Aircraft Radio Communication Principles

Principles of Aircraft Radio Communication

• This covers radio communication systems, including HF, VHF, and UHF
• Also covers Radio Wave Propagation and Types of Modulation
• Finally covers Receivers, including Selectivity, Sensitivity, and Antennas

Radio Waves

• Radio waves have both electric and magnetic fields
• Amplitude is the maximum extent of a vibration or oscillation, measured from the position of equilibrium
• Wavelength is the distance between successive crests of a wave

Wave Propagation

• Propagation means "movement through a medium"
• Radio waves are a form of radiant energy, like light and heat
• They can be detected using sensitive measuring devices
• Radio waves travel at the speed of light
• Eyes can only detect radiant energy within a fixed range of frequencies
• The frequencies of radio waves are below what eyes can detect
• The theory of wave propagation applies to electronic equipment like radar, navigation, detection, and communication devices

Principles of Wave Motion

• A wave is a disturbance (sound, light, radio waves) that moves through a medium (air, water, vacuum)
• Wave motion is a recurring disturbance advancing with or without a physical medium
• Wave motion is a means of moving or transferring energy from one point to another
• When sound waves strike a microphone, sound energy converts to electrical energy
• Light waves striking a phototransistor or radio waves striking an antenna convert to electrical energy
• Sound, light, and radio waves are forms of energy moved by wave motion

Wave Motion in Water

• The water is not moved outward by the wave but moves up and down as the waves move outward
• The up and down motion is transverse or at right angles to the outward motion of the waves
• This type of wave motion is called transverse wave motion

Transverse Waves

• Consists of wavelength, crest, and trough
• Travel in a specific direction

Longitudinal Waves

• Waves in which the disturbance occurs in the direction of propagation
• Sometimes called compression waves

Wave Propagation Essentials

• Source, detector, and medium are all necessary for wave motion and wave propagation
• Electromagnetic waves do not require a medium

Radio Wave Terminology

• CYCLE
• WAVELENGTH
• AMPLITUDE
• FREQUENCY
• REFERENCE LINE
• POSITIVE ALTERNATION
• NEGATIVE ALTERNATION

Characteristics of Wave Motion

• Velocity of propagation is the rate at which the disturbance travels through the medium
• It depends on the wave type (light, sound, radio) and medium type (air, water, metal)
• PERIOD: The time (T) in which one complete vibratory cycle of events occurs
• FREQUENCY OF VIBRATION (f): The number of cycles taking place in one second
• WAVELENGTH: The distance the disturbance travels during one period of vibration.
• λ = v/f, where λ = wavelength, v = velocity of propagation, and f = frequency of vibration

Wave Example

• A sound wave with a frequency of 500 Hz traveling through air at about 340 m/s (meters per second) is used
• Frequency (f) = 500 Hz: The wave completes 500 cycles each second.
• Period (T) = 1/f = 1/500 = 0.002 s: Which takes 0.002 seconds for one complete cycle.
• Wavelength (λ) = v/ f = 340 m/s ÷ 500 Hz = 0.68 m: Each wave cycle stretches 0.68 meters in the air

Electromagnetic Waves

• ELECTROMAGNETIC SPECTRUM: The complete range of frequencies from 3 kHz to beyond 300,000 THz
• Audio frequencies (15 Hz-20 kHz) are not electromagnetic energy

Frequency Bands

• EHF: 30,000 MHz (.01 m)
• SHF: 3,000 MHz (.1 m)
• UHF: 300 MHz (1 m)
• VHF: 30 MHz (10 m)
• HF: 3 MHz (100 m)
• MF: 300 kHz (1 km)
• LF: 30 kHz (10 km)
• VLF: 3 kHz (100 km)

Frequency Use

• VLF: Navigation
• LF: Navigation
• MF: Navigation and shortwave radio
• HF: Shortwave radio
• VHF: FM radio
• UHF: Television, mobile phones, GPS, Wi-Fi, 4G
• SHF: Satellite communications
• EHF: Satellite communications

Frequency Details

• EXTremely Low Frequency (ELF): Ideal for sending short messages great distances and transmitting messages to submarines
• VLF: Reliable path for over great distances over oceans, broadcasts requiring no reply. Used for space vehicle tracking
• LF: Occupies a small part, transmitting installations are large, low-frequency signal reception is seriously hampered by atmospheric noise.
• MF: Used for ground-wave transmission over water and moderately short distances over land; generally for short distance communications.
• HF: Atmospheric variations require multiple frequencies, economic, rapid expansion is the result, some predict a replacement

Atmosphere

• Shows the earth's atmosphere layers in relation to altitude, temperature, and pressure

Antenna Use

• Found on many devices, including aircraft, often low-profile to reduce air resistance
• For transmission, an antenna converts electrical energy into radio waves
• For reception, it turns radio waves back into electrical energy
• According to the law of reciprocity, an antenna works the same way for sending and receiving

