Radio Frequency Propagation: A General Overview from LF to VHF. Presented by: Mike Parkin GØJMI Slide 1
Introduction Mike Parkin: First licensed as G8NDJ in 1977. Became GØJMI in 1988. Interests in Radio have included: - Microwave Bands (Built from 23cm to 6mm) - 6m, 10m and 12m operating SSB/CW - 60m and 80m CW QRP - Building equipment (Tx, Rx, PSU) - Antennas (Designing, Building and Using) - Operating as /P - Satellites - Propagation QTH: Alton, Hampshire. BSC(Hons) MIET CEng MCGI (Electrical, Telecoms & Radio Engineer) Slide 2
Presentation Content 1. Basic Theory. 2. Ground Wave Propagation. 3. Sky Wave Propagation. 4. Space Wave Propagation. 5. Summary and Q&A Session. Annex 1: Ionosphere Height Calculation. Slide 3
1. Basic Theory Useful RF Wave categorisations: Very Low Frequency (VLF): 3 to 30kHz. Low Frequency (LF): 30 to 300kHz. Medium Frequency (MF): 300kHz to 3MHz. High Frequency (HF): 3 to 30MHz. Very High Frequency (VHF): 30 to 300MHz. Ultra High Frequency (UHF): 300MHz to 3GHz. Super High Frequency (SHF): 3 to 30GHz. Extra High Frequency (EHF): 30 to 300GHz. Slide 4
1. Basic Theory Path Loss, in db: 32.5 + 20.Log(Frequency in MHz) + 20.Log(Path Length in km) For a 100Km path length: 472KHz: 66dB Observation: 3.65MHz: 7.1MHz: 14.2MHz: 84dB 90dB 96dB As the frequency increases so the Path Loss increases. 28.5Mhz: 102dB Tx Station Rx Station 50.2MHz: 107dB 100km 144.3MHz: 116dB Slide 5
1. Basic Theory RF Waves Radiated Include: 2. Sky Wave 1. Space Wave, made up of Direct wave and Reflected Waves 2. Sky Wave Tx 1. Direct Wave Rx 3. Surface Wave 1. Reflected Wave 3. Surface wave Slide 6
2. Ground Wave Propagation Ground Wave: Comprises the Space Wave and Surface Wave signals transmitted. Space Wave Surface Wave 1. Ground Waves travel along the surface of the ground due to diffraction. 2. Low mounted Ground Wave antennas suit vertical polarisation because the low resistance of the earth causes the electro-component of horizontally polarised waves to be short-circuited (electric field is horizontal to earth and so induces shorting emf). 3. When used to transmit LF and VLF waves, the Sky Wave component is negligible because reflections cancel the wave out (reflections 180 o out of phase). 4. Ground Wave propagation suits VLF and Long Wave (LF/LW) wave transmission. 5. Ground Wave also suitable for Medium Wave transmissions (MF/MW). Slide 7
2.Ground Wave Propagation Wave front is tilted because the diffracted wave s magnetic component causes currents to flow in the ground that draw power downwards from the wave. This gives the wave a forward and downwards vector that collectively cause the wave to tilt. Ground Wave Ground Wave Propagation becomes of interest for radio amateurs operating on the 136KHz (LF) and 472Khz (MF) and bands. Slide 8
Introduction The Sun: Radiates electro-magnetic energy including emissions in the Ultra-Violent wavelengths (UV ranges from 400nm to 10nm) and X-Rays (10nm to 0.01nm). Earth s Ionosphere: A region surrounding the earth where the Sun s UV emissions are absorbed by the atmosphere s molecules causing them to be ionised creating electrically charged molecules/atoms and releasing electronics. Bursts of X-Rays can significantly effect the Ionosphere s density and structure. The UV and X-Ray emissions are particularly of interest to us as Radio Amateurs because they influence the propagation of RF signals around the planet from LF through to VHF. Not to Scale! Ionosphere Slide 9
The Ionosphere 600km 300km F2-Layer F-Layer F1-Layer E-Layer 500km 250km 220km 200km 150km 600km up 350km up Ionosphere 85km 45km Mesosphere Stratosphere D-Layer 90km 50km 12km Troposphere Slide 10
Concept of Critical Frequency Ionosphere Incident Wave Returned Wave 1. The Critical Frequency (F c ) refers to the highest RF signal that is returned from the Ionosphere back to the Earth s surface when the radiated signal is at 90 o to the Earth s surface. Tx/Rx Station 2. The Critical Frequency varies with time of day, is usually different each day and depends on the level of ionisation within the Ionosphere. 3. When the signal is above Fc then the signal escapes into space. Slide 11
Concept of Maximum Usable Frequency Ionosphere Maximum Usable Frequency (MUF) MUF = F c α o Tx Station Rx Station sine (α o ) Slide 12
Concept of Maximum Usable Frequency Above MUF Ionosphere Maximum Usable Frequency (MUF) Below MUF Zone where Signal Received Tx Station Rx Station Slide 13
