Comprehensive Study Guide on Radio Wave Propagation

RADIO WAVE PROPAGATION FUNDAMENTALS

Introduction to Wave Propagation

Of the various avionic subjects studied by technicians, radio wave propagation theory is uniquely stable, with basic principles remaining largely unchanged over the last $70$ years. Propagation is defined as "movement through a medium."

Radio waves are a form of radiant energy, similar to light and infrared radiation (radiant heat). While they are invisible to the human senses of sight and touch, they can be detected via sensitive measuring devices. A primary characteristic is that all electromagnetic waves travel at the speed of light.

Directionality and Focus

Radio waves can behave in different ways depending on the application:

• Omnidirectional: Spreading out in all directions, similar to light from a bulb in a darkened room.
• Focused: Concentrated into a narrow beam, analogous to a flashlight beam.

Intercom Systems and Sound Principles

Nature of Sound

Audio frequencies are defined as those perceptible by the human ear. Sound is produced by mechanical vibrations that alternately compress and expand the surrounding air.

• Compressions: Regions where air particles move closer together.
• Rarefactions: Regions where air particles move further apart.
• Propagation: These pressure changes are transmitted to adjacent air particles as longitudinal waves.

Perception and Characteristics

• The Ear: The most technically impressive sound receiver, capable of detecting sound density changes of less than one ten-millionth of $1\,\%$ ($0.0000001\,\%$). This corresponds to a particle displacement of less than $0.0000001\,mm$.
• Pitch: Determined by frequency (rate of vibrations).
• Hertz (Hz): The unit of frequency, where $1\,Hz$ equals $1\,\text{vibration per second}$ or $1\,\text{cycle per second}$.
• Human Hearing Range: Sensitive to frequencies between approximately $20\,Hz$ and $20,000\,Hz$.

Transduction: Microphones and Speakers

Microphones

A microphone is a transducer that transforms sound into electrical signals. These signals can be amplified, recorded, and eventually converted back into sound.

• Mechanism: Most microphones contain a thin membrane (a diaphragm or ribbon) that vibrates in response to sound.
• Early Microphones: Consisted of a metal diaphragm attached to a needle that scratched a path onto rotating metal foil.
• Electronic Conversion: Modern microphones convert pressure waves into varying electrical signals, typically a Direct Current ($DC$) signal that varies in strength. Rapid changes in high and low voltages correspond to the compressions and rarefactions of sound.

Microphone Technologies

1. Carbon Microphones: The simplest and oldest type. Sound waves hit a diaphragm that compresses carbon dust, changing its resistance ($R$). Current flow ($I$) varies as resistance changes.
2. Dynamic Microphones: Utilize electromagnetic induction. As a diaphragm moves due to sound, it moves either a magnet or a coil of wire, inducing a small current ($I$).

Speakers and Amplifiers

• Speakers: Reverse the microphone process. Electrical current reaches wire coils, turning it back into physical movement that vibrates the air.
• Amplifiers: Required because microphoned outputs are very low voltage. The amplifier increases the input voltage to a higher-power signal to drive speakers. Volume is controlled by the amplifier gain (the volume knob).

Modes of Communication

Three distinct modes are used in the aircraft industry:

• Simplex: Signal flow is restricted to one direction only. Examples: Airport terminal weather reports, telemetry, marker beacons, and radio broadcasting.
• Half-Duplex: Two-way communication where signal flow exists in one direction at a time. Examples: Air-to-ground and air-to-air radio. Requires procedural words like "over" and "out."
• Full Duplex: Simultaneous signal flow in both directions without interference. Example: Standard telephone conversations.

Radio Communication Frequency Ranges

Voice Transmission Limitations

• Normal Speech Range: From $90\,Hz$ (deep bass) to $10\,kHz$ (soprano).
• Important Speech Frequencies: Fall primarily below $3\,kHz$.
• Fidelity: The ability to faithfully reproduce the input at the output.
• Minimum Range for Speech: $500\,Hz$ to $2000\,Hz$.
• Average Radiotelephone Range: $250\,Hz$ to $2750\,Hz$.

