Conducting the Radio Test
 

 

It has, so far, been determined the requirement lends itself to radio telemetry, the system is technically and economically viable, and the radio paths, on paper, prove feasible. One step remains before deeming the project ready for quoting, this being to determine the radio paths are indeed attainable. This step is required as certain soil types and minerals within the soil can hinder (none are known to assist) radio waves propagating across the top. The higher the ferrous content of the soil the more the radio waves are attracted to it thus shortening the range attainable for a given power.

When shorter distances are involved (under 5km) with VHF frequencies such tests are sometimes not necessary as the lower the frequency the less affected the signal by such soil content. With low power radio systems i.e. licence free types with limited output power operating in the UHF band, it is imperative that such tests are carried out to ensure radio signal integrity.

Accurately determining the signal strength is important, at least a method be used whereby repeatable measurements are made to ensure adequate fade margins are present. Systems offering a signal strength of '1 to 9' without stating the difference (in dB) between each 'signal point' as well as a minimum level will almost undoubtedly result in an unreliable system - this is bitter experience talking.

Conducting the tests in an efficient manner is important if costs are to be minimised. It is imperative to know the true signal level (preferably in dBm or ÁV) of each point on a system using a scale of 1 to 9. Some makes of radio telemetry modules output the level in dBm and it is advised that the accuracy of these measurements be checked before conducting tests. A two-way radio service centre can assist in both of the above. These calibrated units should be kept to one side as repeating the task of calibrating could become a timeous and costly business.

A word of warning regarding the use of radio scanners to determine signal strengths. These receivers are usually designed as cheaply as possible and one of the areas skimped on is the antenna input circuit, known as the "front end". The problem is these cut down circuits (which is the 'economical' approach to creating a large frequency range input system) are not designed to cope with large signal strengths on frequencies other than what you are actually listening to, especially if the signal strength of what you are interested in is rather weak. What happens is the input circuit becomes overloaded with all the signals reaching the antenna which in turn stops the input circuit from correctly amplifying the frequency you are trying to receive. The result is the signal strength can be either enhanced or dampened (reduced). The former will give a false sense of radio path probability, the latter could influence antenna system design thus increasing the costs unnecessarily. Further complications also arise when the overload becomes so severe it forces the input circuit to start mixing the signals together making the readings totally unreliable.

vehicle mounted receiverA personally developed 'one-man operation' technique involves installing the transmitter with a fixed antenna at the proposed host site and the receiver installed in a vehicle with a reasonable magnetic mobile antenna mounted to the roof (see picture). The receiver signal strength is then monitored on a laptop while driving towards each remote site. The antenna in this instance is a 1/4 over 5/8ths collinear offering approx. 2dB gain which happens to be the loss on the 5 metres of RG58 downlead at the operating frequency of 459MHz - this allows us to ignore the downlead loss but the gain of the antenna is also not to be taken into account when documenting final readings.

The reason for monitoring the signal while driving provides a useful indication as to what, if anything other than distance, is causing the degradation in the signal. Sudden drops in signal level while travelling around a bend or corner would more likely indicate a mountain or building becoming a major obstruction. 'Wavy' signals when approaching a site indicates the signal being received is likely to be reflections rather than a direct path and should be carefully analyzed before committing to an installation.

Once all the signal strengths are gathered from the host to the various remote sites it's time to test paths from assigned repeater sites to sites that require repeating as determined when doing the path profiles in the previous section. Tip: Should you have a number of sites available to repeat but are not be sure which of these would do best (highest signal levels) place the transmitter at the site to be repeated and then drive to each proposed repeater site rather than setting up the transmitter at each possible repeater site and then testing through to the site to be repeated. This method may appear obvious to some but there are those who believe radio 'falls' downhill better than going up. Radio is not like this and works equally in both directions. This is known in the radio fraternity as "Law of Reciprocity" i.e. the signal losses are equal in both directions.

Having now ascertained radio telemetry is a viable option technically, economically, and propagationally, we move on to the physical planning stage. Until now, had the radio tests returned non-favourable results, any time spent planning the system would have been futile and a complete waste of time. Note: Using radio telemetry units for path testing as opposed to scanners also offers the security that the signal strength being shown is from the transmitting station and not interference. A high signal level without the "Signal Receive" light lit is a sure indication of interference.

Planning the System  >>


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14.11.00