Concept of Dead Zone Ionosphere Maximum Usable Frequency (MUF) Below MUF Dead Zone Zone where Signal Received Tx Station Rx Station Slide 14
Concept of Skip Distance Ionosphere Skip Distance Can be up to about 4,000km. Tx Station Rx Station Slide 15
Modes of Travel, Single Hop Ionosphere F2 F1 E Tx Station Gradually Diffracted off each Layer and Returned to Earth. Rx Station Slide 16
Modes of Travel, Concept of Multi-Hop F2 F1 Tx Station Rx Station Slide 17
Ionosphere at Night F2 F1 E D Ionisation Reduced F1 Dissolves Ionisation Weakened D Layer Disappears Tx Station Day Night Sunset Rx Station Slide 18
The D-Layer: VLF & LF Propagation Significant Ionisation allows D-Layer and Ground to act like a Waveguide or Duct for VLF and LF Signals. D- Layer Tx Station Rx Station Slide 19
The D-Layer and E-Layer: MF & HF Propagation (to 10MHz) UV and X-Rays Day Sunset Night E-Layer (O2) D-Layer (NO,O 2,N 2 ) D Rapidly Disappears Tx B Sky Wave Tx Station B Tx Station A Sky Wave Ground Wave Sky Wave and Ground Wave algebraically add, causing Fading (QSB). Rx Station Slide 20
The E-Layer: Sporadic E (Propagation to about 200MHz) UV and X-Rays From about May to September intensely ionised clouds can form in the E-Layer (Sporadic E, or E s ). E-Layer (O2) Signals over about 10MHz tend to go though the E-Layer Signals from about 24 to 200MHz E s works well at 50 & 70MHz. Tx Station Skip Distance can be up to 2000km. Rx Station Slide 21
The F-Layer: HF Propagation #1 UV and X-Rays F2-Layer F1-Layer Tx Station If F1 is significantly ionised, the signal is bent by F1 only. Signal is bent by F1 Layer and then by F2. Rx Station Slide 22
The F-Layer: HF Propagation #2 UV and X-Rays Day Sunset Night F2-Layer F1-Layer Can allow MUF to exceed 35MHz and may be even get to 50MHz. Tx Station Skip Distance can be up to 4000km. Rx Station Slide 23
Radiation Level Solar Rotation Variation 1. The Sun s rotation is about 27 Earth days (as seen from Earth). 2. Sunspots and other Solar Events can be seen to track across the surface of the Sun going from limb-to-limb in about 13.5 Days, then to reappear after they have travelled across the remote side of the Sun. 3. A Solar Event that has influenced Sky Wave propagation tends to return after about 27 days and its influence may influence propagation again (assuming it has not disappeared). Sun Sun Sun Sun Day 1 Day 7 Day 14 Day 27 Slide 24
Radiation Level Seasonal Variation Area receives longer duration of radiation = higher ionisation, MUF tends to be lower. Significant E-Layer. Northern Hemisphere Summer (June 21 st ) Area receives shorter duration of radiation = Lower ionisation, MUF tends to be higher. Lesser E-Layer. Northern Hemisphere Winter (December 21st) Earth Sun Earth 23 o Southern Hemisphere Winter (June 21st) F2-Layer predominates during longer nights, e.g. allowing transatlantic skip. Not to Scale! 23 o Southern Hemisphere Summer (December 21st) Slide 25
The Solar Cycle 1. Our Sun is a variable star, its energy output varies on an 11 year cycle. This is referred to the Solar Cycle with each cycle given a number with No. 1 starting in 1755. 2. Solar Cycle No. 25 seems weaker than usual and should have peaked by the end of 2013, however more activity has been observed recently. 3. The Solar Cycle refers to the number of sunspots observable from Earth, with maximum counts occurring about every 11 years. 4. The number of sunspots observed indicates how active the sun is in terms of the level of the UV and X-Ray emissions. Basically, the more sunspots seen the higher the UV and X-Ray levels tend to be. 5. Sunspots are magnetic storms seen on the Sun s Photosphere in visible light. They emit lower levels of UV and X-Rays, however they are associated with Flares and Faculae which are known to emit high energy levels. Slide 26
Solar Events 1.Solar Flares: 1. Very powerful magnetic fields (Sunspots) can result in the sudden eruption of Solar Flare which is an emission of hydrogen gas, charged particles and X-Rays. 2. A Solar Flare can last from a few minutes to several hours. 3. Solar Flares are classified as: A, B, C, M or X depending on their X-Ray level. 4. There are 4 energy thresholds: 2, 10, 50 and 100MeV (denoted 1, 2, 3 and 4). 5. The size of Solar Flares is categorised from 1 to 4 while its brilliance from F (Faint), N (Normal) to B (Bright) with S indicating Sub (e.g: SF = Sub-Faint). 6. M and X classifications are the most powerful and can significantly effect the Ionosphere and propagation (often included in GB2RS news broadcasts). 7. E.g: M4/2B = M Class Flare of up to 100MeV of Brilliance 2 and Optically Bright. Slide 27
