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By Greg Lapin, N9GL
Chairman, ARRL RF Safety Committee
October 16, 2000
We are exposed to RF energy from a number of sources--not just the RF from our own Amateur Radio stations. What are the RF safety implications of these multiple sources of RF?
I was asked an interesting question the other day that made me think about RF safety in the wireless future. Previously, when I thought about RF safety with regard to multiple transmitters, I considered something like a Field Day site (see "RF Safety at Field Day," QST, Jun 1999, p 48) or perhaps a DX packet cluster operating at the same time as an HF station. The new question raised some scenarios that I previously had not considered.
Let's say that your ham shack is set up with a single transmitter. You perform your RF Safety Evaluation and find that your neighbor is below the MPE (maximum permissible exposure) limits based on what your station is transmitting. On the other side of your neighbor, one of the local cellular telephone companies has built a tower that houses one of their base stations. Perhaps they also performed an RF Safety Evaluation and found that they were exposing your neighbor to less than the MPE limits. Your neighbor, on the other hand, is being exposed to the sum of the energy from your station on one side plus the energy from the cell tower on the other side. Lest you and your neighbor get concerned, it is likely that the exposure to your neighbor from each source is many times less than the MPE so, even when added together, the MPE is not exceeded.
Let's start to examine your neighbor's house in this age of the wireless gadget. Perhaps there is a wireless baby monitor. Your neighbor may have a wireless alarm system, where all of the sensors are connected to the controller with RF links. If your neighbor subscribes to a DSL service, HF carriers are found in every twisted pair in the house, and some of that energy is radiated from that unshielded form of transmission line (they often are not even twisted, leading to even more radiation). Perhaps your neighbor is technically advanced and has outfitted his home with a wireless LAN.
Wireless telephones are common in homes. A normal wireless phone can operate at either 49, 900, or 2400 MHz. Your neighbor can also use a cell phone from the house. With the base station right next door, the phone is virtually guaranteed to transmit at its lowest power of 0.06 W. But what if your neighbor subscribes to the other cellular telephone provider in town, with the nearest base station miles away? The transmitter power could climb 10 dB to 0.6 W.
With most of these RF sources, if you do the math you find that even adding up all the exposures you stay well below the MPE limits. Let's think about the future, however. How many additional wireless devices will find their way into our homes in the next decade? Will it rise to the hundreds? I'll leave the horrific RFI implications of this for my friend and ARRL Lab Supervisor Ed Hare, W1RFI, to consider.
How will this scenario affect RF safety?
The FCC deals with the multiple transmitter issue from the point of view of a transmitting site. Atop many tall buildings these days you find lots of antennas--pager transmitters, cellular telephone base stations, PCS sites, fire and burglar alarm systems, and even ham radio repeaters. The FCC has responded to this situation as follows (OET Bulletin 65, Edition 97-01, p 33):
The rules adopted by the FCC specify that, in general, at multiple transmitter sites actions necessary to bring the area into compliance with the guidelines are the shared responsibility of all licensees whose transmitters produce field strengths or power density levels at the area in question in excess of 5% of the exposure limit (in terms of power density or the square of the electric or magnetic field strength) applicable to their particular transmitter. When performing an evaluation for compliance with the FCC's RF guidelines all significant contributors to the ambient RF environment should be considered, including those otherwise excluded from performing routine RF evaluations, and applicants are expected to make a good-faith effort to consider these other transmitters. For purposes of such consideration, significance can be taken to mean any transmitter producing more than 5% of the applicable exposure limit (in terms of power density or the square of the electric or magnetic field strength) at accessible locations. The percentage contributions are then added to determine whether the limits are (or would be) exceeded. If the MPE limits are exceeded, then the responsible party or parties, as described below, must take action to either bring the area into compliance or submit an EA.
The FCC's 5% rule is reasonable in the settings where multiple transmitters are found, particularly since it is unusual to find 20 or more transmitters in one place. This rule would also apply within the home, of course. I would be surprised if any of the devices that I mentioned above produce more than 5% of the MPE. More likely, they expose people in the house to less than 1% (except for portable phones or cell phones).
Thus, no manufacturer would be required to deal with the multiple transmitter situation. I have not seen any manufacturers of wireless devices report the exposure levels from their devices. They are not currently required to provide this information to the consumer, and many have probably never had their exposure levels calculated or measured. The recent exception is the cellular telephone. Just this month the manufacturers have started including SAR measurements for their phones with the device.
A current concern is with cellular telephones. Even though today's cell phones have been tested and certified to expose their users to less than the MPE limits, many of them are very close to the limit. That, in itself, is fine. If the MPE limit is safe, any lesser exposure is just as safe. But, what if you are demonstrating ham radio to a visitor in your shack. Your visitor's cell phone rings and he answers it. You are in the middle of a rag chew QSO, so you keep transmitting while he is on the phone. You know that the exposure in your station is below the MPE limits, but your visitor is being doubly exposed and has probably just exceeded the MPE limits. Throw in any number of RF wireless gadgets in the shack, and exposure can become a concern.
