Electro-Magnetic Waves (EM Wave)

Electro-Magnetic Waves

Electrical and Magnetic Field in EM Waves

• EM waves are created by movement of Alternating current (AC) in a wire
• Radio aids in aviation use Electro Magnetic Waves (EM Waves)
• Alternating current creates a magnetic field perpendicular to electric field
• EM Radiation is transmitted through air if the wire is connected to an Ariel
• EM Radiation consists of electrical and magnetic components
• Electrical field in the same plane of the ariel
• Magnetic field perpendicular to the electrical field

Electro Magnetic Spectrum

• EM waves has a wide spectrum with differing wavelength, frequency and energy
• Visible light is the only part of EM Spectrum seen by a naked eye
• X Ray spectrum have higher frequencies and lower wavelength
• Long wave audio spectrum have lower frequencies and higher wavelength
• Short waves have high levels of energy, hence harmful to body
• EM waves travel at the speed of light in vacuum irrespective of their spectrum
• Speed of light in vacuum is 1,62,000 nm/sec or 3 x 10² m/sec

Horizontal and Vertical Polarisation

• Polarisation is the plane of the electrical component which carries the message
• Electrical and magnetic components of EM waves are perpendicular to each other
• Vertical and Horizontal ariels transmit vertically and horizontal polarised waves
• Vertically polarised wave can be received only by a vertical ariel
• Horizontally polarised wave can be received only by a horizontal ariel

Circular Polarisation

• Circular polarisation can be received by both horizontal and vertical antenna
• Circularly polarised waves are created by a helical antenna
• These are used if the polarisation changes during transmission
• Circular polarisation reduces rain clutter but requires high power transmitters

Speed of EM Wave

• Speed of an EM wave is the same as speed of light
• The speed of EM waves is maximum in vacuum at 3 x 10² m/sec
• Speed over water higher than that its speed over land

Time Period of EM Wave

• Time period is the time taken by the wave to complete one cycle
• EM radiation is transmitted as a sinusoidal or sine wave form
• Amplitude of a wave is the maximum displacement of wave from its mean position
• One cycle of the wave is means one complete set of values
• For example, mean position to maximum to minimum and back to mean position is called one cycle
• Time period is the time time taken to complete one cycle in micro seconds

Frequency – Wavelength

• Frequency of the wave is same as the current which caused the wave
• Frequency in hertz is the number of cycles of the wave in one second
• Wave length in meters is the distance travelled by the wave in one cycle
• Frequency and wavelengths are inversely proportional to each other
• Speed is the distance travelled by an EM wave in one second
• Speed = Wave length x Frequency

Formula connecting Speed, Frequency and Wavelength

• Speed of an EM Wave is a product of its wavelength and frequency
• Speed of EM Waves in Meters per Sec = Frequency in Hertz x Wavelength in Meters
• Frequency in Hertz = Speed of Light in Meters per sec / Wave Length in Meters
• Wave Length in Meters = Speed of Light in Meters per sec / Frequency in Hz

Units of Frequency Measurement

• Frequency is measures in Hertz, Kilohertz,
• 1 Hertz = 1 Cycle Per Second and 1 Kilo Hertz = 1000 Hz = 10³ Hertz
• 1 Mega Hertz = 1000 K Hz= 106 Hertz and 1 Gigha Hertz = 1000 M Hz= 109 Hertz

Radio Wave Frequency Bands

• Frequencies and Wavelengths are used to distinguish between various bands
• VLF Band – Frequency 3-30 KHz – Wavelength 100-10 KM – Used in Submarine Communication
• LF Band – Frequency 30-300 KHz – Wavelength 10-1 KM – Used in Non-Directional Beacon NDB
• MF Band – Frequency 300-3000 KHz – Wavelength 1000-100M – Used in Long Range NDB
• HF Band – Frequency 3-30 MHz – Wavelength 100-10 M – Used in Long Range Communication
• VHF Band – Frequency 30-300 MHz – Wavelength 10-1 M – Used in VHF Communication
• UHF Band – Frequency 300-3000 MHz – Wavelength 100-10 CM – Used in Instrument Landing System
• SHF Band – Frequency 3-30 GHz – Wavelength 10-1 CM – Used in Radar
• EHF Band – Frequency 30-300 GHz – Wavelength 10-1 M – Used in Satellite Communication