Solar Events 2. Solar Flares: 1. Solar Flares can emit X-Rays, UV, RF (usually 3cm to 10m) and release charged particles (electrons and protons) that are carried by the Solar Wind. 2. X-Ray, UV and RF (usually 3cm to 10m) emissions arrive from the Sun after about 8 minutes. The X-Ray and UV emissions immediately increase D-Layer Ionisation causing HF band fade-outs that can last hours (Sudden Ionospheric Disturbance, SID). Generally, the RF noise received can increase on the VHF bands and upper HF frequencies. Strong X-Rays and UV Fade-Outs, HF Band Propagation Lost D-Layer more D-Layer RF Absorptive 90km 50km Slide 28
Solar Events 3. Solar Flares: 1. Solar Flare release of charged particles (electrons and protons) arrive after about 20 to 40 hours. 2. On the earth s day-side the charged particles may interact with the Van Allen belts releasing more electrons into the night-side. If this flow of electrons has the correct magnetic polarity, the flow is towards the Pole causing ionisation of Oxygen and Nitrogen about 80km up. This causes the Aurora (Ionised O and N). Solar Flare s Particles Van Allen Belts Electrons Flow Along Magnetic Field Earth Aurora 1000 60,000km Slide 29
Solar Events Image of the Sun taken at 12.00pm on 27 th February 2014 (in the back garden) before the Aurora Display seen as far south as Reading that evening. Slide 30
Image of the Sun Taken on Saturday 3 rd May 2014 at about 12.00 UTC. Slide 31
Image of the Sun Taken on Friday 9 th May 2014 at about 15.30 UTC. Slide 32
Solar Events 3. Solar Flares (continued): 3. The cloud of charged particles has a magnetic polarity that may interact with the Earth s magnetic field if the polarities align causing a Magnetic Storm. This can last for several days. 4. Ionospheric Storms are associated with Magnetic Storms that can disturb the F2, F1 and E-Layers disrupting Sky Wave propagation (severely limiting the HF bands). 5. Magnetic effects are measured in terms of a Geomagnetic A Index: 1-10 Quiet, 11-20 Unsettled, 21-50 Sub-Storm, 51-80 Storm, 81+ Severe Storm. 30+ usually means poor HF propagation. 50+ can mean an Aurora. 6. Some reports may use a Geomagnetic K Index: K 0 1 2 3 4 5 6 7 8 9 A 0 3 7 15 27 48 80 140 240 400 Slide 33
Solar Events 1. Solar Flux 1. A measure of the RF noise at 2,800MHz, taken at midday. 2. Measurement taken at Penticton, British Columbia, Canada. 3. Has a close correlation to visual Sun in terms of the number of Sunspots observed. 4. The level varies from about 60 units (Sunspot Cycle Low) to 300 units (Sunspot Cycle High). HF Sky Wave propagation conditions are usually good when the A Index is LOW (e.g. under 15) and the Solar Flux is HIGH (e.g. over 100 Units). Slide 34
Solar Events 1. Coronal Holes: 1. Gaps in the Sun s Corona (outer atmosphere) that allow solar material to be ejected. 2. A Coronal Hole can exist for several weeks and seen to travel across the Sun as it rotates, returning to the same point after about 27 days. 3. Corona Holes in Sun s equatorial regions can eject material that can cause magnetic disturbances. If the magnetic disturbance has a South Pole polarity, it can couple with the Earth s Northward Magnetic Field. 4. Charged particles ejected from the Coronal Hole (and other Solar Wind material) can then flow towards the Earth s North Pole and set up an Aurora (sometimes called a Scottish Aurora). 5. The Aurora may allow 2m, 4m and 6m openings, but severely effect HF conditions. 6. The passage of Coronal Holes are often included in GB2RS news broadcasts. Slide 35
4. Space Wave Propagation Basic Theory: 1. The Space Wave is made up of the Direct Wave and the Reflected Wave with the Tx Station and Rx Station generally considered to be Line-of-Sight. Tx 1. Direct Wave 1. Reflected Wave Rx 2. As the frequency increases so the Path Loss increases. E.g. For 100km: 28.5Mhz: 102dB. 50.2MHz: 107dB. 144.3MHz: 116dB Slide 36
