Chelmsford Amateur Radio Society
Advanced Course
(6) Propagation
Part 2 – Propagation Modes
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
1
Introduction
Syllabus covers a wide range of propagation topics:-
Key Aspects:• Solar Radiation creates the Ionospheric Layers
• Understand the layers and their variation and influence on HF
• Understand various ionospheric propagation modes/terms
• Other affects/modes that affect VHF and higher frequencies
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
2
The Ionosphere
• The Ionosphere comprises
layers of ionised gases
• Ionisation occurs due to
input from Solar emissions
• Sources include:– Ultra-violet radiation
– Solar wind particles
– X-Rays
• Whilst Light/UV is fairly
constant, others do vary
• The Earth’s rotation, orbit,
and magnetic field also
have a role
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Advanced Licence Course
Solar Influence is key factor
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
3
Sunspots & Flares
• A major long term variation is from
the sunspot cycle (~11 year period)
• More sunspots lead to higher
ionisation in the ionosphere
• These higher levels increase the
range of refraction and usable
frequencies at HF
• However: If a solar flare gives a
major Coronal Mass Ejection, this
will upset the earths magnetic field
leading to poor conditions on HF
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
4
HF and the Ionosphere
F2
F1
E
VHF/UHF
D
400km
Earth
HF
70km
•
•
•
•
Ionosphere is layers of Ionised Air: 70 - 400km above earth
HF is bent by ionosphere (refraction) - VHF+ passes through
Four Layers: D, E, F1, F2 – created by and vary with Solar input
Layers change with day/night, season, flares, sunspots etc
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
5
Refraction
C
• The velocity of radio
waves are slightly
lower in air than in a
vacuum
• Ionised particles
affect the velocity
• They cause a small
increase in velocity,
causing the wavefront to veer and
change direction
Velocity change
results in a change
of direction
Ionised Air
A
Wave in the ionosphere
gets slightly faster
D
Normal Air
B
Wave Front
• Higher frequencies
are affected less,
reducing their
refraction
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
6
Ionosphere – D Layer
D Layer: ~ 80km Height
• The D layer tends to absorb the
lower radio frequencies
• During daylight hours it absorbs
most radio energy below 3-4 MHz,
though it can sometimes extend
up to 14MHz
<
>
• At night, it virtually disappears,
making 80 metre (3.5 MHz)
DX communications usable
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
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Ionosphere – E Layer
E Layer: ~ 120km Height
• The E layer is more densely
ionised and tends to refract rf
• It varies with UV and X-rays, and
quickly disappears, at night
>
<
• Mainly affects up to 14MHz
• Bursts of radiation can cause
more intense refraction in the
summer months
• Sporadic E: Can occur from
patches of highly ionised gas and
refract 10m and VHF (6m, 4m, 2m)
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• Sporadic-E gives single-hop
VHF QSOs of ~2000km
• It is a difficult to predict
short-lived event.
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
8
Ionosphere – F Layers
F Layers: ~200 - 400km
• The F Layers are highest and give
longer distance refraction
<
>
• During the day it ionises into two
distinct layers:• F1 at 200km
• F2 at 300 - 400km
• At night the two layers combine
into a single F-Layer
• F2 gives long distance
propagation over 1000s of km
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• F2 enables 4000km distance
in a single hop
• Multi-hop gives worldwide
communications
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
9
HF Band Examples
3.5 MHz / 80 metres
• Ionospheric propagation influenced by D-Layer absorption
• Can be noisy, especially at night
• Daytime:
Ranges limited to a few 100 km
• Nighttime: D-layer dissipates giving greater distances.
Over 1600km may typically be achieved
21MHz / 15 metres
• Sunspot Cycle has significant influence. Poor if numbers are low
• The MUF can be below 21MHz – giving no propagation
• Sunspot peaks will raise MUF for F-Layer DX propagation during the
day and often into the evening up to 1000s km
• After midnight, F-Layer thins further and propagation ceases
Book has a fuller review, but only 3.5 and 21MHz is need for exam
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
10
Critical Frequency
• The critical frequency is the highest frequency that will
returned to earth from an overhead vertical path
• It is directly dependent on the level of ionisation above the
observer – may be measured by ionospheric sounders
• Sometimes called: Critical Frequency of Vertical Incidence
• Typical figures are:
– Summer: High 9MHz,
Low 4MHz
– Winter: High 14MHz, Low 3MHz
• Note: Near Vertical Incidence Skywave (NVIS) exploits this
for local communications coverage
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
11
Maximum Usable
Frequency (MUF)
• The Maximum Usable Frequency (MUF) is the highest frequency
that will be refracted over a particular path.
