Isolating the problem
 
 

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INTERMITTENT COMMS:

One's mind can find the area of intermittent comms a real playing field. It is not uncommon to find almost every fault heard of becoming a fault that is explored.

ANTENNA SYSTEM FAILURES:

There are many components to an antenna system and the system will only be as reliable as the weakest link. Testing techniques vary from person to person but overall there are proven methods of carrying out the basic tests to ensure the system is healthy. Unfortunately these proven techniques are not without pitfalls and being unaware of them can lead to costly return visits and severe frustration.

The first thing that needs to be checked is the orientation of the antenna system. Personal experience has shown many installers to not sufficiently tighten masts carrying directional antennas. High winds then change the direction these are facing and suddenly the transmission path is suffering undue loss. Both the direction and polarization of the antennas of the whole system must be checked as there may have been two installation/repair teams and the one mounted the antenna vertically, the other horizontally. Birds, in horizontal systems, have been known to change the polarization too.

If not visible mechanical failure then one needs to start testing the electrical properties of an antenna system. Tools for testing such systems are not complex and neither are the testing methods. If nothing else, a test sheet drawn up and completed at every commissioning should ensure the system is healthy to start with.

Multimeter:
It may appear strange that a multimeter be the first instrument associated with RF but there is nothing handier for measuring whether or not a coax has gone open circuit, short circuit, has developed leakage etc.

SWR meter:
It may have been heard that measuring the Standing Wave Ratio (SWR) of an antenna system can be used as an indicator of an antenna system's general state of health, especially when the coax cable becomes old, has suffered water ingress, or a bad connection has developed. This is usually the case but there are pitfalls associated with this type of test especially when there are long coax runs between the radio and the antenna. This is fully explained in the section Antenna System SWR. Testing coax loss need not involve expensive equipment and a successful method is described here.

Bias T Coupler:
At first it may appear strange to want to inject DC onto an antenna system that may very well be short circuit but this device, when used with a multimeter and audio amp, is extremely handy for determining both breakdown and operating faults on an antenna system. The section on Connectors deals with tracing a faulty connector without having to remove the connector and possibly even any waterproofing from the coax.

WRONG READINGS:

Tackling any form of bad outputs i.e. outputs showing the wrong value, should be done by analyzing the comms between the stations as a first measure. This way you are absolutely sure the command is 1) coming from the correct input and module and 2) is being directed to the correct output, and finally 3) the value being relayed is correct (this also isolates whether the input or output side is at fault).

If points 1 are 2 are questionable then have a look at "Wrong Output Programmed" and see if this is your problem. If points 1 & 2 are fine then point 3 requires a little interpreting. If the value is correct then the output is not converting the received value into the correct current or state. If the incoming received value is incorrect then the input side requires attention.

Non-linearity (where for some of the input range readings are correct and then go wrong for the remainder) is an issue all on its own and has been dealt with in a following section. What is of importance here is to determine whether the fault originates at the input or output module. Sure, if the input module is in a dark, dingy, wet, cold site it will not be surprising to 'wish' the fault to be at the clean, airconditioned, output end but a lot can be done to minimise the time spent at the 'undesirable' side.

Take a spare module with you together with tools (laptop, current sources etc.) so you can modify analogue inputs to test for non-linearity and accuracy. Also learn any debugging or self test features the modules have. Try turning the output on or setting the level (especially at the problematic point/current) and see if the values are correct while in this 'local' mode. If such test facilities are not available to you then program the spare you brought with, oh no, you didn't leave it behind did you? Well, once you have it with you program it to send to the output being tested and see if the results are satisfactory. They are? Off you go to that horrible site.

Ah! We nearly forgot, there is one more input type that falls between digital and analogue, this being pulse. These inputs are digital inputs relaying an analogue value but they too are susceptible to wrong values. The typically fault being over-counting although missing counts are not uncommon. Two sections are written, the first with over counting, the second with missing counts.

SCADA (MMI/HMI) ISSUES:

Most modern radio telemetry systems finally interface into a Supervisory Control And Data Acquisition (SCADA) software package. These are also often referred to as a Man-Machine Interface (MMI) or Human-Machine Interface (HMI) software package or suite. A personally favoured MMI software suite is 'Citect' from Ci Technologies, Australia. There are many 'front end' process packages designed to interface to Citect, and example of this is SCADA-C by Elpro Technologies, also in Australia.

The standard means of interconnecting a radio telemetry system to the computer equipment is usually by means of the RS232 port (better known as a COM port), commonly using a protocol known as MODBUS (a trademark of Gould Modicon PLCs). Although there are some inherent weaknesses with MODBUS it has managed to become a world standard, probably through the fact Gould published it when it was written.

Stating MODBUS has weaknesses would do it injustice without explaining a main problem the protocol faces. Any noise introduced on either RS232 port (PC or PLC/radio) that could be deemed as a character, will be interpreted as the start of a message. Furthermore, should the transmit and receive be looped back on each other some cheaper equipment may accept the string as an acknowledgement. It is therefore imperative that all MODBUS carrying RS232 lines be of the highest quality shielded cable specifically designed for this type of comms. The use of twisted pairs with each signal having a ground return is strongly advised.

With MODBUS being so widely used, software tools are available to assist with determining the cause of link failures. A brief explanation of RS232 and very useful test software is given here.

POWER FAILURES:

This is usually an area, if a system is unrealiable, that is either intensely scrutinised or almost overlooked. It is often disregarded how large a part the quality of power fed to a radio telemetry system can play in a system's reliability.

Quality may not necassarily mean power dips or surges but can also mean the amount of power delivered and for what time. It may be perfectly accepted that power will only be available for some time of the day e.g. a privately run generator scheme, but the quality may be that it was assured the generator will run for six hours per day but when monitored it was found to only be four hours. Such errors could result from asking the questions during winter when night falls earlier, but the system was installed in the summer when the demand for power is reduced.

This scenario would have the charging time reduced by a third which could easily result in the battery not being charged enough to survive the other twenty hours of the day. Again, the top most portion of the solar calculator is the perfect tool to ensure the charging pattern will maintain a system.

Further areas where power quality can be questioned is when smaller transformers than should be are used during an economy drive. This usually results in lower charge currents being available than what is documented. This is false economy and should be avoided. If a module requires a 1amp transformer in order to power the module and charger successfully, then a 1amp or greater transformer should be employed. If the transformer is found to be at fault, replacing it with one of double the required capacity is not a bad idea as this builds in a little safety factor.

Not all power issues are related to design. External influences are discussed in Power Failures- External Influences