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HF Propagation
Nick Massey
VA7NRM
8-Nov-2004
NSARC HF Operators - HF Propagation
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HF Communicators
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! “Professional” Users
" Require predictable, reliable, communications
" Have a small number of assigned spot frequencies
" Cannot move off assigned frequencies
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Amateurs
" Exploit opportunistic communications
" Have well-spaced bands of frequencies
" Can move frequency to avoid other users
! Professionals are interested in moving to amateur style
“Adaptive” operation within assigned bands of frequencies
8-Nov-2004
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Electromagnetic
Spectrum
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" Radio Waves are a form of Electromagnetic Radiation
" Visible Light is also a form of Electromagnetic Radiation
" Radio Waves behave a lot like light
! … but, the wavelength of radio waves is 1000 m to 0.01 m whereas
the wavelength of light about 500 x 10-9 m
" HF Radio has wavelengths from 200 m to 10 m
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Path Loss
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! Good Path
" Transmit 100 Watts (+50 dBm)
" Receive an S9 signal, 0.00000000005 Watts (-73 dBm)
" Path loss is 1: 2,000,000,000,000 (123 dB)
! 20 Trucks carry 2,000,000,000,000 grains of sugar
! Poor Path
" Transmit 1000 Watts (+60 dBm)
" Receive an S4 signal, 0.00000000000005 Watts (-103 dBm)
" Path loss is 1: 20,000,000,000,000,000 (163 dB)
! 40 Ships carry 20,,000,000,000,000,000 grains of sugar
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NSARC HF Operators - HF Propagation
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Classification of
Propagation Waves
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Propagation Wave
Non-Ionised Layer
Ionised layer
Direct Wave
Free Space Wave
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Reflected Wave
Ground Reflections,
Reflectors
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Refracted Wave
Atmospheric Refraction
Refracted Wave
Diffracted Wave
Obstruction / Earth
Diffraction
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Surface Wave
Surface Wave
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Scatter Wave
Troposcatter
Ionospheric Scatter
Applicable to HF Communications
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The Ionosphere
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Ionisation
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" Atmospheric gas particles become positively charged
by removal of negatively charged electrons - ions
" In sufficient quantity, ions affect the velocity of radio
wave propagation
" Energy to produce ionisation comes from ultraviolet and
x-ray radiation from the sun
" In the absence of ionising energy, the ionised particles
die away due to collisions between ions and electrons
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Ionised Layers
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! D Layer
" 50 – 90 km, Only during daytime
" Attenuates radio waves passing through it
! AM broadcast stations stronger at night
! E Layer
" 90 – 130 km, Always present
" Primarily useful for daytime 1000 – 2000 km links
" Sporadic E
! F Layer
" Day F1 130 - 210 km and F2 250 – 400 km, Night F2 300 km
" F2 is most used layer for HF communications
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The Ionosphere
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Skywave Propagation
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! Radio waves appear to be reflected back to earth
by the E or F Layers
" In fact, due to refraction. The velocity of the radio wave
is changed as it passes through the ionised layer
" In ionised layer, the refractive index (n) < 1 and the wave
speeds up, bending away from the normal
" As ionisation increases n decreases and wave bends
further and further until it “reflects” back towards earth
" For any given frequency, the amount of refraction
depends on ionisation density and angle of incidence
8-Nov-2004
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Reflection by Refraction
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Critical Frequency
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" In ionised layer, refractive index depends on amount of
ionisation and frequency
" As frequency increases, n increases and amount of
refraction decreases
" Critical Frequency = Highest frequency which will reflect
vertically from the ionosphere
" Different critical frequencies for different layers, f0E,
f0F1, f0F2 etc.
" Vertical Ionogram from HAARP Alaska
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Vertical Ionogram
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http://www.haarp.alaska.edu/haarp/data.fcgi
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Oblique Propagation
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! Frequency
Increasing
! Angle
Increasing
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Pederson Rays
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MUF, OWF and LUF
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" Maximum Usable Frequency (MUF)
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Calculated from (Predicted) Critical Frequency
Depends on Location and Range
Critical Frequency depends on Ionospheric Conditions
Actual maximum operating frequency can be higher
" Optimum Working Frequency (OWF)
! 0.85 x MUF
! Predicted 90% Probability of propagation
" Lowest Usable Frequency (LUF)
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8-Nov-2004
Depends on Location and Range
Depends on (Predicted) Ionospheric Conditions
Depends on Transmitter Power
Depends on Antenna Gain (Directivity)
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Oblique Ionogram
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http://www.rcru.rl.ac.uk/ionospheric/hori_iono.htm
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Propagation Modes
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Predictable Ionospheric
Variations
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! Temporal
" Diurnal (Daily) Cycle
" Seasonal Cycle
" Sun Spot Cycle
! Geographic
" Latitude
! “Grey Line”
" Day/Night Terminator
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Diurnal Cycle
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f0F2 at Slough (UK)
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Seasonal Cycle
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Winter Anomaly
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f0F2 at Slough (UK)
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Sun Spot Cycle
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Latitudinal Variation
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Day/Night Terminator
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Can provide stable communications along “grey line”
just after sunset
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Unpredictable
Ionospheric Variations
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" Solar Flares – Sudden Ionospheric Disturbances (SID)
! Immediately
! Sudden X-ray emissions from the Sun
! Cause abnormally high absorption in the D Layer
" Solar Flares – Polar Cap Absorption (PCA) Events
! 10 mins to several hours after Flare
! Protons arrive at earth and magnetic field directs them to poles
! Cause abnormally high D Layer absorption for polar links
" Solar Flares – Ionospheric Storms
! 1 to 3 days after flare
! Shock Wave and Flare Cloud arrive
! Electric currents induced in Magnetosphere disturb Ionosphere
8-Nov-2004
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Unpredictable
Ionospheric Variations
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" Sporadic E
! Dense ionisation forms in the E Layer
! Can disrupt predicted communications
! But can be exploited for unpredicted
communications
" Auroral Effects
! Unpredictable disturbance
mechanisms
! Reliable HF communications is very
difficult
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Distortion
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! The signal that is received is not very often the
same as the signal that was transmitted!
" Fading
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Groundwave / Skywave interaction
Multiple skywave paths (including magneto-ionic splitting)
Movement of ionospheric irregularities
“Flutter” fading
" Frequency Shift and Frequency Spread (Doppler)
" Time Dispersion and Delay Spread
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DAMSON
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http://www.cpar.qinetiq.com/images/damson_custard.gif
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Noise and Interference
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! The signal that is received is usually
“contaminated” by Noise and Interference
" Noise
! Galactic Noise
! Atmospheric Noise
! Man-made Noise
" Interference
! Other Users
! Sounders, Radars etc.
! ISM and other “License Free” or Licensed operations
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Great Circle Maps
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