• The MUF varies with 24hr day/night cycle, season etc
• The MUF will always be higher than the critical frequency
• Longer paths (with lower angles) will have a higher MUF
• The MUF may be up to five times the critical frequency, depending
on the angle
• It is usually advantageous to use highest available frequency
• The MUF varies with solar ionisation:– Overnight the ionisation steadily falls resulting in much lower MUF,
to as low as around 2MHz during a sunspot minimum.
– At mid-day during the maximum of the sunspot cycle, it may reach
40MHz for a long hop.
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
12
Lowest Usable
Frequency (LUF)
• Lower frequencies are more liable to absorption in the D layer
• Some propagation charts give a lowest usable frequency to
allow for this effect
• If the LUF is greater then the MUF, No propagation by the
ionosphere is possible
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
13
Skip / Dead Zone
• Between the skip distance
and ground wave range is a
region that can not be
covered
• This is known as the Skip or
Dead Zone
• It is quite easy to observe...
• Tune to a distant station in
QSO with someone in the UK
• The distant station may be a
strong signal, but the UK
station is often totally
inaudible, despite being
located nearer to you
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
14
Fading
• Fading is caused by signals arriving at the receiver by
slightly different paths - Multipath
• The path lengths will vary, changing the received phase from each path
• Differences in phase cause the signals to add or cancel
• SSB, CW will fade/drop out, FM can become severely distorted
• If two signals are 180° out of phase, fading results in full cancellation
• The paths with vary with time and propagation mode leading
to variable fading
• Fading rates may be slow, fast or a hybrid combination
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
15
Propagation Modes
Main Propagation modes:• Ground wave – at LF
• Sky/Ionospheric waves – at HF
• Tropospheric (space) waves – at VHF
Shorter wavelength VHF/Microwaves can be affected by:• Ducting from moist/warm air causing ‘lifts’
• Edge-diffraction
• Aurora
• Meteor trails
• Building scatter / multipath
• Scatter from aircraft, heavy rain
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Aurora
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
16
Main Propagation Modes
Ground Wave
• Ground wave hugs the curvature of the earth but quickly gets weaker
• Range over land is relatively short – but usable below 2MHz
• Losses influenced by ground conductivity – best over sea water
Sky or Ionospheric Wave
• Sky wave is the primary mode of propagation from 1 - 30MHz
• It is very dependent on the level of ionisation
Tropospheric Wave (or space wave)
• Primary propagation mode at frequencies above 30 - 40MHz
• Occurs below the ionosphere but above the influence of the terrain
• Water vapour and temperature variations cause radio waves to refract
downwards slightly, following the curvature of the earth
• Enables contacts somewhat greater than line of sight
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
17
LUF, MUF, Paths Summary
Above 30MHz lost to space
F2
Tropospheric
Wave
F1
Skip/Dead
Zone
E
LUF
Absorption
Ground
Wave
Critical
Frequency
D
MUF Signal
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Earth
Christopher Chapman G0IPU
Tx
Station
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
18
VHF/UHF Modes
• Ducts: Moist or warm air layers, often associated with high atmospheric
pressure, bend or trap waves giving propagation over longer distances
• Ducts can be in mid-air or just above the sea surface, but antennas need
to be in the duct to get strong signals
• Knife Edge Diffraction: Waves bend around corners or hill tops, enabling
communication between stations that may otherwise be obstructed
• Shadowing: from buildings, hills gives patchy coverage
• Scatter: can give signals for unlikely paths – in between buildings, from
aircraft wings, rain clouds, moonbounce etc
• Meteor Scatter: Ionised meteor trails reflect signals. Good on 2m and 6m
• Aurora: On 6m, 4m and 2m, SSB voice loses tonal content, giving a
whisper-like sound
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
19
Re-Cap
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Ionosphere & Propagation
11 Year Sunspot Cycle
HF and the Ionosphere
Refraction
Ionosphere Layers: D,E,F (F1,F2)
HF Band Examples: 80m (3.5MHz) and 15m (21MHz)
Critical Frequency
Maximum Usable Frequency (MUF)
Lowest Usable Frequency (LUF)
Skip/Dead Zone
Fading
Main Propagation Modes
LUF, MUF & Propagation Paths
VHF/UHF Propagation Modes
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 16: v2.1 30-Jan-2013
(6) Propagation – Part-2
20