Presentation is loading. Please wait.

Presentation is loading. Please wait.

Introduction Syllabus covers a wide range of propagation topics:-

Similar presentations

Presentation on theme: "Introduction Syllabus covers a wide range of propagation topics:-"— Presentation transcript:

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 > <

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) > < 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 > < 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

Download ppt "Introduction Syllabus covers a wide range of propagation topics:-"

Similar presentations

Ads by Google