Practical Industrial Data Networks:Design, Installation and ...

292 Practical Industrial Data Networks: Design, Installation and Troubleshootingobstruction. It is important to calculate the amount of attenuation introduced by thediffraction and determine the affect it has on the availability (i.e. fade margin) of the radiolink. The mathematics for doing this calculation is relatively complex and has beenincluded in the software provided.As an example, Figure 17.10 shows a hill obstructing the radio path. Therefore acalculation is required to be carried out to determine the attenuation due to diffraction atthis hill. This would be then added to the total path loss to determine if the link will stilloperate satisfactorily.One further point of note is that this discussion and the formulas and software providedare for fixed radio links only. Mobile (moving RTUs) radio uses a completely differentset of criteria and formula. The greater majority of telemetry links are fixed and analysisof mobile radio is not provided with this book.17.7.2 RF path loss calculationsThe next step is to calculate the total attenuation of RF signal from the transmitterantenna to the receiver antenna. This includes:• Free space attenuation• Diffraction losses• Rain attenuation• Reflection lossesFree space attenuation and diffraction losses are calculated using the industry standardformulas. Rain attenuation is negligible at frequencies below 1 GHz.Reflection losses are difficult to determine. First of all, the strength of the reflectedsignal depends on the surface it is reflected off (for example, water, rock, sand).Secondly, the reflected signal may arrive in phase, out of phase or at a phase angle inbetween. So reflected waves can be anything from totally catastrophic to enhancing thesignal. Good engineering practice should always assume the worst case, which would becatastrophic failure. Therefore when designing a link, a check is made for reflections andif they exist, measures should be taken to remove the problem.This can be done by moving antennas or masts to different locations and heights or byplacing a barrier in the path of the reflection to absorb it. For example, place the antennabehind a hill, house, billboard, etc.Line of Sight PathReflected PathBillboardor BuildingFigure 17.9Removing potential reflections using barriersWith reference to Figure 17.10, the total loss would be:Therefore the total RF loss is (a) + (b).(a) A = 32.5 + 20log 10F + 20log 10D = free space loss= 32.5 + 53.1 + 24.6+ 110.2 dB