4. Space Wave Propagation The Troposphere: 1. The Troposphere is the main area of interest within the Atmosphere. This is the portion of the Atmosphere from ground level to about 12km up. 2. The influence of the Troposphere on propagation becomes significant when the frequency in use exceeds 28MHz, i.e. at VHF and above. 3. The Troposphere is the portion of the Atmosphere where most of the weather exists in terms of the impact of High Pressure and Low Pressure Systems. 4. Water vapour within the Troposphere has the most significant influence on propagation at VHF, UHF, SHF and above. 85km 45km 12km Mesosphere Stratosphere Troposphere Slide 37
4. Space Wave Propagation The Troposphere: Factors Influencing Tropospheric Propagation: 1. Air temperature ( o C) 2. Atmospheric Pressure (milli-bars, mb) 3. Moisture (Relative Humidity, RH). Altitude Temperature lowers (about 2 o C/1000ft) 10,000ft (~ 700mb) 3,300ft (~ 850mb) 0ft (~ 1012mb) Not to Scale Pressure Increases 1mb/30ft) Air becomes drier Slide 38
4. Space Wave Propagation Conditions, Refractive Index N 1. One of the stranger properties of moist air is that it can diffract electromagnetic waves (e.g. light and RF). 2. The Radio Refractive Index, N, is used as a measure as to how much Radio Frequencies are bent through refraction within moist air. 3. N is a function of: The Air Moisture Content (e), The Pressure (p) and the Temperature (T). The higher the value of N, the more the RF is diffracted. N = (77.6/T) x (p + (4810 x e)/t)). Where T in o K, p in mb, e is water vapour pressure in mb 4. When measured N usually has a very small value, e.g. 1.000345. However, rather than quoting 1.000345 it is usual to quote N as 345 (i.e. the last three digits). Slide 39
4. Space Wave Propagation Conditions, Refractive Index N 5. From ground level N usually decreases at 40 units/km (40 units/3300 ft) 6. A decrease of 40 units/km allows RF to be diffracted beyond the horizon and gives rise to the VHF concept of the 4/3 radius Earth. These are often referred to as FLAT CONDITIONS! 7. When N lapses at 157 units/km allows RF to follow the curvature of the Earth. 360N 380N 400N 1km Space Wave diffracted over the horizon Space Wave Slide 40
4. Space Wave Propagation Conditions Generally: 1. High Pressure Regions: Stable Air = Tends to give better conditions for Space Wave propagation. High, 1030mb 2. Low Pressure Regions: Unstable Air, Windy = Tend to give poorer conditions for Space Wave propagation. Low, 970mB Good Tropospheric Conditions tend to be associated when the pressure is High. Slide 41
4. Space Wave Propagation Conditions High Pressure: Air descends in a High Pressure Region Moister Air As Air descends it dries out Temperature Lowers Dew Point, where cloud forms because moisture condenses out of air Drier Air Slide 42
4. Space Wave Propagation Conditions, When Land is Cold High Pressure: o C Moist Air descending in a High Pressure Region Dew Point Reached Air Cools at Usual rate Warmer Air Cool Air Land is Cooler Temperature Inversion 1 o C 7 o C 5 o C This mechanism is called a Subsidence Inversion. Slide 43
4. Space Wave Propagation Conditions, When Land is Cold High Pressure: o C 1. Temperature Inversion s moist layer causes a high rate of change in N. When N exceeds 157 units/km allows RF signal diffraction back to earth. 2. RF signal bounces off the Earth s surface then is diffracted again. 1 o C 3. This process is referred to as Ducting and gives excellent dx conditions on VHF/UHF allowing contacts over 100 s of km. Dx er on top of Hill with no conditions!!!!!!! Temperature Inversion 7 o C 5 o C Tx Rx Slide 44
5. Summary and Questions 1. Basic Theory. 2. Ground Wave Propagation. 3. Sky Wave Propagation. 4. Space Wave Propagation. 5. Summary and Q&A Session. Annex 1: Ionosphere Height Calculation. Slide 45
Radio Frequency Propagation: A General Overview from LF to VHF. - End - Thank you for Listening Slide 46
Annex 1: Ionosphere Height Calculation IO91MD /G0JMI to IL18AT/EA8CQS on 50MHz (6m) Assumptions: Radius of Earth is 6378km. Take-Off Angle is a Tangent (90 o ) to. Single Hop contact. A knowledge of trigonometry. 1. Physical Distance between Stations is 2842km. 2Ɵ = (2842/2.Π.6378). 360 o = 25.53 o So, Ɵ = 12.76 o F A 12.76 o B IL18AT IO91MD 90 o C 90 o 77.24 o D 156.5km 2. Length AC = 6378. Sin(12.76) = 1387km. 3. Length DC = 1387. Tan(12.76) = 314km. 4. Length CE = 6378. Sin(77.24 o ) = 6220.5km. 5. Length FC= 6378 6220.5 = 157.5km. 6. Length DF= 314-157.5 = 156.5km. 12.76 o Ɵ E Ɵ 6378km Slide 47