Measurement and assessment of interferenMeasurement and assessment of interference

Measurement and assessment of interference

The interference ratio limits discussed above specify levels that are measured at the input terminals of a reference receiver. While this definition is closely tied to the receiver performance degradation due to interference, it does present difficulties in relating these values to field strength measurements.

Field strength measurements can be subject to considerable variability. Even when measured at 10 metres above ground, the field strength may vary considerably with small changes in location or frequency; this is particularly so if the environment is subject to clutter. Field strength measurements can also be subject to short term and seasonal time variations. Variations in the field strengths of wanted and interfering signals will not necessarily be well correlated. Ideally, to obtain reliable and repeatable results it would be necessary to perform a large set of wanted and interfering signal field strength measurements. Notwithstanding time and cost considerations, in many situations terrain or access considerations would make this difficult or impractical. Experienced field strength surveyors may, however, be able to select representative sites where clutter and other confounding effects are minimised and where reproducible results can be obtained from a limited number of field strength measurements.

Calculated field strength predictions do not suffer the variability of measured field strength values. However, as with any mathematical or empirical model, the accuracy of predicted field strength values depends on how well the prediction model represents the real situation. Predicted field strength values can be a convenient method by which to assess claims about potential interference but their use should be tempered by knowledge of how well the prediction model fits the actual situation. Predictions can be used with greatest confidence where they are supported by field strength measurements from nearby areas.

In the final analysis judgements may need to be made based on the combination of data that is available. This could include whatever field strength measurements are available, comparative measurements made when the suspected interfering source is cycled on and off, or reduced in power by a known number of dB, and field strength prediction calculations for wanted and unwanted signals (especially where these predictions have been validated by field measurements in nearby areas). It is noted that while cycling of power of suspected digital interference sources could be a very useful technique for investigation of interference during the start up period, it may be more difficult to apply this technique for high power digital services once they commence normal operation.