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• Electromagnetic Spectrum
• Wireless Communication – Different Frequency Bands and Their Applications

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• As per Maxwell’s theory accelerated charges radiate electromagnetic waves.
• Key contribution of various scientists:
  • Hertz: He experimentally demonstrated that accelerated charged particles emitted electromagnetic waves. [Hertz Experiment 1887] (He did it for low frequency – Radio waves)
  • JC Bose working at Kolkata succeeded in producing and observing electromagnetic waves of much shorter wavelength (25 mm to 5mm). His experiment like that of Hertz was confined to the laboratory.
  • Guglielmo Marconi followed Hertz work and succeeded in transmitting electromagnetic waves over distances of many kilometers. Marconi’s experiment marked the beginning of the field of communication using electromagnetic waves.
• Key Features of Electromagnetic Waves:
  • Electric and Magnetic field are perpendicular to each other, and to the direction of propagation.

The adjacent figure shows a linearly polarized electromagnetic wave propagating in the z-direction with the oscillating electric field E along the x direction and the oscillating magnetic field B along the y-direction.

• • They are self-sustaining oscillations of electric and magnetic fields in free space or vacuum.
  • It can travel in vacuum and no material medium is involved in the vibrations of the electric and magnetic fields.
  • In vacuum (free space), electromagnetic wave travels with a speed of light 2.99792458 x 10⁸ m/s (or roughly 3 x 10⁸ m/s).
    • The constancy of the velocity is EM waves in vacuum is so strongly supported by experiments and the actual value is so well known now that this is used to define a standard of length.
  • Hertz has also established wave nature of the radiation. He demonstrated that the waves, which had wavelength ten million times that of the light waves, could be diffracted, refracted, and polarized.

1)  Electromagnetic Spectrum

At the time Maxwell predicted the existence of electromagnetic waves, the only familiar electromagnetic waves were the visible light waves. The existence of ultraviolet and infrared waves was barely established. By the end of the nineteenth century, X-rays and gamma rays had also been discovered

• Electromagnetic Waves include radio waves, microwaves, infrared, visible light, ultraviolet, x rays and gamma rays. The classification of EM waves according to frequency is the electromagnetic spectrum. Note, that there is no sharp division between one kind of wave and the next. The classification is based on roughly on how the waves are produced and/or detected.

Different Types of Electromagnetic waves, in order of increasing frequency/decreasing wavelength:

A) Radio waves:

• They are produced by accelerated motion of charges in the conducting wires.
• Uses: They are used in Radio and Television communication.
• Wavelength: They range from around a foot long to several kms.
• Frequency ~: 500 KHz to 1000 MHz
  • The AM (Amplitude Modulated) band is from 530 KHz 1710 KHz.
  • The FM (Frequency Modulated) band is from 88 MHz to 108 MHz.
  • The TV waves range from 54 MHz to 89 MHz.
  • Cellular phones use radio waves to transmit voice communication in the Ultra High Frequency (UHF) band.
    • For e.g., in 2014, the DoT auctioned 2G telecom spectrum in the frequency range of 900 MHz and 1800 MHz.
    • For e.g., In 2022 auction, Jio bought frequencies in 700 MHz as well as in 1800 MHz band.
    • In 2022, 700 MHz was sold for the first time. Jio bought the spectrum,

B) Microwaves

• Microwaves (short wavelength radio waves) are produced by special vacuum tubes (called klystrons, magnetrons, and Gunn Diodes).
• Frequency: GHz range
• Applications:
  • Radar: There short wavelength makes them suitable for Radar system in aeroplanes. Due to their short wavelength, they are suitable for Radar systems used in aircraft navigation. Radar also provides the basis for the speed guns used to time fast balls, tennis serves, and automobiles.
  • Microwave Ovens are an interesting application of these waves. In such ovens, the frequency of microwaves is selected to match the resonant frequency of water molecules so that energy from the waves is transferred efficiently to kinetic energy of the molecules. This raises the temperature of any food containing water.

Details of how microwave work: When the temperature of a body rises, the energy of the random motion of atoms and molecules increases and the molecules travel or vibrate or rotate with higher energies. The frequency of rotation of water molecules is about 300 crore hertz, which is 3 gigahertz (GHz). If water receives microwaves of this frequency, its molecules absorb this radiation, which is equivalent to heating up water. These molecules share this energy with neighbouring food molecules, heating up the food. One should use porcelain vessels and not metal containers in a microwave oven because of the danger of getting a shock from accumulated electric charges. Metals may also melt from heating. The porcelain container remains unaffected and cool, because its large molecules vibrate and rotate with much smaller frequencies, and thus cannot absorb microwaves. Hence, they do not get heated up. Thus, the basic principle of a microwave oven is to generate microwave radiation of appropriate frequency in the working space of the oven where we keep food. This way energy is not wasted in heating up the vessel. In the conventional heating method, the vessel on the burner gets heated first, and then the food inside gets heated because of transfer of energy from the vessel. In the microwave oven, on the other hand, energy is directly delivered to water molecules which is shared by the entire food.