Antenna Gain

• How much stronger a signal is compared to a reference antenna
• Expressed in decibels (dB) and uses two references
• dBi: Compared to a theoretical isotropic radiator (radiates equally in all directions)
• dBd: Compared to a half-wave dipole (a common real-world antenna)

Antenna Varieties

• Isotropic Radiator (a):
  • An isotropic radiator sends signals equally in all directions, like a perfect sphere of radiation.
  • It doesn't favor any particular direction and spreads its energy uniformly
• Directional Antenna (b):
  • A directional antenna focuses its radiation for stronger signal in that area
  • It illuminates only a part of the sphere, unlike the isotropic radiator.

Aircraft Antenna Locations

• Typical antenna locations are numbered on an aircraft
• 1, VOR
• 2, HF comms
• 3, 5 and 6, VHF comms
• 4, ADF
• 7, TCAS (upper)
• 8, weather radar

Frequency Ranges

• VHF (VERY HIGH FREQUENCY): Frequencies above 30 megahertz, ground-wave range is minimal
• Frequencies at the lower end of this band work around the shielding effects of hills and structures
• Transmitting antenna must be in a direct line with the receiving antenna but may not be over the horizon
• The source, detector, and medium are necessary for wave motion and propagation
• ELECTROMAGNETIC SPECTRUM – AVIATION APPLICATION lists frequency and related detail

How Signals Get From Antenna to Air

• Signals are an electric current travelling through a wire with an electromagnetic field
• Electrons also move in the transmitters antenna and a field is created around the antenna
• The wire carrying the signal from the transmitter is shielded (co-axial cable) to keep the electromagnetic field in
• The antenna is designed to radiate the electromagnetic field

Receiving Signals

• Electromagnetic fields travel from the antenna in all directions and at the speed of light.
• Electromagnetic field produces current in a wire (or antenna).

Modulating Signals

• In order to be transmitted and received, useful information carrier wave is altered or modulated
• Modulation varies the characteristic of one wave in accordance with the instantaneous value of the other
• A Carrier or RF (radio frequency), otherwise called CW (continuous wave)

Signal Types

• Modulating signal, which can be an audio signal amplified from the microphone
• Amplitude modulation (AM)
• Single sideband modulation (SSB)
• Frequency modulation (FM)
• Continuous wave (CW)

Amplitude Modulation

• The information signal can rarely be transmitted unless it is processed and converted from audio to an electric signal
• After conversion it is used to modulate a carrier signal.
• A carrier signal is used for two main reasons:
  • Reduce the wavelength for efficient transmission
  • allow simultaneous use of the same channel, called multiplexing
• The process of modulation means to systematically use the information signal (what you want to transmit) to vary some parameter of the carrier signal
• Typically a simple, single-frequency sinusoid (varies in time like a sine wave)

Voltage and Information

• V(t) is the voltage of the signal as a function of time
• Vo is the amplitude of the signal (represents the maximum value achieved each cycle)
• f represents the frequency of oscillation, the number of cycles per second (also known as Hertz = 1 cycle per second)
• $ Represents the phase of the signal

Common Modulations

• AM: amplitude modulation
• FM: frequency modulation or
• PM: phase modulation
• Transmitter uses info signal, Vm (t) to vary the amplitude of the carrier, Vco to produce a modulated signal, VAM (t).
• Formula is: VAM (t) = {Vco +Vm(t) }sin (2πfct + φ)
• Modulation depends on amplitude, Modulation Index: m =Vm/Vc

Interpreting the Modulation Index

• Expressed as a fraction
• m = 0.5, carrier amplitude varies by 50% above and below its original value
• if m= 1.0 then it varies by 100%.
• fSB = fc ±n fm relates to frequency calculation

Waves

• Visual examples are provided of Under-Modulated and Over-Modulated waves

Modulation Spectrum

• BW = 2 fm. where fm is now the maximum modulating frequency used

AM Efficiency

• Can measure efficiency as follows: η= efficiency = Pѕв / Ртот -PSB = the power in all the side-bands -PTOT = the total transmitted power (includes carrier and side-bands)
• The more strongly modulated, the more power is put into the side-bands.
• Modulation index (m)is the measure of the strength
• Increasing the modulation index increases the efficiency

AM Summary

• A carrier is used to make the wavelength smaller for practical transmission and to permit multiplexing
• Spectrum is used to measure bandwidth, efficiency
• Bandwidth predicted using BW = 2 fm where fm = the maximum modulating frequency.
• Efficiency depends only on the modulating index, m
• AM is limited to 33% efficiency, index cannot be > 1.0 without introducing distortion

Conventional AM Problems

• Bandwidth is wasted by having two identical side-bands on either side of the carrier
• Efficiency is limited to 33% to prevent distortion when demodulating
• Carrier signal is present even if nothing is being transmitted

