Level probing radars can use the 5.925-7.25, 24.05-29, and 75-85 GHz bands without an FCC license.

The FCC has adopted rules that allow unlicensed use of the “level probing radars” (LPRs) used for measuring the levels of liquids in big tanks, the heights of piles of rock and wood chips at industrial sites, and water levels at treatment plants and nuclear reactors.

Traditional radars use a single frequency at relatively high power. Their signal looks much like that from an ordinary radio transmitter, and has long fit easily into the FCC’s regulatory scheme.

LPRs are different. To achieve high precision over a short range, they emit a sequence of very narrow pulses. For reasons familiar to every electrical engineering student, narrow pulses distribute energy over a relatively wide range of spectrum, while placing relatively low power on any one frequency. This broad spectrum occupancy, combined with the “peaky” nature of the pulsed signal and certain quirks in the FCC rules, had long disqualified LPRs from routine approval. Still, understanding the need for the devices, the FCC has worked with manufacturers on various rule waivers. Twenty months ago, it proposed new rules that would allow the approval of LPRs in the ordinary course.

Those rules have now been adopted, allowing LPRs to use the 5.925-7.25, 24.05-29, and 75-85 GHz bands without an FCC license. Although all of these bands have other users, few objected to the prospect of LPRs. The FCC carefully reviewed the interference concerns and concluded that the low power and industrial locations of LPRs pose little risk.

In many respects the new rules parallel those in effect in Europe. That simplifies manufacture for companies that market products globally.

The rules include a novel approach to compliance testing. An LPR aims its radar beam downward, onto the material being measured. Potential interference victims are somewhere off to the sides, well outside the main beam. The possibility of interference arises partly from radar signals being reflected or scattered off the material below. But measuring those reflections tends to give erratic results. Besides, the compliance engineers don’t want piles of rocks and wood chips on their clean floors – and neither, we suspect, do the engineers at the FCC’s test facility in Columbia, Maryland.

So the FCC will allow the testing of LPRs with the main beam aimed directly at the test antenna – a so-called “boresight” measurement. Technically this is easy to do, with no mess on the floor, and it yields consistent measurements. But it gives readings much higher than the reflected signal would produce. Having estimated the difference, the FCC will allow correspondingly higher readings in the boresight.

Some applications, particularly in closed metal or concrete tanks, have long operated under a different FCC rule section that affords more flexibility as to frequency use. The FCC has accepted the practice because the tank walls shield most of the radar signal from causing interference. This option continues to be available as an alternative to the new rules.

Besides the boresight limits, a few other provisions are likewise designed to control interference: no hand-held operation; no operation in a moving vehicle; downward-pointing installation only; and no marketing to residential consumers. This last is bad news for that personal nuclear power plant in your back yard: you’re going to need some other way to monitor water levels in the cooling pond.

FHH represents clients in this proceeding.