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



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



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



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



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



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



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

>> <<



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)



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



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



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



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.



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



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



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



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



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



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



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



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

chelmsford amateur radio society advanced course (6) propagation part 2 – propagation modes

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