Side Band Modulation

• Band pass filter removes a side band

Side Band Recovery

• The receiver must restore the signal before demodulation
• The receiver in a SSB system has its own carrier signal that is puts back in from a local oscillator
• Then the signal is processed normally

Side Band Summary

• Two mods:
  • Transmitter adds a band pass filter before amplification for transmission
  • Receiver adds a local carrier signal back into the signal prior to processing

Receiver Sensitivity

• A signal is transmitted at a very high level of power, measured in watts or kilowatts and will diminish in amplitude along the receiving antenna
• The signal can be as low a a few micro volts depending on distance
• A broadcast receiver may need an input described in milli volts to produce a satisfactory output.
• An aircraft receiver may can need a few micro volts to produce a good output.
• Sensitivity is a characteristic that is described as the minimum signal strength
• This will produce a satisfactory output in a specified receiver
• Signal input is reduced until noise is apparent

Receiver Selectivity

• When tuned into only the selected frequency, the rest is ignored

Frequency Modulation

• Frequency modulation uses the information signal, Vm(t) to vary the carrier frequency within some small range about its original value.
• Formula is: VFM (t) = Vco sin (2π[fc+(Af/Vmo)Vm(t)]t + φ)
• New term is Af, for peak frequency deviation
• Af can be that it is the farthest away from the original frequency that the FM signal can be and referred to as the "swing" in the frequency.
• Can also define a modulation index for FM: β = ∆f / fm, where fm is the maximum modulating frequency used.
• The simplest interpretation of the modulation index, ẞ, is as a measure of the peak frequency deviation, Af.
• Af = β fm .

Radio Frequency Example

• suppose in FM radio, the audio signal to be transmitted ranges from 20 to 15,000 Hz.
• If the FM system used a maximum modulating index, ẞ, of 5.0,
• Then the frequency would "swing" by a maximum of 5 x 15 kHz = 75 kHz above and below the carrier frequency.
• BW = 2 (β + 1) fm

Radio Receivers

• To successfully receive a signal, an antenna is required
• Aircraft have several receivers to receive the various types of transmissions
• The transmitter has a predetermined frequency, the receiver has to extract relevant voice notes for reproduction
• The basic elements of a receiver system: -an antenna (reception) -a receiver (amplification and detection) -speaker or headset (reproduction)

Bandwidth

• Bandwidth is one of the limitations of a communication
• The greater the bandwidth, the more information to be delivered
• For instance, in an AM receiver the bandwidth has to be wide enough to allow all frequencies transmitted to be accepted.
• These frequencies are the RF carrier, {Fc + Fm) and (Fc - Fm}.
• The RF input of a TX receiver may have an RF tuned resonant circuit
• The formula for resonant frequency is Fr =1/2π√LC
• At resonance, maximum output,

Formulas

• BW of a receiver must be wide enough to intelligence
• BW of an AM receiver needs to be wide enough to pass the upper and lower sideband frequencies
• BW characteristics are measured at the half power points
• BW of a receiver is determined by the characteristics of the receiver circuitry.
• Voltage Gain (in dB) = 20 log Vout/Vin
• Power Gain (in dB) = 10 log Pout/Pin

Receivers and Loss

• Total gain is where stages are multiplied
• Receivers generally have two-stage amplifiers.
  • The first stage amplifies the signal 10 times
  • The second stage amplifies the signal 100 times -Total gain equals 10 x 100 = 1000.
• Stage loss is where input power is greater, loss will exist -dB =10 log1/10
  • = 10 log 0.1
  • = -10dB -A stage loss will always result in a negative dB number

TRF Receiver

• TUNED RADIO FREQUENCY RECEIVER (TRF) is a very simple receiver with a number of RF amplifiers, a detector and an audio amplifier stage
• Signal and selection begins ate the antenna
• Each tuned circuit is mechanically tuned by varying the capacitance
• Capacitors are ganged together to tune simultaneously -Next, amplified at each RF amplifier point -The detector extracts the intelligence and the RF is removed Audio is led via amplifier from the speakers

Super Heterodyne Receiver

• Operates on principles of the the mixing of two frequencies, to achieve a operating within the receiver
• RF stage tunes and selects, bandwidth must accommodate -The local oscillator (LO) will match the RF input -The IF is 455 kHz, the incoming signal is 2 MHz, the LO frequency will be 2.455 MHz

Audio

• The purpose of the mixer is to combine the RF signal and the local oscillator to obtain an intermediate frequency (IF) which contains the modulation information
• The IF stage utilizes a number of amplifiers and is selective, limiting adjacent problems
• The detector produces both audio and voltage to the earth
• The output is the fed to the audio amplifier
• AGC automatically controls the gain of receiver amplifier
• The strength varies
• AGC is designed to maintain output