies and wavelengths.
- Frequencies in the microwave range are often referred to by their IEEE radar band designations: S, C, X, K band or by similar NATO or EU designations.
- Microwaves are “small”, compared to the radio waves used prior to microwaves technology, in that they have shorter wavelengths.
- Microwaves travel by line-of-sight: unlike lower frequency radio waves they do not diffract around hills, follow the Earth’s surface as ground waves or reflect from the ionosphere, so terrestrial microwave communication links are limited by the visual horizon to about 40 miles (64 km).
- Microwaves are absorbed by moisture in the atmosphere and the attenuation increases with frequency, becoming a significant factor (rain fade) at the high end of the band.
- Beginning at about 40 GHz, atmosphere gases also begin to absorb microwaves, so above this frequency microwave transmission is limited to a few kilometres.
- More easily focused into narrower beams than radio waves, allowing frequency reuse (that’s why it’s used for point-to-point telecommunication).
- Microwave frequency can be measured by either electronic or mechanical techniques.
- Frequency counters or high frequency heterodyne systems can be used. Here the unknown frequency is compared with harmonics of a known lower frequency by use of a low frequency generator, a harmonic generator are a mixer. (Accuracy of the measurement is limited by the accuracy and stability of the reference source.)
- Microwaves do not contain sufficient energy to chemically change substances by ionization and so are an example of non-ionizing radiation. But long-term exposure may have a carcinogenic effect.
How to form images - Microwave imaging is a science which has been evolved from older detecting/locating techniques (e.g. radar) in order to evaluate hidden or embedded objects in a structure (or media) using electromagnetic (EM) waves in microwave regime (i.e., ~300 MHz – 300 GHz). Engineering and application oriented microwave imaging for non-destructive testing is called microwave testing.
A general view of a microwave imaging system. Applications - Microwave imaging has been used in a variety of applications such as:
- Nondestructice testing and evaluation (NDT&E)
- Medical imaging
- Concealed weapon detection at security check points
- Structural health monitoring
- Through-the-wall imaging
Nondestructive testing and evaluation - Microwave thermography (MWT) has many advantages including strong penetrability, selective heating, volumetric heating, significant energy savings, uniform heating and good thermal efficiency. MWT has received growing interest due to its potential to overcome some of the limitations of microwave nondestructive testing (NDT) and thermal NDT. Moreover, during the last few decades MWT has attracted growing interest in materials assessment.
An example for NDT&E Concealed weapon detection at security check points - It is monumentally difficult to timely detect concealed weapons carried on an individual with conventional metal detectors.
- Since microwave possesses a unique property of passing transparently through materials such as heavy clothing, it is possible to identify the presence of hidden threatening objects under clothing with microwave imaging technique.
- In addition, microwave imaging systems eliminate the use of ionizing radiation such as those present in X-ray systems and therefore, pose no known health hazard at a low power.
- In the past few years and increased interest has been addressed to three-dimensional microwave imaging.
References - Microwave – Wikipedia
- Microwave Frequency - an overview | ScienceDirect Topics
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