Chelmsford Amateur Radio Society Advanced Course (6) Propagation Part 2 – Propagation Modes
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Transcript of Chelmsford Amateur Radio Society Advanced Course (6) Propagation Part 2 – Propagation Modes
1Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
Chelmsford Amateur Radio Society Chelmsford Amateur Radio Society
Advanced CourseAdvanced Course(6) Propagation(6) PropagationPart 2 – Propagation ModesPart 2 – Propagation Modes
2Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
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
3Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-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
Solar Influence is key factor
4Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
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
5Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
HF and the Ionosphere
• 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
400km
70km
Earth
F2
F1E
D
VHF/UHF
HF
6Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
Refraction
• 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 wave-front to veer and change direction
• Higher frequencies are affected less, reducing their refraction
Ionised Air
Normal Air
A
B
C
D
Wave Front
Wave in the ionospheregets slightly faster
Velocity change results in a change of direction
7Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
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
>> <<
8Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
Ionosphere – E Layer
>> <<
• Sporadic-E gives single-hop VHF QSOs of ~2000km
• It is a difficult to predict short-lived event.
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)
9Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
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
• F2 enables 4000km distance in a single hop
• Multi-hop gives worldwide communications
10Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
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
11Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
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
12Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
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.
13Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
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
14Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
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
15Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
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
16Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
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 Aurora
17Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
Main Propagation ModesGround 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
18Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
F2
Earth
F1
E
D
LUF, MUF, Paths Summary
Critical Frequency
Skip/Dead Zone
Above 30MHz lost to space
Tropospheric Wave
Ground Wave
MUF Signal
LUF Absorption
Tx Station
19Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
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
20Chelmsford Amateur Radio SocietyAdvanced Licence Course
Christopher Chapman G0IPU Slide Set 16: v2.1 30-Jan-2013 (6) Propagation – Part-2
Re-Cap
• 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