Ionospheric Propagation

Transcription

1 Ionospheric Nick Massey VA7NRM 1

2 Electromagnetic Spectrum Radio Waves are a form of Electromagnetic Radiation Visible Light is also a form of Electromagnetic Radiation Radio Waves behave a lot like light but, the wavelength of radio waves is 1000 m to 0.01 m whereas the wavelength of light about 500 x 10-9 m HF Radio has wavelengths from 200 m to 10 m 2

3 Path Loss Good Path Transmit 100 Watts (+50 dbm) Receive an S9 signal, Watts (-73 dbm) Path loss is 1: 2,000,000,000,000 (123 db) 20 Trucks carry 2,000,000,000,000 grains of sugar Poor Path Transmit 1000 Watts (+60 dbm) Receive an S4 signal, Watts (-103 dbm) Path loss is 1: 20,000,000,000,000,000 (163 db) 40 Ships carry 20,000,000,000,000,000 grains of sugar 3

4 Polarisation (Linear) 4

5 Circular Polarisation E Field Vector Rotates Right-hand (CW) or Left-hand (CCW) Elliptical Polarisation - amplitudes are not equal Linear polarised wave can be created from two counter-rotating Circular polarised waves 5

6 Space Weather Parts I and II Ionosphere UV Radiation Magnetosphere Solar Wind Cycles Sun Spot (~11 Yrs) Seasonal Diurnal Storms Geomagnetic Storms - Solar Wind Blackouts - X-rays Solar Radiation Storms - Protons 6

7 Ionisation Atmospheric gas particles become positively charged ions by removal of negatively charged electrons Plasma In sufficient quantity, free electrons affect the velocity of radio wave propagation Energy to produce ionisation comes from ultraviolet and x-ray radiation from the sun In the absence of ionising energy, the ionised particles die away due to collisions between ions and electrons 7

8 Ionised Layers D Layer km, Only during daytime Attenuates radio waves passing through it (Collision Freq.) AM broadcast stations stronger at night E Layer km, Always present Primarily useful for daytime km links Sporadic E F Layer Day F km and F km, Night F2 300 km F2 is most used layer for HF communications 8

9 The Ionosphere 9

10 The Ionosphere To scale! 10

11 The Magnetosphere 11

12 Skywave Radio waves appear to be reflected back to earth by the E or F Layers In fact, due to refraction. The velocity of the radio wave is changed as it passes through the ionised layer In the ionised layer, the refractive index (n) < 1 and the wave speeds up, bending away from the normal As ionisation increases n decreases and wave bends further and further until it reflects back towards earth For any given frequency, the amount of refraction depends on ionisation density and angle of incidence 12

13 Reflection by Refraction 13

14 Refractive Index of Cold Magnetised Plasma Appleton Hartree Formula n is refractive index ± means there are two solutions i.e Two differing refractive indices Birefringent Ө angle between wave vector and ambient magnetic field 14

15 Why Two Solutions? Plasma Physicists forgive me! Free electrons are moved by the magnetic field Gyrofrequency MHz Ions are much heavier and move correspondingly slower Electric Field parallel to Magnetic Field - electrons move more easily Electric Field perpendicular to Magnetic Field electrons "push back" 15

16 Magneto-Ionic Splitting On entering the birefringent ionosphere an incident radio wave splits into two eliptically polarised waves The exact polarisation depends on the angle between the wave vector and the magnetic field In the unusual case that the wave vector is parallel to the magnetic field the two waves will be counter-rotating circular polarisation When wave vector and magnetic field are not parallel Ordinary Wave (O) behaves as if there is no magnetic field Extraordinary Wave (X) modified by magnetic field 16

17 Did you get that? 17

18 Not Always Great Circle Not Always Reciprocal 18

19 Critical Frequency In ionised layer, refractive index depends on amount of ionisation and frequency As frequency increases, n increases and amount of refraction decreases Critical Frequency = Highest frequency which will reflect vertically from the ionosphere Different critical frequencies for different layers, f 0 E, f 0 F1, f 0 F2 etc. Different critical frequencies for the O-mode and the X-mode Vertical Ionogram from HAARP Alaska Red O-mode Green X-mode 19

20 Vertical Ionogram Alaska 20

21 Vertical Ionogram UK 21

22 Oblique Frequency Increasing Angle Increasing 22

23 Pederson Rays 23

24 MUF, OWF and LUF Maximum Usable Frequency (MUF) Calculated from (Predicted) Critical Frequency Depends on Location and Range Critical Frequency depends on Ionospheric Conditions Actual maximum operating frequency can be higher Optimum Working Frequency (OWF) 0.85 x MUF Predicted 90% Probability of propagation Lowest Usable Frequency (LUF) Depends on Location and Range Depends on (Predicted) Ionospheric Conditions Depends on Transmitter Power Depends on Antenna Gain (Directivity) 24

25 Oblique Ionogram (3000 km, Sun Spot Min.) 25

26 Interpret Ionogram 26

27 Many Modes 27

28 Flimsy Ionosphere Mass ~ 1 tonne Diameter ~ 13,000 km Density ~ 1/10 15 (1/(1000* * ) ) Air at Sea Level Hugely susceptible to disturbances Magnetic Solar Wind Weather (below) Aurora NOT a sphere Moving Lumpy "Tilted" Travelling Ionospheric Disturbances (TIDs) 28

29 Distortion The signal that is received is not very often the same as the signal that was transmitted! Fading Groundwave / Skywave interaction Multiple skywave paths (including magneto-ionic splitting) Movement of ionospheric irregularities Spread-F / Aurora Frequency Shift and Frequency Spread (Doppler) Time Dispersion and Delay Spread 29

30 DAMSON 30