Radio Propagation PDF

RADI0 PROPAGATION  Name Frequency Range Very Low Frequency (VLF...

RADI0 PROPAGATION  Name Frequency Range Very Low Frequency (VLF) 3-30 kHz RADI0 PROPAGATION Low Frequency/Longwave 30-300 kHz Medium Frequency/Medium Wave 0.3-3 MHz RF bands and bandwidths High Frequency (HF)/Short Wave 3-30 MHz Very High Frequency (VHF) 30-300 MHz Previously, the different RF bands and their names and frequencies ranges were discussed (above). Ultra High Frequency (UHF) 0.3-3.0 GHz -Different antennas were suitable for the Super High Frequency (SHF)/Centimetre 3-30 GHz different bands. Wave Extra High Frequency (EHF)/Millimeter 30-300 GHz -These bands relate to the carrier frequency of the signal. Name Frequency Range Telegraphy 1.25kHz Signals also have a bandwidth (BW or frequency Sound Broadcast: AM in LF & MF band 9kHz range of operation). Sound Broadcast: AM in HF band 10kHz -The BWs for various applications and their associated signals are below. FM 180kHz TV (Video Part) 5.5MHz TV (Audio Part 20kHz Point-to-Point Radio Systems 250Hz-3kHz  RADI0 PROPAGATION Modes of propagation Signal transmitting information can move from Tx to Rx by means of five modes of propagation: -Ground (surface) wave, -Skywave, -Space wave, -Tropospheric scatter, -Communications satellite. Satellite communications involve sending signals outside the Earth’s atmosphere to be bounced by a satellite. Skywaves use the Earth’s ionosphere as natural satellite. Ground/surface waves follow the Earth’s curvature. Space waves are line-of-sight propagation.  RADI0 PROPAGATION Ionosphere & troposphere Before discussing these modes of propagation, two aspects of the Earth’s atmosphere need to be differentiated: -Troposphere, -Ionosphere. As can be seen the ionosphere & troposphere are differentiated by their respective heights in the sky. -The troposphere exists as far as 10km where it is replaced by the ionosphere. -As its name suggests, the ionosphere contains many charged particles (ions).  RADI0 PROPAGATION Ionosphere The ions arise because UV light from the sun causes electrons to be liberated from the atoms in the atmosphere of the ionosphere. -Losing an electron results in an imbalance of positively charged protons in the atom and hence that atom is now ionic. -The degree of ionization is measured by the amount of free electrons that exist per 𝑚3 of volume. As the UV light permeates through the ionosphere, it loses energy. -This results in different levels of free electrons circulating in the ionosphere thus providing different layers of ionosphere.  RADI0 PROPAGATION Ionosphere The different layers of ionosphere are depicted above. -Their respective electron densities are depicted below. The (sub-) layers of the ionosphere are called: 𝐹2, 𝐹1 , 𝐸 & 𝐷. -Below 𝐷 is the troposphere. As can be expected, the electron density increases with height above the Earth’s surface (above). -This is because the UV light has more energy the higher it is with respect to the Earth’s surface.  RADI0 PROPAGATION Ionosphere – Sub-layers The different layers of ionosphere can be described as follows: D-layer: This is where the electron density in electrons/𝑚3 is smallest. -This is to be expected as it is the nearest to the Earth’s surface of them all. -At night, the UV disappears and thus, so too does the D-layer. -There is a high rate of electron-atom recombination in the D-layer as the UV light is at its weakest. -Thus, it is the most likely of the layers to disappear at night.  RADI0 PROPAGATION Ionosphere – Sub-layers E-layer: The E-layer has a lower rate of electron- atom recombination. -It therefore does not disappear at night but rather becomes weaker. -It occurs at a height of 100km above the Earth’s surface. 𝑭𝟏 & 𝑭𝟐 layers: The 𝐹1 layer is at a constant height of 200-220km in the daytime. -However, the 𝐹2 layer’s height varies depending on the season of the year. -The 𝐹2 layer’s height is 250-350km in winter. -The 𝐹2 layer’s height is 350-500km in summer.  RADI0 PROPAGATION Ionosphere – VLF (3-30kHz) & LF (30-300kHz) behaviour In the VLF & LF bands, the ionosphere acts like a highly reflective lossless medium. -This means that signals using VLF & LF carrier frequencies will bounce off the ionosphere and the Earth. -This allows them to propagate long distances in a guided fashion (opposite). Because, the ionosphere is a lossless medium, there is little loss in the signal and only what would be accounted for by distance, i.e., path loss. Ionosphere – MF (0.3-3MHz) behaviour The D-layer causes attenuation to MF waves before they reflect off the E-layer (opposite). -The D-layer is said to act as a lossy medium to MF waves.  RADI0 PROPAGATION Ionosphere – MF (0.3-3MHz) behaviour This attenuation is at a maximum at 1.4MHz and this is called the gyro frequency. However, at night this lossy layer disappears and MF waves can reflect from the E-layer with a lot less attenuation. Ionosphere – HF (3-30MHz) behaviour The D-layer attenuation decreases in the HF band. -Thus, there is reflection from the E-layer with less loss. Ionosphere – VHF (30-300MHz) behaviour At VHF and higher, the ionosphere is no longer able to reflect EM waves (opposite)  RADI0 PROPAGATION Ionospheric effects - Faraday rotation At frequencies above VHF, Faraday rotation 𝜃𝐹 can occur in the ionosphere, which has a rotative effect on polarisation state (below). -Faraday rotation will occur whenever there is magnetic field in the presence of linearly polarised EM radiation (opposite). The degree to which the rotation occurs is quantified by: 2.36 × 1020 𝜃𝐹 = 𝐵𝑎𝑣 𝑁𝑇 𝑓2 Where: 𝐵𝑎𝑣 = 7 × 10−21 Wb/m is the average magnetic field of the Earth, 𝑁𝑇 is the amount of electrons encountered in a column of cross-sectional area 1m2 (typically 1016 − 1019), 𝑓 is the frequency of transmission. The effect does not affect systems using circularly polarised signals, which is why circular polarisation is often used in satellite communications.  RADI0 PROPAGATION Ground waves Signals above MF are limited by the Earth’s horizon that they observe in terms of the maximum distance that they can travel. -The Earth’s curvature blocks them from travelling further -This is typically 80-90km. However, at LF & MF frequencies, the signal is seen to, ‘hug’, the Earth’s surface. -This phenomenon is known as, ‘ground wave’. -This means that the Earth’s curvature is no longer blocking the signal anymore and the signal can travel over 1000km.  RADI0 PROPAGATION Ground waves To exploit this effect, vertical polarisation must be used. Dry ground can offer significant frequency dependent attenuation. -The least propagation occurs when the ground wave propagates over water. Frequency Range (km) 100 kHz 200 Also, the propagation is frequency dependent, which in turn limits its distance (range) differently at 1MHz 60 different frequencies. 10MHz 6 -The table opposite, shows figures for maximum 100MHz 1.5 range (due to attenuation) for the case of ground waves propagating over ground of average dampness with a Tx power of 1kW.