The question arises whether your visitor is considered to be part of the occupational or general population when in your shack. Since exposure of cell phones has been characterized in terms of the general population, you may have an additional factor of five to play with when you are in the occupational population.
If this confuses you, you are not alone. One of the reasons behind making different limits for occupational and general populations, however, is that people in the former group are aware of their exposure and how to limit it. Thus, it might be argued that someone in the occupational exposure group should not choose to talk on a cellular telephone while in a ham shack.
Now say that you are operating on 10 meters with the full gallon and someone standing at your property line is being exposed from your transmissions at 80% of the MPE for the general population. Suppose they are talking on a cell phone that exposes them to 95% of the MPE. Their total exposure would be 175%, or 75% above the MPE!
You may ask, "Do you really think this type of exposure will harm you?" I'd have to answer, "No." That's because the MPE limits have been set to be many times below recognized dangerous levels of exposure (10 times below for occupational exposure, 50 times below for general population exposure). There are many good reasons for this, some of which I covered last month. Mainly, calculation or measurement of exact localized exposure is very complicated. All kinds of resonances can be set up in different tissues of the body. Focal points of RF lead to "hot spots" that have much higher absorption than other parts of the body.
Just to be on the safe side, factors have been added to make sure that no localized exposures exceed the actual safe levels. I don't think that going from 50 times to 28 times below the limit presents a dangerous situation, particularly over the duration of one phone call. However, since we can't be sure of what localized resonances arise in every situation, it is best to follow the published MPE limits wherever possible.
Of course, these scenarios are greatly simplified. Estimating exposure is a highly complicated process, and there are a number of exposure definitions that may not be compatible. For instance, the exposure from a cellular telephone falls under the category of partial-body exposure, since most of the energy from that device is absorbed in the head. The applicable exposure regulation states that (OET Bulletin 65, Supplement C, Edition 97-01, page 30) "partial-body SAR ... averaged over any 1 g of tissue defined as a tissue volume in the shape of a cube" must be less than 1.6 W/kg. Exposure from antennas in the far field, such as ham radio antennas and cellular base stations, is usually evaluated with the whole-body exposure standards, that has a limit of 0.08 W/kg (general population).
To combine exposure from both types of sources, you need to put them both in the same system of measurement. One way is to calculate the whole-body exposure from the cell phone and add it to the whole-body exposure from the far-field antenna. The other way is to determine the partial-body exposure from the far-field antenna and add it to the exposure from the cell phone for every 1 g of tissue in the head.
The exposure from a cell phone appears to be extremely low when averaged over the entire body (worst case of about 0.004 W/kg for a 70 kg person). It is unlikely that this will be a significant factor in causing overexposure. The partial-body exposure from a far-field antenna will have points of high absorption throughout the body. It is unlikely that a 1 g (a cube that measures 1 cm× 1 cm× 1 cm) "hot-spot" from your ham antenna will be at the exact same 1 g cube of tissue that absorbs the most energy from the cell phone. Thus, very high localized exposures from multiple sources are not a concern. However, if one source, such as a cell phone, generates localized exposures that are near the MPE, it will not take much additional absorption to exceed that limit.
As hams, we have been given the honor--and the responsibility--of being considered by the FCC as being part of the occupational population. This also applies to our families. The FCC assumes that we understand something about the potential for danger from RF exposure and can use that knowledge to make sure that we are not overexposed. Be aware of all RF transmitters in your house and try to stay out of situations where overexposure may occur. The only one that may be valid today is the use of a cell phone in your house simultaneously with your station transmitting.
As the world of technology changes, however, stay aware of the transmitters in your house. Particularly if your station produces exposure levels that approach the MPEs, it may not take many additional wireless devices to push your exposure over the limit.
Editor's note: Greg Lapin, N9GL, started working in the RF safety world after spending many years first studying cardiac function imaging and then brain tumor kinetics. He serves as chairman of the ARRL RF safety Committee and as a member of the IEEE Committee on Man and Radiation. A former professor of Biomedical Engineering and Neurology at Northwestern University, Lapin now works as a consulting professional engineer in the electronics industry. He was first licensed while a teenager in 1969 and continues to be fascinated by virtually all aspects of Amateur Radio. One of his many interests is electronic design, and he is the author of Chapter 8, "Analog Signal Theory and Components" in The ARRL Handbook for Radio Amateurs. His non-ham interests include making things grow in his garden and serving as commissioner of the local children's softball league. At other times--when he is not working or helping his kids with their homework--you might find him with the local emergency services agency, climbing his tower, building a new QRP rig, playing with his APRS setup, responding to QSL cards, going off on a DXpedition, or trying to get that "new one." You can reach him by email at g.lapin@ieee.org.