C) Infrared Waves

• Produced by hot bodies and molecules. They are sometimes also referred as heat waves. This is because, water molecules produced in most materials readily absorb infrared waves (many other molecules, for example, CO2, NH3, also absorb infrared waves). After absorption, their thermal motion increase i.e. they heat up and heat up their surroundings.
• Infrared lamps are used in physical therapy.
• Infrared waves also play a crucial role in maintaining the earth’s warmth or the average temperature through Greenhouse Effect.
• Infrared Emitting Devices (IrEDs) are used in remotes of TV, AC

D) Visible Rays

• Part of the spectrum detected by Human eyes.
• Frequency range: 4 * 10¹⁴ Hz to about 7*10¹⁴ Hz.
• Wavelength: 700 nm to 400 nm (note: Speed of light = frequency * Wavelength)
• Note: Different animals are sensitive to different electromagnetic spectrum. For e.g. snakes can detect infrared waves, and the ‘visible’ range of many insects extends well into ultraviolet.

E) Ultraviolet Rays

• Wavelength: 400 nm to 0.6 nm
• UV radiations are produced by special lamps or very hot objects. For e.g. Sun is an important source of ultraviolet rays, but fortunately, most of the radiation is absorbed in the ozone layer. This is because UV radiation in large quantities will be harmful for human health and other forms of biodiversity.
• Applications:
  • Due to very short wavelengths, UV radiation can be focused on very narrow beams for high precision application such as LASIK (Laser assisted in situ keratomileusis) eye surgery.
  • UV lamps are used to kill germs in water purifiers.

F) X-Rays

• Wavelength: 10 nm to 10^-4 nm (10^-13m)
• One common way of generating X-Rays is to bombard a metal target by high energy electrons.
• Applications:
  • They are used in diagnostic tools in medicine and as a treatment for various kinds of cancer.

G) Gamma Rays

• Wavelength: 10^-10 m to 10^-14 m
• Produced in nuclear reactions and also emitted by radioactive nuclei. They are used in radiative cancer therapies.

H) Penetration of various EM Waves in Earth’s Atmosphere

• The Earth’s atmosphere stops most type of EM radiation from reaching earth’s surface. This illustration shows how far into the atmosphere different parts of EM spectrum can go before being absorbed. Only portions of radio and visible light reach the surface.
• Radio frequencies, visible light and some part of ultraviolet lights makes it to sea level. These wavelength ranges are called atmospheric window. Ground based astronomical observation employs optical and radio telescopes that take advantage of atmospheric windows.
• Astronomers can observe some infrared wavelengths by putting telescopes on mountain tops.
• But, earth’s atmosphere absorbs the majority of ultraviolet, X-Rays, and gamma rays. So they can only be absorbed using balloons and astronomical satellites outside the earth’s atmosphere.
• Note: Long wavelength radio waves and infrared rays also don’t reach the surface.
• Note:

1)  Wireless Communication – Different Frequency Bands and Their Applications

A) Radio Waves (500 KHz – 1 G Hz)

• The AM (Amplitude Modulated) band is from 530 KHz 1710 KHz.
• The FM (Frequency Modulated) band is from 88 MHz to 108 MHz.
• The TV waves range from 54 MHz to 89 MHz
• Used for broadcasting radio and TV Programmes. Anyone with receiver can tune it to the radio frequency to pick the signal. When radio stations use similar transmission frequencies the waves sometimes interfere with each other.
• Medium wavelength radio waves are reflected from the ionosphere so they can be used for long distance communication, but not for communicating with satellite above the ionosphere. Thus, they can only be used for low earth orbit satellite communication.
• AM vs FM

	AM	FM
Full form	AM stands for Amplitude modulation	Frequency modulation
First used	AM method of audio transmission first carried out in the mid-1870s	FM radio was developed in the US states in the 1930s, mainly by Edwin Armstrong
Modulating difference	In AM, a radio wave known as the “carrier” or “carrier wave” is modulated in amplitude by the signal that is to be transmitted. The frequency and the phase remain same	In FM, a radio wave known as carrier wave is modulated in frequency by the signal that is to be transmitted. The amplitude and phase remains the same.
Pros and Cons	AM has poorer sound quality compared with FM but is cheaper and can be transmitted over long distances. It has lower bandwidth so it can have more stations available in any frequency range.	FM is less prone to interference than AM. However, FM signals are impacted by physical barriers. FM has better sound quality due to higher bandwidth.
Frequency Range	AM radio ranges from 535 to 1705 KHz (OR) Up to 1200 bits per second.	FM radio ranges in a higher spectrum from 88 to 108 MHz. (OR) 1200 to 2400 bits per second
Bandwidth requirement	Twice the highest modulating signal. In AM radio broadcasting, the modulating signal has bandwidth of 15 KHz, and hence the bandwidth of an amplitude-modulated signal is 30 KHz	Twice the sum of the modulating signal frequency and the frequency deviation. If the frequency deviation is 75 KHz and the modulating signal frequency is 15 KHz, the bandwidth required is 180 KHz.
Zero crossing in modulated signal	Equidistant	Not Equidistant
Complexity	Transmitter and receiver are simple but synchronization is needed in case of SSBSC AM carrier.	Transmitter and receiver are more complex as variation of modulating signal has to be converted and detected from corresponding variation in frequencies (i.e. voltage to frequency and frequency to voltage conversion has to be done)
NOISE	AM is more susceptible to noise because noise effects amplitude, which is where information is stored in AM.	FM is less susceptible to noise because information in an FM signal is transmitted through varying of frequency, and not amplitude.

