Radio Propagation PDF

RADI0 PROPAGATION  RADI0 PROPAGATION NORMAL ↓ AIR S...

RADI0 PROPAGATION  RADI0 PROPAGATION NORMAL ↓ AIR Sky wave – Refractive index in ionosphere HORIZONTAL -7 When a wave passes from one medium (material) to CLASS another medium, it will bend (opposite). All media have a refractive index 𝑛, which sets the ratio of how fast light travels through it with respect to the speed in vacuum, i.e., 3 × 10 m/s. -When an EM waves passes from a medium of high refractive index to low, the wave is bent towards the horizontal (above). GLASS -When an EM waves passes from a medium of low refractive index to high, the wave is bent away from AIR the horizontal (below). This bending is called refraction. -The ratio of incident angle 𝜙 to refracted angle 𝜙 also sets the refractive index 𝑛 as: 𝜙 𝑛= 𝜙  RADI0 PROPAGATION Sky wave – Refractive index in ionosphere Within the ionosphere, there is a variation in 𝑛. -This has a number of dependencies. 𝑛 will vary with height with 𝑛 decreasing with increasing height. -This is due to the fact that there are more free electrons the higher one is in the ionosphere. -This is caused by the greater energy of UV light observed as mentioned previously. 𝑛 will also vary with frequency with 𝑛 increasing with increasing frequency. As a result, 𝑛 has a relationship with both frequency 𝑓 and number of free electrons per 𝑚 𝑁 as: 81𝑁 𝑛= 1− 𝑓  where it Frequency Fruit-pentetrates CONUSPHERE RADI0 PROPAGATION Fmuf Fouf Sky wave – Critical frequency The critical frequency 𝑓 of the ionospheric layer is the maximum frequency that the layer can return to Earth when propagated upwards. -The height at which this reflection occurs is known as the, ‘virtual height’. 𝑓 will depend on the maximum electron density per 𝑚 𝑁 observed in the ionosphere at a given point in time. -This may be written as: 𝑓 =9 𝑁 This parameter is important because: -It is measurable from the ground, -It sets what is called the maximum usable frequency (MUF), 𝑓.  RADI0 PROPAGATION Sky wave – Critical frequency The maximum usable frequency (MUF) 𝑓 is the highest frequency that can be used to establish communication using a sky wave between two points. It is dependent on: -The angle of 𝜙 that the EM wave makes with respect to the ionosphere, -𝑓 at the time of launching the EM wave (opposite). As a result, the 𝑓 may be expressed: 𝑓 USE THIS & TEETA 𝑓 = => cos 𝜙 Since by definition: 1 sec 𝑎 = cos 𝑎 Thus: 𝑓 =𝑓 × sec 𝜙  RADI0 PROPAGATION Sky wave – OWF, LUF & maximum incidence angle Reliable skywave propagation occurs when the communications system operates at the optimum working frequency (OWF) 𝑓 , which is expressed as: 𝑓 =𝑓 × 0.8 Furthermore, the lowest frequency at which a reliable skywave link can be established is called the lowest useful frequency (LUF). -The LUF and MUF vary similarly. Also, the angle of incidence 𝜙 with which a sky wave makes with the ionosphere has a maximum limit 𝜙 (below). -This occurs when the sky wave travels to ionosphere in a tangential manner as depicted.  -+ 4 RADI0 PROPAGATION Sky wave – Skip distance/dead zone The minimum distance at which a sky wave can be propagated is known as the, ‘skip distance’, or, ‘dead zone’. -This is because communications cannot be established at a shorter distance. -The skip distance is a function of frequency of the EM wave. -The frequency will also determine which ionospheric layer will be used and some typical skip distances per layer are shown in the table below Layer Skip distance -It is also greatest at night when the ionosphere is 𝐸 2000 km higher. 𝐹 3400 km It is equivalent to 2𝑑 and may also be calculated as: 𝐹 4000 km NOT In 2𝑑 = 2𝑅𝜃 = 2𝑅𝐶𝑜𝑠 EXAMS  RADI0 PROPAGATION Sky wave – Multi-hop Further to this, the required angle of incidence 𝜙 with the ionosphere varies with respect to the frequency of EM wave signal used as: 81𝑁 sin 𝜃 > 1− 𝑓 If this criterion is not met, then the sky wave is not effective (above). Another option for sky wave propagation if the distance required is long, is multi-hop sky wave (below). In this case, two hops can be used with a reflection from the Earth’s surface forming part of the communications link. -Two and three hop systems have been implemented. -If the frequency used is just below 𝑓 , the losses can often be quite small thus facilitating multi-hop.  RADI0 PROPAGATION Sky wave – Fading In the context of frequencies suitable for skywave, i.e., VLF, LF & MF, general fading is said to occur when the entire signal fades. -Recall that a signal will have an associated bandwidth, therefore general fading is drop in power that is consistent across that bandwidth. -Anything that causes fluctuations in the ionosphere, such as solar flares, can cause the D-layer to totally absorb the wave leading to a general fade lasting hours in some cases. Selective fading occurs when only part of the signal's bandwidth experiences a decrease in power. -This often occurs when the signal arrives by a second unintended path (opposite).  RADI0 PROPAGATION Sky wave – Diversity A solution to fading is that of diversity and this can be undertaken in two main forms: -Frequency diversity, -Spatial diversity. Frequency diversity involves transmitting the signal twice simultaneously on two different carrier frequencies. -This idea being that fading has a random component to its behaviour and it rarely happens on two different frequencies simultaneously. -At the Rx, the power in the signals at both frequencies is measured and the one with the higher power is chosen as the signal to be further processed to establish the communications link.  RADI0 PROPAGATION Sky wave – Diversity Spatial diversity is a similar concept except that instead of operating at two different frequencies, there two Rx antennas in different places. -Again, a similar principle is at play. -Fading on both links is unlikely to happen simultaneously. -The link with higher receive power is chosen to be processed and establish the link. -This advantage to this is the need for a large site and a second antenna.  RADI0 PROPAGATION NVIS - NEAR VERTICAL INCIDENT SILY Sky wave has so far been discussed in relation to long propagation distances, i.e., ~1000km or more. -However, there is another version of it where a range of distances, amounting to a coverage area, is served. This is called near vertical incidence sky wave (NVIS) and is achieved by transmitting the signal at a very steep launch angle (70 − 90 ). it Everything near ↑ -This use of the steep launch angle is how it differs will work from standard sky wave (opposite) -The signal goes up to the ionosphere and is scattered downwards to create a coverage area. EXAM -As a result, NVIS does not have a skip distance or Q dead zone. NVIS is useful in disaster zones to create a coverage area when mobile phone operations have ceased.