Basic Radio Wave Theory Flashcards
Amplitude
Measure of max deflection in oscillation or wave
Frequency
Number of cycles in one second in hertz
Kilohertz
Khz
1 x 103
(1000)
VLF
LF
MF
Megahertz
MHz
1 x 106
1,000,000
HF
VHF
UHF
Gigahertz
GHz
1 x 109
1,000,000,000
SHF
EHF
Formula for wavelength/speed of light/frequency
Speed of light
——————-
Wavelength x frequency
Wave length = speed of light / frequency
Speed of light = wave length x frequency
Frequency = speed of light / wavelength
Relationship between wavelength and frequency
Wave length up frequency down
Wave length down frequency up
Inversely proportional
Very little mice have very usual sandwiches everyday
Very Low frequency
Low Frequency. KHz
Medium Frequency
High frequency
Very high frequency MHz
Ultra high frequency
Super high frequency. GHz
Extremely high frequency
3 - 30
30 - 300
300 - 3000
Doppler Effect
Frequency increases or decreases due to relative motion between transmitter and receiver
Moving away - frequency decreases
Moving towards - frequency increases
How does a antenna work
Convert electrical power into radio waves and vice versa
Voltage applied to make current
Em field created around the antenna
Current switched off
Em collapses
Current switched on again
Em field bounces off the expanding em fiel into space rapidly
Enduces voltage on receiver
Antenna must be
Half the wavelength
Polarisation
Orientation of electromagnetic wave with direction of propagation
Linear vs circular vs elliptical types of polarisation
Plane of oscillation rotates:
Circular
Elliptical
Plane of oscillation fixed:
Linear
Loop Antenna
Used in ADF
Detect direction a signal is coming from (null seeking)
In line with electromagnetic wave = different phase angle = current
(Sideways)
- No signal/no phase difference either side of loop
Not in line = same phase angle = no potential difference = no current flowing
(Front on)
- Strong signal/Significant phase different
Parabolic Antenna
Common in old AC/weather radars
Collects EM waves and reflects them back to a single focal point
Used in UHF and SHF
Directional
Can amplify weak signals
Beam width = size of dish/frequency/wavelength
Slotted Planar Array Antenna
Flat antenna matrix
Slots that behave like dipoles
Electronically scanned rather than mechanically
Simpler/lighter more suitable for aircraft
Used in modern aircraft weather radars
+ Narrower beams than parabolic - more efficient
+ less side lobes - requires less power
Helical Antenna
Wire coiled into helix
Diameter and distance of coils determine wavelength sensitivity
Used for GPS
Antenna shadowing
Roll/pitch shadow signal via body
Terrain/buildings/other AC also block line of sight between transmitter and receiver
Most likely at low elevation/built up areas
Wave Propagation
Means by which signal is sent from transmitter to receiver
Bending etc
Categories of radio waves
Space waves - line of sight waves (straight)
Surface waves - move along differing media with different electrical properties
Ground waves - follow curvature of earth between surface and ionosphere
(SW + GW used interchangeably)
Sky waves - transmitted skywards and refracted by ionosphere back to surface
Factors effecting propagation
Weather
Solar activity
Ionosphere
Frequency
Antenna used
Reflection
Angle of incidence is equal to reflection
Can degrade quality of signal if reflected
Terrain/buildings/vehicles/ac/water droplets
Refraction
One medium to another and speed up or slow down (density etc)
Higher frequency = less refraction (move higher into ionosphere)
Diffraction
Signal bending to match curvature of earth (ground waves)
Knife edge diffraction
EM waveform contacts sharp edge of terrain (mountains)
Results in propagating of new diffracted wavefront into free space
Secondary transmitting source in forward direction only
Multi path propagation
Arrives at receiver via more than one path
Change in both direction and length of signal path
- navigational anomalies
Absorption
Radio wave energy changing to another form such as heat
(Attenuated)
Attenuation
Weakening of radio signal
Terrain/building/aircraft/dust/water droplets
Interference
Vary with time of day and position
Noise or unusable signal
Out of phase signal or anti phase cancel out
Superposition
2 or more signals with similar carrier waves interfere with each other
Fading
Two or more signals with same frequency interfering changing signal strength and polarisation
Layers of ionosphere
D region - weakest
E region (Kennelly Heaviside layer)
F1 layer (Appleton Layers)
F2 layer - strongest (Appleton Layers)
Ionosphere day vs night + interaction with radio waves
Strongest in day via solar activity
Weaker at night
radio waves move electrons in ionsphere which cause collisions so loss of energy (attenuates)