Phase Difference between EM Waves

• Phase Difference is the difference in Phase between two waves
• Phase of a EM wave is a defined position on the sine wave form expressed from 0 to 360 degrees of a circle
• Phase difference is obtained by phase comparison of two waves of same frequency and wavelength
• Principle of phase comparison is used in some radio navigational equipments

Power of Radio Wave and Polar Diagram

• Inverse square law of electro magnetic radiation defines the power required
• Power of a radio wave reduces with increase in distance from the transmitter
• Power available is inversely proportional to the square of the range
• Therefore, power transmitted has to be increased four times to double the range
• Polar diagram is the line joining points of equal power or intensity
• Polar diagram can be drawn for both transmitter as well as receiver
• Polar diagram of transmitter connects points of half of the the power transmitted
• Polar diagram of receiver connects points of half the power received
• Shape of ariel determines the shape of polar diagram

Surface Attenuation

• Surface attenuation occurs due absorption of energy by the earth’s surface
• Attenuation is the weakening of radio wave during transmission with distance
• Attenuation of radio waves follow inverse square law of electro magnetic radiation
• Vertically polarised waves have lesser surface attenuation since they have lesser contact with earth
• Horizontally polarised waves have greater surface attenuation due to constant contact with earth surface
• Surface attenuation is least in very low frequency and increases with frequency of the radio wave
• Therefore, Very Low frequency bands are used for submarine communications

Factors affecting Surface Attenuation

• Surface attenuation greater over land surface than over smooth sea surface
• Land range is proportional to twice times the square root of power
• Sea range is proportional to thrice the square root of power transmitted
• Reflection attenuation is a form of surface attenuation which is caused due reflections from buildings and hills
• Installation of tall ariels for transmission reduces reflection attenuation

Fading of Signals

• Fading causes Waxing and Waning of audio signals or increase and decrease in voice signals
• Fading is caused by out of phase signals from the same transmitter received through different paths

Atmospheric Attenuation

• Atmospheric attenuation causes weakening of radio waves due to Atmospheric Absorption and Static Interference
• Atmospheric absorption or scatter is caused by small particles in atmosphere
• High frequency radio waves are affected more by atmospheric absorption
• Frequencies around 5 GHz are affected badly by absorption due to their wavelength similar to water droplets

Static Interference

• Static interference is reduction in signal strength caused due to negatively charged particles in atmosphere
• Negative charges build up due to rain, thunderstorm or solar activity
• Static interference is higher in lower frequencies affecting voice clarity
• Equipment’s electronic circuits also cause static interference

Signal to Noise Ratio

• Signal to noise ratio is the ratio of strength of audio signal to static noise in decibels
• Better signal to noise ratio is achieved by increasing Transmission Power or improving Receiver Sensitivity
• Increasing transmission power would be as per Inverse Square Law of Electromagnetic Radiation
• Therefore, doubling the range requires four times increase in power
• Improving receiver sensitivity reduces internal noise by better manufacturing quality
• Receiver sensitivity improves receiver’s ability to receive weaker signals

Diffraction and Refraction

• Radio waves alter their path during propagation by Diffraction and Refraction
• Diffraction is bending of radio waves around the surface of earth
• High diffraction is seen in lower frequencies like VLF, LF and HF
• Refraction is bending of radio waves due to change in speeds with density of medium
• For example, density of air over land and sea surface have different densities

Ionospheric Attenuation and Refraction

• Radio waves are Attenuated and Refracted by the Ionosphere and a part of radio wave returns to earth
• Ionosphere is a layer of free electrons in the upper levels of Stratosphere
• Ionospheric Refraction is caused by slowing down of radio waves in ionosphere
• Increasing frequency reduces Ionospheric Refraction and exit the atmosphere as Escape Rays

Super and Sub Refraction

• Super and sub-refraction increase the range of VHF band unexpectedly
• Normally only lower frequencies are affected by ionospheric refraction
• Special atmospheric conditions cause super and sub refraction in higher frequencies
• Super refraction is caused due to High pressure system and Warm air flowing over cold surface
• Sub refraction is caused due to Low pressure system and Cold air flowing over warm surface

Coastal Refraction

• Coastal refraction causes an error in radio aids located close to coast line
• Bending of radio waves while crossing coast line result in coastal refraction
• Speed of radio waves is faster over sea due to change in medium
• Radio waves bend towards land which is the slower speed medium
• Coastal refraction is lesser in comparatively higher frequencies
• Therefore, Non directional beacons near coastline use higher frequencies

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