Electro-Magnetic Waves (EM Wave) – Ibex Aviation Academy

Properties of Electro-Magnetic Waves – Basics of EM Waves used in Radio Navigation

Electromagnetic (EM) waves are waves that carry energy from one place to another. They do not need a material medium, so they can travel through air, water, and even the vacuum of space. EM waves include radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. They are widely used in communication, medicine, navigation, and aviation.

Electrical and Magnetic Field

Radio aids in aviation use Electro-Magnetic (EM) Waves.
EM waves are created by the movement of alternating current (AC) in a wire.
Alternating current creates a magnetic field perpendicular to the electric field.
EM radiation is transmitted through air if the wire is connected to an aerial.
EM radiation consists of electrical and magnetic components.
The electrical field lies in the same plane as the aerial.
The magnetic field is perpendicular to the electrical field.

Electro-Magnetic Spectrum

EM waves can travel through a vacuum and have a wide spectrum.
Spectrums differ in wavelength, frequency, and energy.
Visible light is the only part of the EM spectrum visible to the naked eye.
X-rays have higher frequencies and shorter wavelengths.
Long-wave audio signals have lower frequencies and longer wavelengths.

Short Wave and Long Wave

Short waves have high levels of energy and are therefore more harmful to the body.
EM waves travel at the speed of light in a vacuum regardless of their spectrum.
Speed of light in a vacuum is approximately 3 × 10⁸ m/s (about 186,000 miles/s).

Polarisation of Electromagnetic waves

Horizontal and Vertical Polarisation

Electrical and magnetic components of EM waves are perpendicular to each other.
Polarisation is the plane of the electrical component, which carries the information.
Vertical and horizontal aerials transmit vertically and horizontally polarised waves respectively.
Vertically polarised waves can only be received by a vertical aerial.
Horizontally polarised waves can only be received by a horizontal aerial.

Circular Polarisation

Circularly polarised waves are created by a helical antenna.
Circular polarisation can be received by various types of antennas.
Used when polarisation changes during transmission.
Reduces rain clutter but requires higher transmitter power.

Relationship between Time-Period, Wavelength and Speed of EM Waves

Speed of EM Wave

EM radiation is transmitted as a sinusoidal (sine) waveform.
The frequency of the wave is the same as the alternating current producing it.
The speed of an EM wave is equal to the speed of light.
The speed is maximum in a vacuum.
Radio waves travel faster over water than over land.

Time Period of EM Wave

One cycle is one complete waveform.
A cycle extends from the mean position to maximum, minimum, and back to the mean position.
The time period is the time required to complete one cycle.
Time period is usually expressed in microseconds (10^-6 seconds).

Frequency and Wavelength

Amplitude is the maximum displacement from the mean position.
Frequency (Hz) is the number of cycles completed each second.
Wavelength (m) is the distance travelled during one cycle.
Frequency and wavelength are inversely proportional.

Speed – Frequency – Wavelength

Speed is the distance travelled by an EM wave in one second.
Wave speed is the product of wavelength and frequency.
Speed = Wavelength × Frequency
In a vacuum, the speed equals the speed of light.

Formula

Speed = Frequency × Wavelength

Frequency = Speed of Light ÷ Wavelength

Wavelength = Speed of Light ÷ Frequency

Units of Frequency Measurement

1 Hertz (Hz) = 1 cycle per second.
1 Kilohertz (kHz) = 1,000 Hz = 10³ Hz.
1 Megahertz (MHz) = 1,000 kHz = 10⁶ Hz.
1 Gigahertz (GHz) = 1,000 MHz = 10⁹ Hz.

Classification of Radio Waves

Radio Wave Frequency Bands

Band	Frequency	Wavelength
VLF	3–30 kHz	100–10 km
LF	30–300 kHz	10–1 km
MF	300–3000 kHz	1000–100 m
HF	3–30 MHz	100–10 m
VHF	30–300 MHz	10–1 m
UHF	300–3000 MHz	100–10 cm
SHF	3–30 GHz	10–1 cm
EHF	30–300 GHz	10–1 mm

Power and Phase of Radio Waves

Phase Difference Between EM Waves

Phase is a defined position on a sine waveform.
It is expressed from 0° to 360°.
Phase difference is the difference in phase between two waves.
It is determined by comparing waveforms.
Phase comparison is only possible between waves of the same frequency.
This principle is used in some radio navigation equipment.

Power of Radio Wave

Radio wave power decreases as distance from the transmitter increases.
The inverse square law governs electromagnetic radiation.
Available power is inversely proportional to the square of the distance.
To double the range, transmitter power must be increased fourfold.

Polar Diagram

A polar diagram joins points of equal signal strength.
The antenna shape determines the polar diagram.
For transmitters, it joins points receiving half the transmitted power.
Polar diagrams are used for both transmitters and receivers.
Receiver polar diagrams connect points receiving half the received power.

Attenuation – Loss of Power during Transmission

Surface Attenuation

Attenuation is the weakening of radio waves during transmission.
It follows the inverse square law.
Surface attenuation results from absorption by the Earth’s surface.
Vertically polarised waves experience less surface attenuation.
Horizontally polarised waves experience greater attenuation.
Surface attenuation increases with frequency.
Very Low Frequency (VLF) bands are used for submarine communication.

Factors Affecting Signals

Surface attenuation is greater over land than over sea.
Land range is proportional to three times the square root of power.
Sea range is proportional to twice the square root of transmitter power.
Reflection attenuation is caused by buildings and hills.
Taller aerials reduce reflection attenuation.

Fading of Signals

Fading causes fluctuations in signal strength.
This is known as waxing and waning.
Fading occurs when out-of-phase signals arrive via different paths.

Atmospheric Attenuation

Atmospheric attenuation weakens radio waves.
It is caused by atmospheric absorption and static interference.
Absorption results from particles in the atmosphere.
Higher frequencies suffer greater atmospheric absorption.
Frequencies around 5 GHz are heavily affected.
The wavelength at these frequencies is similar to the size of water droplets.

Static Interference

Static interference is caused by electrical charges in the atmosphere.
It increases during rain, thunderstorms, and solar activity.
Lower frequencies experience greater static interference.
Electronic circuits can also generate static noise.

Signal-to-Noise Ratio (SNR)

SNR is the ratio of signal strength to noise, expressed in decibels (dB).
It can be improved by increasing transmission power.
Doubling range requires four times the transmitter power.
Receiver sensitivity can also be improved by reducing internal noise.
This enables reception of weaker signals.

Change in Direction of Radio Waves during Transmission

Diffraction and Refraction

Diffraction is the bending of radio waves around the Earth’s surface.
Lower frequencies (VLF, LF, HF) experience greater diffraction.
Refraction is the bending of waves due to changes in propagation speed caused by changes in medium density.

Ionospheric Refraction

Occurs when radio waves slow down in the ionosphere.
Some refracted waves return to Earth.
Ionospheric refraction decreases as frequency increases.

Super Refraction

Normally affects only lower frequencies.
Special atmospheric conditions may refract higher frequencies.
Caused by high-pressure systems.
Also caused by warm air flowing over a cold surface.

Sub Refraction

Caused by atmospheric conditions.
Associated with low-pressure systems.
Occurs when cold air flows over a warm surface.
Super and sub-refraction can unexpectedly increase VHF range.

Coastal Refraction

Coastal refraction affects radio aids located near coastlines.
Occurs when radio waves cross the land-sea boundary.
Radio waves travel faster over sea than over land.
They bend toward the slower medium (land).
Coastal refraction is less significant at higher frequencies.
Non-Directional Beacons (NDBs) near coastlines often use higher frequencies to minimize this effect.