October 7, 2021 by Omna Solomon, Senior Consultant
Antenna systems for wireless networks serving the public safety and mission critical industries are designed with a great deal of redundancy. During initial commissioning, an in-depth coverage test is additionally conducted to validate performance. However, this initial baseline test is often the last robust measurement of RF performance. Like any hardware, particularly those that are exposed to the elements, antenna systems can deteriorate over time; recurring tests are essential to quantify and preserve network coverage. Numerous approaches for regular testing of RF and antenna performance are available; here we discuss an RF test that can be done with limited equipment and time.
Recurring testing can uncover issues stemming from gradual deterioration or equipment damaged from bad weather or poor maintenance work. The Measured Power Distribution figure depicts a few minutes of drive test data in the near vicinity of a tower site with multiple transmitting antennas. Each curve represents a probability distribution function (PDF) of signal levels from one of the three independent combiners and antennas. While all three antennas were originally balanced to transmit equal output power, Tx 2’s output power is clearly 18 dB (on average) less than the other two antennas. A closer inspection uncovered significant water damage to antenna 2, which had to be replaced. Even egregious issues like this can go unnoticed on well-designed systems. For example, this could be a trunked system and the channels on Tx 2 are only used a fraction of the time. Recurring testing can uncover the problems before it’s too late.
Gathering and processing the data takes only a few minutes with the right equipment. Our field engineers drove about a few hundred feet away on all sides of the tower. It’s important to gather RF data over time interval and distance to smooth out the effects of fading. Averaging multiple points gathered at the same location could also suffice but will not account for other physical and environmental differences, such as antennas mounted on different legs of a tower. Measuring and documenting RF data in this manner on an annual basis can be valuable in monitoring trends and implementing timely solutions.
Televate recommends periodic cable sweeps of the lines and antennas that would have found this issue, but there are other problems that would not have been identified by sweeping. Conducting this type of test captures RF performance data from the transmitter through combiners, lines, and antennas to obtain a more comprehensive picture of the RF transmission system quality. For LMR system operators who have their hands full with daily duties, a short drive test annually around a tower site, which can be scheduled during other site maintenance trips, can be an efficient approach to performance verification and troubleshooting, as well as extend the life of antennas that are performing well. Periodic benchmarking of the coverage systemwide can also identify the impacts of new construction on your service and highlight new issues to address.
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