Antenna Constraints

Transmitting audio frequencies directly via radio is impractical due to their extremely long wavelengths ($\lambda$). Wavelengths of voice frequencies ($250\,Hz$ to $2750\,Hz$) range from $1,200,000\,m$ to $109,090\,m$.

Antennas are most efficient when they are $1/4, 1/2, 1$ or more full wavelengths of the frequency being radiated. To make antennas practical and portable, voice signals are modulated onto higher frequency carrier waves.

Modulation and Carrier Waves

To radiate information from a practical antenna, the voice signal (modulating wave) is impressed upon a high-frequency radio-frequency wave (carrier).

• Modulated Wave: The resulting complex wave composed of the carrier and modulating wave.
• Demodulation: The process at the receiver where the original component waves are reproduced.
• Amplitude Modulation (AM): Varying the amplitude while frequency remains constant.
• Frequency Modulation (FM): Varying the frequency while amplitude remains constant.
• FM Bandwidth: FM communications require high bandwidth and are thus restricted to VHF ($30-300\,MHz$) or UHF ($300\,MHz - 3\,GHz$) bands.

THE ELECTROMAGNETIC FIELD AND SPECTRUM

Generation of Electromagnetic (EM) Waves

EM waves are produced whenever electric current passes through a conductor. Radiating EM waves requires an alternating current ($AC$) to produce an expanding and collapsing field.

• Antenna vs. Wire: A shielded coaxial cable keeps the field contained, whereas an antenna is designed specifically to radiate the field.
• Physics of Radiation: Electrons moving back and forth in an antenna create an electromagnetic field that travels at the speed of light ($3 \times 10^8\,m/s$).

Spectrum Specifics and Interference

Low-Frequency Challenges

• VLF Band ($3\,kHz$): Wavelength is approximately $100\,km$. An effective half-wave antenna would need to be $50\,km$ long.
• $400\,Hz$ Power Systems: Used in aircraft because high frequency allows efficient power transfer over thinner wiring (via smaller transformers). However, at $400\,Hz$, the wavelength is $750\,km$, meaning unintentional EM radiation is small. This radiation is known as Electromagnetic Interference (EMI) and must be shielded from sensitive circuits.

Mathematical Characteristics of Radio Waves

Three critical characteristics define a radio wave:

1. Frequency ($f$): Number of cycles passing a point in one second. Measured in Hertz ($Hz$).
2. Velocity ($v$): Speed through a medium. In a vacuum, standard velocity is $300 \times 10^6\,m/s$ (approx. $186,000\,miles/second$).
3. Wavelength ($\lambda$): Distance between two consecutive maxima, measured in meters ($m$), kilometers ($km$), etc.

Fundamental Formulas:

• Velocity Calculation: $V = \lambda f$
• Wavelength Calculation: $\lambda = \frac{c}{f}$ where $c$ is the speed of light ($3 \times 10^8\,m/s$).

Wave Components and Polarisation

Transverse Electromagnetic Waves

A radiated wave consists of two periodic, oscillating fields mutually perpendicular to each other and the direction of travel:

1. Electric Field ($E$): Parallel to the conductor (antenna).
2. Magnetic Field ($H$): At right angles to the Electric Field.

Polarisation Definition

Polarisation describes the plane of the Electric Field ($E$) with respect to the Earth's surface.

• Vertical Polarisation: The $E$ field travels in a plane perpendicular to the Earth's surface.
• Horizontal Polarisation: The $E$ field travels in a plane parallel to the Earth's surface.
• Principle of Induction: For maximum energy absorption, the receiving antenna must be in the same plane of polarisation as the transmitting antenna.

