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Testing AM Antennas
There are different ways, techniques, and equipment available to test antennas. As with any antenna system, regular testing as a component of an overall maintenance program will ensure maximum performance, and limit failure related downtime.
Generally speaking, we can cluster testing methodologies under two main headings: basic and advanced. Basic procedures can and should be performed by the station engineer. These procedures are supplemented by more involved methods usually performed by consultants or at the corporate level. This second group tends to be undertaken beyond the local level because of the substantial monetary investment required in equipment, often not possible at the local level.
At the heart of basic testing procedures are acquisition of phase monitor readings, common point and/or base current values, and monitor point or reference location field strength readings. Obviously, the availability and necessity of certain portions of this data will depend on the configuration of your antenna. Although no longer required by the Commission, maintenance of base current measurement equipment in directional antennas is highly recommended, as this data provides another cross-check of other parameters.
Frequent acquisition of this data will provide a good historical record, and will aid in determining if an anomaly is seasonal, or an indication of a more sinister issue. As a consultant, it can be somewhat frustrating assisting a facility where there is no such record. Even worse is the station where the locations of the monitor points are completely unknown. Don't be that guy.
Anomalous readings in one sector of the data are not necessarily an indication of a substantial problem with the system. For instance, if the phase monitor readings are well out of tolerance, but the transmitter is still happy, base current ratios are within norms, and monitor points do not exceed limits, it is a good bet that the problem lies in the sampling system itself. Similarly, if a couple monitor points have drifted outside their limits, then we need to look at the environmental conditions to see if a point has gone haywire, or if there truly has been an increase in the radiated field along that azimuth. A series of field strength measurements on those azimuths compared to the last full proof will usually provide that answer.
Probably one of the most glaring exceptions to this may deal with common point current (for a directional) or base current (for a non-directional). Sudden changes in these values, coupled with an unhappy transmitter, almost certainly indicate an acute issue with the system, requiring additional testing, maybe even down to the component level. A more gradual change over time is likely more indicative of system aging, such as ground system deterioration. Either way, swapping the meter or inserting a similar meter in series will tend to confirm or exclude measurement error.
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