A) MICROWAVES

• Microwaves have shorter wavelength and thus can pass through ionosphere. They can thus be used for long distance satellite communications.
• Line of sight -> Prerequisite:
• Signals are sent to and from satellites, which relay signals around the Earth. This may be for TV programmes, telephone conversations or monitoring the earth, for example weather forecasting.
• Types

L Band: 1-2 GHz

• Low bandwidth -> not suitable for streaming applications like video, voice, and broadband connectivity.
• Radars, GPS signals
• Other advantages -> least expensive and easiest to implement.

S Band:

• It is a part of microwave band of the electromagnetic spectrum. It is defined by IEEE standard of radio waves with frequencies that range from 2 to 4 GHz, crossing the conventional boundary between UHF (Ultra High Frequency) and SHF (Super High frequency) at 3.0 GHz.
• Used by
  • Weather radar
  • Surface ship radar
  • Some Communication Satellites

C Band

• The C Band is the name given to certain portions of the electromagnetic spectrum, including wavelength of microwaves that are used for long-distance radio-telecommunication.
• The IEEE C Band (4 – 8 GHz) and its slight variations contain frequency ranges that are used for many satellite communication transmission, some Wi-Fi devices, some cordless telephones, and some weather radar system.

K_u Band

• Name given to 12-18 GHz portion of electromagnetic spectrum in the microwave range of frequencies.
• Uses
  • Primarily used for satellite communication, most notably for fixed and broadcast services

K Band (18-27 GHz)

Ka Band (27 – 40 GHz)

V Band (40 – 75 GHz)

W Band (75-110 GHz)

Millimeter Band (110-300 GHz)

D) Deep Space Optical Communication

1. Why in news?
   1. NASA’s Deep Space Optical Communication Demo sends, receives first data (Nov 2023)
2. Need of Deep Space Optical Communication:
   1. Low bandwidth of radio frequency communications: Future space missions are going to required higher bandwidth of communication as they will need to transmit higher volumes of science data, images, videos
   2. Higher frequencies (shorter wavelengths) which can carry more data suffer from the problems of getting blocked by atmosphere, and higher scattering when it is contacted with any interference.
3. NASA’s Psyche Spacecraft is on its way to Psyche asteroid and will reach there by 2029. But in between it is involved in experiments related to Deep Space Optical Communication (DSOC).
4. Primary Objective of DSOC is to give tools and technology to future NASA initiatives to communicate at much higher bandwidth.
5. Demo:
   1. DSOC has achieved ‘first light’ sending data via laser to and from far beyond the Moon for the first time.
   2. NASA’s DSOC experiment has beamed a near-infrared laser encoded with test data from nearly 16 million kms away – about 40 times further than the Moon is from Earth – to the Hale Telescope at Caltech’s Palomar Observatory in San Diego County, California. This is the farthest ever demonstration of optical communication.
6. Key features:
   1. It is pioneering the use of near-infrared laser signal for communication with spacecraft.
   2. Its bandwidth is more than 10 times higher that the state of art radio-telecommunication system of comparable size and power. This enables higher resolution images, larger volumes of science data, and streaming of videos.
7. Advantages: Higher Bandwidth, faster data transmission, improved image resolution, reduced power consumption, potential for streaming video and real-time communication
8. How were the limitations of high frequency communication overcome?
   1. Extremely precise pointing: To achieve this, the transceiver aboard the spacecraft needs to be isolated from the craft’s vibration.
   2. Compensating for movements of spacecraft and Earth: The targeting has to adjust for this continuous movement.
   3. Extracting information from weak signal: Since the signal will travel several million kms, the received signal will be very weak. New Signal processing tools have to be utilized to extract precise information from the communication.
9. Psyche spacecraft is the first to carry a DSOC transceiver and will be testing high bandwidth optical communications to Earth during the first two years of the spacecraft’s journey to the main asteroid belt.
   1. Achieving the first light is one of many critical DSOC milestones in the coming months, paving the way toward higher-data-rate communication.
10. Has Space based optical communication happened in past?
    1. In 2013, NASA’s Lunar Laser Communications Demonstration tested record breaking uplink and downlink rates between Earth and the Moon using similar technology.
    2. But DSOC is taking optical communication to Deep Space, paving the way for high-bandwidth communication far beyond the Moon and over 1,000 times farther than any optical communication test to date.
11. Significance:
    1. The DSOC holds the key for future space missions. As humans travel deep into space, they would want fast way of sending and receiving large amount of data from earth.
    2. It would pave the way for high data rate communications capable of sending scientific information, high-definition imagery, and streaming video in support of humanity’s next giant leap: Sending humans to Mars.