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

Radio and wireless communications overview 297(e) It is beneficial to obtain mean time between failure (MTBF) figures frommanufacturers because if the radio is to be placed in a remote location that is difficult toregularly access, it is vital that the radio is reliable.(f) If the telemetry communication protocol requires the radio is to be switched on and offat regular intervals, it is best to avoid having relays in the inline RF circuit. Discretetransistor RF switching is preferred.17.8 TroubleshootingWhen troubleshooting an existing system, it is worth checking on a few issues discussedin the previous section. These are as follows:Frequency selectionIs the frequency selection good? Look at Table 17.2 to see whether this is correct as far asabsorption of wet vegetation, fading issues and diffraction properties are concerned.Interference from other radio equipmentBesides the issue of intermodulation which can be addressed and corrected, if theinterference on the radio equipment is due to another radio user operating close by on thesame frequency or from a noisy radio transmitter emitting spurious frequencies outside itsallocated bandwidth, the only fix is to terminate the other radio’s transmissions in someway.Intermodulation problemsThis is discussed in the previous section. If you find considerable interference from otherradio equipment, this could be the problem. Do the analysis using the software to find outwhether this is a problem for your particular application.Incorrect path loss calculationIf you find the received signal is weaker than anticipated, it is worth running a path losscalculation and checking that the fade margin is adequate. A fade margin of at least 30dBshould be targeted.Radio modemsAs far as troubleshooting radio modems is concerned, once the issues above have beenaddressed; often the problems are related to the RS-232 or RS-485 interfaces. Thetroubleshooting techniques here are indicated in chapters 3 and 4.