Unstrung has been asking around recently to find out if there are any potential health risks associated with the growing use of wireless LAN technology. We wondered if the growing use of 802.11b technology -- in our homes, cafés, and other public areas -- with the resultant increase in radio traffic all around us should be a cause for concern.
The answer seems to be... Maybe -- if you are one of the growing band of people who use home-made equipment to increase the range of 802.11b hotspots.
Understandably, in the wider world, there isn't the same level of concern about the radiation emissions from relatively low-power WLAN kit as there is about, say, living next to a cellular tower. And apparently, as long as you're using standard-issue commercial wireless LAN kit, the emission levels are tightly controlled by the Federal Communications Commission (FCC) in the U.S. and other regulatory bodies worldwide.
However, some of people working outside of the strictly commercial WLAN market -- boosting the range of their local wireless community sites with Pringle can antennas, for instance -- could potentially be taking a risk, according to David Sifry, CTO at open-source WLAN mavens, Sputnik Inc.
"I'm not a doctor, nor do I play one on TV," he writes in an email reply to Unstrung. "So please take this for what it is worth -- one engineer's understanding of the current issues, and some extrapolation of current physical research into the 2.4GHz spectrum.
"Current FCC regulations limit power output to 1 Watt EIRP (Effective Isotropic Radiated Power) for 802.11b (2.4GHz) devices. Most cards are 30 milliwatt, and there are a few 100mW and 200mW cards out there. Compare this with microwave ovens, which can emit 500 to 700 Watts to heat up your dinner. Of course, microwave ovens are shielded, but even a small amount of leakage would emit more radiation than these 802.11 devices."
However, Sifry fears that some of the more outré methods being used to boost the range of WLAN systems could prove to be hazardous: "Of course, if you illegally crank up the power or narrow the beam [exactly as the Pringles can antenna does], the EIRP goes way up, so there are potential health threats associated with using illegal equipment."
Now, these kinds of booster devices tend to be placed on rooftops and thus generally away from people, in order to get the best line-of-sight access. So it is hard to gauge exactly how much of a risk there is from such signal amplification. And common sense suggests that you don't walk in front of your home-made antenna if you are really trying to crank up the range of your 802.11b system (Unstrung health tip number one).
Despite the low emissions of commercial WLAN equipment, Jack Gold, VP of the mobile and pervasive computing group at Meta Group sums up just how much we don't know about the effects of widespread use of wireless technology.
"I'm not sure anyone knows the long-term effect of walking around in lots of radio fields… I think it may take years before we have proof one way or the other about the effects," he says. "Of course, I am not a medical researcher, so I am not able to give a medical opinion -- but from all that I've read, the jury is still out on this."
The power out minus the transmission line is the power at the intentional radiator. Power at the intentional radiator times the gain offered by the antenna is EIRP. You can still use the formula offered by alkuiper with EIRP whether the antenna is a point source (isotropic), or a big dish with loads of gain.
So if you connect a 10dB gain antenna (like a pringles can) to your 100mW card, you need to be at least 15cm away from the front of it. Mind you, staring down the throat of the pringles can is not its normal operating position.
Even if you buy a 150 Watt Ham Radio 2.4 Ghz amplifier, and connect it to a 12 foot satellite dish, you have to be in the focal point of the antenna or in the rear lobes to feel the heating effects. Yes, you have to be further away from the front of it. Thus. it's probably not a good idea to put your kid's trampoline in front of your moonbounce experiment.
Consider the flux of the sun for a minute. At 1300 Watts/square meter, the FCC's formula will tell you the sun is hazardous. In fact it is. It's know to cause skin cancer.
WRONG. If you add a high gain antenna then you need to multiple all of this by the gain factor. For example a 10dB gain antenna means the power is now only spread over 1/10 of the sphere and hence the range for the same power flux (and SAR) will increase by a factor of 10. That's the whole point of the "Pringle can" and other such tricks.
Basically, there is no such thing a free lunch. If you replace the omni antenna with a directional one then you can go further but you also have to stay further away.
Milliwatt at 2.4 GHz Safe distance in cm 300 mW 5,0 cm 100 mW 2,9 cm 55 mW 2,1 cm 35 mW 1,7 cm 32 mW 1,6 cm 9 mW 0,9 cm 1 mW 0,3 cm
This identifies that the safety is an (albeit small) issue. --> A user must at least remain at a distance of 2 cm from a unit. Specifically this give boundaries to the use of Bluetooth that is worn as apparel. --> Users should not keep their hand on the WLAN unit. --> If an external antenna is used, the power increases, but users will never realistically be in reach of the boundary distance indicated with the formula. Thus these are safe.
Source:
The FCC has published a formula for safety in electromagnetic radiation.
IRPA formula calculates the safe distance from a radio source. In the range of 2.4 Ghz the safe field strength is 60 V/m. The formula for the safe distance is: E2 = 30 P/r2 thus r2 = 30 P/E2
The signal strength is 30 mWatt of a card unit. The safe distance is r2 = 30 *30.10-3/602 = 0,00025 m2; r = 2 cm.
The real issue is not EIRP (more an interference consideration) but rather the resulting power flux density due to amplifiers and/or high gain antenna.
"I am not expert" but, if I recall correctly, the normal safety limit for this band is 200 microwatt per cm2 if you are to safely meet the limits of specific absorption rate. The easy way to calculate this is divide the total transmit power by the area of piece of a sphere, that has a solid angle the size of your radio beam, and a radius equal to the distance your head is from the antenna. (Naturally Nth. Americans should calculate all this with centimetres to avoid horrible inch - cm conversions!)
You should also watch out for "near field" effects (funny things happen close to antenna).
In summary, community WLAN is a fun idea, there are lots of people trying lots of tricks but please do the sums before you start living too close to these "souped-up" WLAN cards!
P.S. To measure these things correctly you should follow the procedures in the standards from IEC/TC106
The blogs and comments are the opinions only of the writers and do not reflect the views of Light Reading. They are no substitute for your own research and should not be relied upon for trading or any other purpose.
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Green Broadband 2010 Tuesday, September 21, 2010
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