Fields Around the Antenna

1. Induction Field: Pulsates between magnetic and electric energy as fields collapse and expand. It surrounds the antenna but diminishes rapidly (inversely proportional to the square of the distance, $1/D^2$). It is useless for communication and contributes to antenna losses.
2. Radiation Field: The energy released that actually travels through space. Significant radiation occurs only when the antenna size is a substantial portion of the wavelength.

PROPAGATION PHENOMENA AND MODES

Wave Behaviors in the Atmosphere

Reflection

Waves bounce off objects of comparable or larger size than the wavelength. At the point of reflection, a phase change of $180^{\circ}$ occurs.

• Incident Wave: The wave approaching the surface.
• Reflected Wave: The wave bouncing off.
• Normal: An imaginary line perpendicular to the surface at the point of impact.

Refraction

The bending of a wave when passing from one medium to another of different density. In the atmosphere, air density decreases with altitude, causing gradual refraction.

• Mechanism: The part of the wavefront entering a less dense medium travels at a different velocity, causing the wave to bend back toward the denser medium (the Earth).

Diffraction

The ability of a wave to curve or bend around obstacles (Huygen’s Theory).

• Knife-Edge Diffraction: Allows propagation around mountains and buildings.
• Shadow Zones: Areas on the opposite side of steep obstacles where the diffraction is not sharp enough to penetrate.

Doppler Effect

The shift in frequency of a reflected wave caused by relative motion between the transmitter and the reflecting object. This is used in aircraft radar and police speed detection.

Radio Frequency Spectrum Bands

Core Propagation Modes

1. Ground Waves (Surface, Direct, and Reflected)

Ground waves exist in the troposphere and travel along the Earth's surface.

• Surface Wave: Follows Earth's contours via diffraction but suffers attenuation due to induced ground voltages. Vertical polarisation is superior for surface waves because the $E$ field merely "dips" into the ground rather than being in constant contact.
• Direct Wave: Travels line-of-sight between antennas (Space wave).
• Ground-Reflected Wave: Reaches the receiver after bouncing off the ground. If it arrives simultaneously with the direct wave, they may add or cancel (fading).

2. Skywaves (Ionospheric Waves)

Used for HF communications ($3-30\,MHz$). The wave is radiated upward and refracted back to Earth by the ionosphere.

• Range: Can reach thousands of miles by "skipping" between the ionosphere and the Earth.
• Variability: High frequency bands like VHF/UHF pass through the ionosphere into space and do not return as skywaves.

3. Space Waves (Line-of-Sight)

Typical for VHF ($30\,MHz$) and above. Range is limited by the curvature of the Earth.

• Radio Horizon: Due to slight diffraction, the radio horizon is approximately $1/3$ further than the optical (natural) horizon.
• Range Formula:$Range \approx 1.23(\sqrt{H_r} + \sqrt{H_t})$ (Range in $Nautical\,Miles$, heights in $feet$ above sea level).

THE IONOSPHERE AND HF PROPAGATION

Structure of the Ionosphere

Extending from $50\,km$ to $500\,km$, this region contains ionized gases (ions and free electrons) created by ultraviolet ($UV$) radiation from the sun.

Ionospheric Layers

• D Layer ($50-90\,km$): Present only in daylight. It primarily absorbs and attenuates HF signals, especially at the lower end of the band.
• E Layer ($90-140\,km$): Refracts HF waves up to $20\,MHz$ for distances up to $1200\,miles$. Intensity decreases significantly at night.
• F Layer ($> 140\,km$): The most critical for long-distance HF. During the day, it splits into F1 ($140-210\,km$) and F2 ($> 210\,km$). At night, they merge into a single F layer.

