Answer to Question #11901 Submitted to "Ask the Experts"

Category: Micro/Radio Waves, Radar & Powerlines — Microwaves and Radiofrequency

The following question was answered by an expert in the appropriate field:

Q

I work in the television industry. Today while training a fellow employee on using our satellite truck, the transmit switch was accidently thrown causing the dish to beam straight down at the truck with an estimated 50 watts (W) of power. Unfortunately this wasn't found for about 20 minutes. We were mainly inside the truck during this time. My question is how harmful was this? Did the inside of the truck serve as any protection? Just how concerned should I be? Needless to add, steps have been taken to ensure this doesn't happen again.

A

First of all, based on my evaluation below, it would appear there is no need for concern about any harm as the radiofrequency (RF) exposure was below the Federal Communications Commission (FCC) occupational limit.

Second, congratulations on taking the correct steps for dealing with any similar situation: hazard recognition, evaluation, and control. When you say you have taken steps to "ensure this doesn't happen again," I assume you have a written procedure or protocols to follow and that's good. This information, however, should be included in an RF safety plan for the truck. This safety plan should be written and available to anyone working around the transmitting RF sources. For guidance, refer to IEEE standard C95.7-2014, "IEEE Recommended Practice for Radio Frequency Safety Programs, 3 kHz to 300 GHz."

Now for the evaluation. While there is not enough specific information regarding your particular situation to make an exact, forensic, RF dose assessment, we can make some close approximations and assumptions using the FCC Office of Engineering & Technology's (OET) Bulletin 65 (Edition 97-01), "Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields."

First, let's assume the truck is the standard uplink truck with one 2.4 meter (m) diameter (1.2 m radius) C/Ku satellite antenna to fit the description of a "dish." The Ku frequency is designated solely for communications use via satellite and operates at 11.7–12.2 gigahertz (GHz, downlinks) and 14.0–14.5 GHz (uplinks). C-band operates at the lower frequencies of 3.7–4.2 GHz (downlinks) and 5.9–6.4 GHz (uplinks). It is important to note that there is no difference between the FCC allowable exposure "guidelines" for C vs. Ku band. The FCC occupational (and "controlled" environments) limit, assuming an exposure greater than six continuous minutes, is 5.0 milliwatts per square centimeter (mW cm-2). This is the same RF field value that your microwave oven is allowed to "leak" 2.45 GHz fields. For every other area, the FCC general population limit is 1.0 mW cm-2, this time assuming an exposure greater than 30 continuous minutes.

Let's also assume basically no distance between you and the surface of the antenna, as it was folded down but still able to transmit 50 W of power. There is no doubt the truck's metal roof construction attenuates the RF signals, but we have no idea exactly how much. A factor of 10 reduction is a good, albeit conservative, approximation.

According to FCC's OET Bulletin 65, the "maximum power density directly in front of an antenna (e.g., at the antenna surface) can be approximated" by multiplying the power by four and dividing by the area of the dish. Thus 50 W × 4 = 200 W or 200,000 mW. The area of the dish is pi times the radius squared, or 3.14 × (1.2 m)2 = 4.52 m2 or 45,239 cm2. One tenth of 200,000 mW divided by 45,239 cm2 is 0.44 mW cm-2. This calculated intensity compares favorably to the FCC occupational limit of 5.0 mW cm-2. While there is no doubt the truck's roof provided even greater attenuation of the signal, it would appear there is no need for concern about any harm as the RF exposure was well below the FCC occupational limit.

Donald L. Haes, Jr., CHP, CLSO

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