Measuring radio frequency interference is key to avoiding it

(Ed.: This article originally appeared in print as "Know the Enemy.")

Land-mobile-radio system efficiency depends on clean and clear use of wireless spectrum. This article takes an advanced look at radio-frequency interference, explains how it affects system performance and coverage — as well as resource management — and suggests tactics for mitigating it.

In interference-limited RF environments, the understanding and measurement of causes and ramifications are very important. The effective receiver sensitivity at the radio base-station sites — and where the mobiles and portables roam — is paramount to the optimum use of the spectrum and controls the number of sites required.

A high noise floor will limit the input range of a site. The extent of the decrease in range is a function of interference type and intensity. We have discussed the various types of interference and how to mitigate them in previous articles in this series and in the previous LMR 100 series, but the impact on resource management has not been considered in the prior articles. If the interference can be located and the range of a system can be extended, this will translate into needing fewer sites to provide a given area of coverage, which is especially important for public-safety agencies.

Because radio systems are proliferating in many areas, the noise floor in most cities and towns has risen. Consequently, many sites require that all transmitters at the site have a bandpass cavity filter to limit the transmitter noise that accompanies the desired signals. However, most radio service technicians set the insertion loss at the minimum setting; as a result, these filters do not provide much protection. There must be cooperation by everyone at every site to lower the noise floor.

The cellular and PCS operators have brought to the LMR community the quantitative measurement of passive intermodulation (PIM), which robs a site of its desired range. New test sets on the market today have proven that, if you build a site correctly, you can control a good bit of its PIM. Even though these test sets operate in the 700-1000 MHz range or the 1700-2100 MHz range, dummy loads can be created for VHF and UHF systems, and the entire transmission line and jumper connections still can be tested for PIM problems.

Meanwhile, the newer spectrum analyzers and communications service monitors have better sensitivity specifications. Coupled with a directional antenna, they can be used to track the source of interference that could not be detected before. Most of the older test sets have a sensitivity of 2.0 microvolts (-100 dBm); in contrast, the latest generation of test equipment on the market today offers sensitivity down to -130 dBm or greater. In addition, the newer test sets are smaller, lighter in weight and powered by batteries.

With better detection, it is now possible to find more sources of potential interference. Some interference is unavoidable and may require that a targeted base-station site not be used. On the other hand, if the site was to be used and the interference either was not detected or was ignored, the system would not perform properly. In both cases, an additional site would be needed.