Lower frequency has greater attenuation due to greater displacement of electrons = more collisions
D Layer
50-90km above surface
Greatest in day/disappears at night
MF/HF attenuated here
Sun up - frequency up (high frequency used in day)
Sun down - frequency down (lower frequency used at night)
E Layer
90km to 150km
Thinner at night time/can even disappear if weakened in day
Base rises after sunset
F layers
150km to 500km - Appleton Barnett layer
Most influence on radio wave propagation
F1/F2 in the day and F2 at night
Absorption layer
Lower frequency radio waves
VLF/LF/MF/HF - ground wave propagation
Higher frequency radio waves
VHF/UHF/SHF/EHF propagate as space waves via line of site
Max Theoretical range formula
1.23 x (Square root Transmitter height + square root receiver height)
Sky waves
Refracted from E/F layers
VLF/LF/MF/HF
Higher frequency increases length of surface wave less diffraction
Higher frequency greater sky wave range less accuracy
2MHz to 30 MHz
Critical angle
Highest angle at which wave returns to earth
Skip distance
Distance between transmitter and point on surface of earth where first sky wave returns
Dead space/zone
Distance between the limit of surface waves and sky waves where no signal received
Sporadic E
VHF usually limited to line of sight but can be refracted in E layer
Abnormal event where transmitted over long range
Can happen in summer
Super refraction
Occurs in VHF+
Increases radio range
Not reliable
Caused by marked inversion/rapid humidity changes
Modulation
Process of altering signal to carry info
Impressing and transporting info via radio waves
Modulator to be transmitted and demodulator to return to raw form via receiver
Carrier wave modulated to higher frequency with audio at lower frequency
Pulse modulation
On/off to produce pulse radio waves (pulse strings) - used in radar
Pulse width - length of pulse in unit of time
Pulse power - power contained in single pulse
Continuous - power consumed over time including periods of no transmission
Keying - pulses for morse code
Amplitude Modulation
Altering amplitude of the carrier
Modulating audio applied to CW
Audio wave mixed in modulator with CW of constant frequency and power
- noisy/static
+ simple design
USB/LSB side bands
Upper side bands + lower side bands
Mirror images of each other
Together known as double sideband transmission
Single sideband modulation
Removing one of the side bands to improve single efficiency
Used in HF transmissions
+ power saving
+ energy saving
+ maintain range
+ less bandwidth occupation
+ less risk of interference
Frequency modulation
Altering frequency of the carrier
Adjusted in line with amplitude of audio signal
+ more efficient
+ less power wastage
+ better signal quality
+ less effected by noise than AM (clips excess)
Used on frequencies above 30MHz (VHF FM)
- Less frequency bandwidth as use more than AM
Phase Modulation
Used in GPS
Phase of carrier waves reversed
- require complex demodulator
Aviation FM vs AM
AM:
+ More frequencies
+ simpler and less expensive
+ technology and cost problems changing to FM
N0N
Carrier without modulation (NDBs)
A1A
Carrier with keyed morse code modulation
BFO required
Frequency and amplitude of wave stay the same
A2A
Carrier with amplitude morse code
A3E
Carrier with amplitude modulated speech VHF
Phase Angle
The fraction of a wavelength expressed in degrees from 000 to 360
GPS Antennas are
On top of the fuselage
Antennas for ground based facilities are located where on aircraft
On the underside of the aircraft
Propagation of all wave types
VLF = Ground wave
LF = Ground wave
MF = Ground wave
HF = Sky Wave
VHF = Space wave
UHF = Space Wave
SHF = Space Wave
Systems using the Doppler principle
VOR
GPS
MTI
AWR
Single sideband two way communication is used in what frequency
HF - 3-30MHZ
Primary Radar
Used to detect ac without secondary radar transponder
One Directional antenna for transmitting and receiving - greater power and less interference
TNT - Classifications of radio signals
Type of modulation - AM/FM/Pulse etc
Nature of signal - digital/analogue etc
Type of information - morse code/voice etc
Dead space “skip zone” gets bigger when
Between dawn and dusk when ionosphere becomes less bouncy
Polarisation
Orientation of the plane of oscillation of the electrical component of the electromagnetic wave
Light direction well defined = polarised (laser for example)
Linear - confined to single plane
Circular - rotates in circle
Elliptical - differing amplitudes/phase
Modulation does what to frequency
Addition of a low frequency signal (tone/voice) onto a high frequency carrier wave
Input signal onto carrier wave
Reason: adding low frequency human voice onto a high frequency carrier wave
Pulse Length
Measure in time
