Cell Phone Radiation: Taboo Topic, Interesting Science

Amid a growing consumer concern for possible health issues associated with radiation, more attention is being paid to what’s being generated by mobile devices—particularly the ones we hold next to our brains. Legislation has been introduced in some parts of the country, with varying success, aimed mainly at informing consumers about the potential risks and giving suggestions for reducing exposure to radiation.

The FCC said it worked closely with federal health and safety agencies, such as the Food and Drug Administration (FDA), to develop and adopt limits for safe exposure to radio frequency (RF) energy. These limits are defined as the Specific Absorption Rate (SAR), which is a measure of the amount of radio frequency energy absorbed by the body when using a mobile phone. “The FCC requires cell phone manufacturers to ensure that their phones comply with these objective limits for safe exposure. Any cell phone at or below these SAR levels (that is, any phone legally sold in the U.S.) is a ‘safe’ phone, as measured by these standards. The FCC limit for public exposure from cellular telephones is an SAR level of 1.6 watts per kilogram (1.6 W/kg),” according to the FCC on its Web site.

The antenna in a mobile device is what emits the bulk of the radiation and one phone can emit more than another due to devices having different RF amplifier or antenna designs, said Wolfgang Kainz, chair of the Computation Techniques subcommittee at the IEEE International Committee on Electromagnetic Safety. Also, different antennas have different gain and direct the RF radiation in different directions.

Aveek Sarkar, vice president of product engineering and support at Apache Design, noted that there are actually two parts of energy radiation from a cell phone. “One is when you transmit the data out of the phone using its antenna, like when you have a voice conversation or you’re doing texting. Then, when the phone is used as a computer, it is doing its own operations internally it is generating lots of noise, a lot of heat, etc., and it is doing communications between one small part of the chip and another. There, the distances are small so the radiation coming out of that may not be that significant.”

Cell phone makers surely do not want their customers being concerned about the safety of mobile devices, but they view this as a “policy” issue and declined to comment for this article.

Interestingly, Renee Sharp, director of the California office and senior scientist at the Environmental Working Group (EWG), pointed out that the IEEE was advising the FCC in its development of the standard. “They are not a health agency. Their job is to try to make things work well and have regulations that allow that. Right now there is a standard of a maximum of 1.6 watts per kilogram. The IEEE for years has been lobbying the FCC to change it to the way the Europeans do it, which is slightly different and is a little bit less protective, and also just to raise it, period. What’s interesting about that is that it is happening at the same time that there is research coming out showing that children’s brains absorb more radiation than adults’.”

The EWG, a lobbying group focused on human health and the environment, has conducted its own research on cell phone radiation and has produced a cell phone radiation report as a resource for consumers.

A SAR rating has everything to do with where the antenna is placed versus where the user holds it and the metals and plastics around it. “Everything that’s directly near the antenna, which is the ultimate point of transmission out of the handset, will affect the radiation pattern. It affects the directionality of the antenna. It affects the transmit efficiency of the antenna to the base station. It affects a lot of different things. That radiation pattern is affected by a lot of things and one of the things it’s affected by is the skull that is sitting next to it, so how it’s being held relative to where that antenna is has a big effect. That’s one of the reasons why a SAR rating is higher or lower on phones,” explained Erick Olsen, director of marketing for mil/aero at NXP.

As such, the design of the system has an impact on SAR ratings in addition to the cellular technology being used, EWG’s Sharp said. “One thing that basically no one knows, because this information appears to have been suppressed by the industry, is that the amount of radiation that you are exposed to varies on a whole bunch of different factors. But it varies very strongly on your network, basically, GSM versus CDMA. If you have a GSM phone, that phone is going to be operating near the top of its radiation range a lot of the time versus the CDMA phones, which have much better ability to do adaptive power control, and those phones operate at the top of their radiation range much less frequently. So in fact, you could have the same phone on two different networks and you could be exposed at different radiation levels.”

Olsen concurred. “GSM and that lineage of technology, which is still the backbone of most networks in Europe and Asia and increasingly so in the U.S., typically transmit at much higher power levels, whereas CDMA and it’s evolution to WCDMA and LTE use lower power levels. Part of it is the physical design of the handset and the relative positioning of the antenna, and the other is which transmitter modulation technology they are using. That will have a big effect as well.”

So what does this have to do with the designer? “If you crack open a cell phone the first thing you’ll see is a lot of the various regions of the phone,” Olsen said. “In order to reduce the unintentional coupling of one function of the phone to another function of the phone (the EMI problem), they put metal cages around everything. By sectioning off the key parts of each phone—the RF from the radio from the baseband from the wireless LAN section from the Bluetooth section, and so forth—they are able to segregate in a small area the fields that are generated by each area and thereby mitigating, not eliminating, the EMI.”

Once the signal gets to the antenna and is transmitting it to free space, basically all around the phone, the signal is going everywhere, Olsen added.

David Johns, vice president of engineering and support at CST of America, pointed out that the power levels transmitted by cell phones are relatively small and cell phone manufacturers are required by the FCC to comply with standards for SAR levels. “These place a limit on the amount of power that can be transmitted and absorbed by the human body, which electromagnetically-speaking is formed from layers of lossy-dielectric materials. Electromagnetic field simulation is routinely used to predict the SAR performance of cell phones and other wireless devices.”

As if cell phone radiation weren’t a big enough concern, he also said that the conducting body of an aircraft creates a cavity that can trap and store electromagnetic fields. “Field strengths can build up over time and this may increase the exposure to passengers inside. The SAR distribution may be affected by the aircraft environment in which the passenger is situated.”

Thankfully, electromagnetic simulation tools have recently been used to model complete airframe structures including the internal seating and passengers, Johns added.