Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 15: v1.2, 22-Dec-2012 (6) Propagation – Part-1 1 Chelmsford Amateur Radio Society Advanced Course (6) Propagation Part-1: Electromagnetic Waves

Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 15: v1.2, 22-Dec-2012 (6) Propagation – Part-1 2 Electromagnetic Waves Magnetic Field, H Electric Field, E Direction of Propagation Electromagnetic radiation comprises both an Electric and a Magnetic Field The two fields are at right-angles to each other, and the direction of propagation is at right-angles to both fields The Plane of the Electric Field defines the Polarisation of the wave The relationship between the magnitudes of the E and H fields is constant z x y

Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 15: v1.2, 22-Dec-2012 (6) Propagation – Part-1 3 EM Wave Fronts head-on Magnetic Field (H) Electric Field (E)

Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 15: v1.2, 22-Dec-2012 (6) Propagation – Part-1 4 EM Wave Fronts head-on Magnetic Field (H) Electric Field (E)

Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 15: v1.2, 22-Dec-2012 (6) Propagation – Part-1 5 Electromagnetic Radiation In free space conditions Electro-Magnetic (EM) waves travel in straight lines and spread out Both Power Flux Density (PFD) in W/m 2 and Field Strength in V/m, drop with distance Double the distance from an antenna and you will halve the field strength, but also quarter the Power Flux Density (PFD), according to an inverse square law

Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 15: v1.2, 22-Dec-2012 (6) Propagation – Part-1 6 EM Field vs Distance Tx Rx

Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 15: v1.2, 22-Dec-2012 (6) Propagation – Part-1 7 Inverse Square Law for Distance and Intensity Relationship between distance and intensity of electromagnetic radiation (rf or light) Inverse square gives a 1/d 2 reduction Example D= 1m - Intensity = 1 D = 2m - Intensity = ¼ Doubling the distance spreads same amount of energy over four times the area D=1 One unit of area D=1 Four units of area

Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 15: v1.2, 22-Dec-2012 (6) Propagation – Part-1 8 D=1 Another PFD Example D=1

Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 15: v1.2, 22-Dec-2012 (6) Propagation – Part-1 9 Velocity Of Propagation Propagation velocity depends on the medium in which EM waves travel: –Free Space  Maximum velocity –Air  slightly less than free space and dependent on water vapour –Coax  ~0.66 Times that of Free Space –Water  far less, and lossy v = f. vis the speed of radio waves light :- v = 300,000,000 m/s f is the frequency in Hz. is the wavelength in metres For /4 for 2m 300 /145.5 x 0.66 / 4 = Coax length 340.2mm v f

Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 15: v1.2, 22-Dec-2012 (6) Propagation – Part-1 10 Linear Polarisation Transmit and receive antennas should have the same polarisation –To be efficient... Otherwise, signal strength may be reduced by up to 32dB –Polarisation losses can be up to 5 S-points Note that after travelling through the ionosphere, the polarisation may have changed significantly Horizontal Antenna Vertical Antenna

Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 15: v1.2, 22-Dec-2012 (6) Propagation – Part-1 11 Circular Polarisation Magnetic Field, H Electric Field, E Direction of Propagation Used for satellite comms where the orientation of the EM field from the satellite is indeterminate Make sure you receive correct circular orientation or high polarisation losses will result The E-field of the wave rotates as it propagates, with either: Left-handed polarisation (LHCP) (anticlockwise from behind) as shown, or Right-Handed polarisation (RHCP) (clockwise from behind)

Chelmsford Amateur Radio Society Advanced Licence Course Christopher Chapman G0IPU Slide Set 15: v1.2, 22-Dec-2012 (6) Propagation – Part-1 12 Helical Antennas for Circular Polarisation 2.4 GHz Helical Antenna Designed for OSCAR 40 Omni-directional Quadrifilar Helical RHCP antenna