standardsarefun 12/4/2012 | 10:18:06 PM
re: Does WLAN Pose a Health Risk? 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
standardsarefun 12/4/2012 | 10:18:05 PM
re: Does WLAN Pose a Health Risk? 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.
alkuiper 12/4/2012 | 10:18:05 PM
re: Does WLAN Pose a Health Risk? There is no great risk.

Milliwatt at 2.4 GHzSafe 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.


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.

For a Bluetooth unit at 10 mWatt this is : 1 cm.

lrmobile_kr 12/4/2012 | 10:16:50 PM
re: Does WLAN Pose a Health Risk? 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.

Konrad Roeder

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