1. Chelmsford Amateur Radio Society Advanced Course (6) Propagation Part 2 – Propagation Modes

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
1) Demo using torch 1/dSq *d

3. 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
1) Demo using torch 1/dSq *d

4. 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

5. HF and the Ionosphere 400km Earth 70km
VHF/UHF HF
Ionosphere is layers of Ionised Air: km 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

6. 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 wave-front to veer and change direction
Higher frequencies are affected less, reducing their refraction
Velocity change results in a change of direction
Wave in the ionosphere gets slightly faster A
Ionised Air
Normal Air D B
Wave Front

7. 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
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8. 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.

9. 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 km
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

10. 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

11. 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

12. 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.

13. 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

14. 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

15. 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

16. 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

17. 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 MHz
It is very dependent on the level of ionisation
Tropospheric Wave (or space wave)
Primary propagation mode at frequencies above MHz
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

18. LUF, MUF, Paths Summary Earth F2 F1 E D Tx Station
Above 30MHz lost to space
MUF Signal
LUF Absorption
Tropospheric Wave
Skip/Dead Zone
Ground Wave
Critical Frequency
Tx Station

19. 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

20. 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