Signal Quality Terms

• Critical Frequency: The maximum frequency that can be transmitted vertically and still be refracted back to Earth. Frequencies above this pass into space.
• Critical Angle: The maximum angle from the vertical that allowing a wave to return to Earth. Higher angles (more vertical) result in waves penetrating the layer and being lost.
• Maximum Usable Frequency (MUF): The highest frequency that can be used for communications between two locations at a specific time and angle.
• Lowest Usable Frequency (LUF): The minimum frequency usable, limited by absorption, atmospheric noise, and refraction rates.
• Optimum Working Frequency (FOT): The most reliable frequency, typically set at $85\,\%$ of the MUF ($FOT = 0.85 \times MUF$).

Skip Zones and Coverage

• Skip Distance: The distance from the transmitter to the point where the skywave first returns to Earth.
• Skip Zone (Zone of Silence): The area between where the ground wave ends and the first skywave returns. Receptions in this zone are usually impossible.
• Scattering:
  • Backscatter/Side Scatter: Small amounts of energy scattered in the atmosphere that can sometimes fill in the skip zone with weak, distorted signals.

Fading Phenomena

1. Absorption Fading: Caused by variations in ionospheric absorption rates over time.
2. Multipath Fading: Occurs when signals take multiple paths (e.g., ground wave vs. skywave, or a double-hop skywave) and arrive out of phase, causing cancellation.
3. Selective Fading: Occurs when transmitting multiple frequencies. Since each frequency fades differently, the original phase and amplitude of the broadband signal are distorted.

ATMOSPHERIC LAYERS AND DISTURBANCES

Troposphere ($0 - 18\,km$)

• Contains virtually all weather.
• Temperature decreases with altitude (Lapse Rate: $6.5^{\circ}\,C\,per\,km$).
• Tropospheric Scattering: Small turbulences (eddies) can refract/scatter VHF, UHF, and SHF waves ($100\,MHz$ to $10\,GHz$) far beyond the horizon, enabling ranges of $50-500\,miles$.

Stratosphere ($18 - 50\,km$)

• Relative constant temperature, although it contains an Inversion Layer in the upper parts where temperature increases with height due to Ozone ($O_3$) absorbing $UV$ radiation.
• Largely calm with little effect on radio waves.

Tropospheric Ducting

Temperature inversions (warm air over cool air) create ducts that trap and guide VHF/UHF waves follows the Earth's curvature. This can extend signal range by hundreds of miles.

Factors Affecting Ionisation

1. Daily (Diurnal): Ionisation peaks at noon and reaches a minimum just before dawn.
2. Seasonal: D, E, and F1 layers are densest in Summer. The F2 layer is densest in Winter.
3. 11-Year Sunspot Cycle: Maximum sunspot activity increases ionisation and MUF, requiring higher operating frequencies.
4. 27-Day Rotation Site: Corresponds to the sun's rotation on its axis.
5. Sudden Ionospheric Disturbances (SID): Caused by solar eruptions; intense $UV$ bursts cause the D layer to become so dense it "blanks out" HF propagation by total absorption.
6. Ionospheric Storms: Magnetic field disturbances associated with solar particle radiation. They lower critical frequencies and make skywave propagation erratic.

Precipitation Attenuation

• Frequency Dependence: Attenuation increases as frequency increases (wavelength decreases).
• Rain: Most significant attenuator above $100\,MHz$. Absorption and scattering both occur.
• Fog: Suspend moisture; significant attenuation above $2\,GHz$.
• Snow/Hail: Generally less dense than rain ($Density_{Rain} = 8 \times Density_{Snow}$), thus causing less attenuation for an equal fall rate.

RADIATION ANGLE AND ANTENNA PLACEMENT

• Angle of Radiation (Take-off Angle): The angle at which energy leaves the antenna relative to the mounting plane.
• Height Effect: Height controls the take-off angle. The higher the antenna is mounted, the lower the radiation angle.
• Long-Distance Strategy: Lower angles ($10^{\circ}$ to $20^{\circ}$) are preferred for long-distance skywave (fewer hops, less attenuation) and groundwave communication.
• Short-Distance Strategy: Higher angles (from low-mounted antennas) result in shorter "skips," useful for communications within an $800$ to $1600\,km$ range.