Jan the Ham wrote: >But there are many orders of magnitude difference
between them!
True, but they are also cumulative. I work in an office building which has
hundreds of wireless 2.4gHz headsets in use, along with an extensive WiFi
network (24 AP's visible from my desk). Not to mention the no less than 30
microwave ovens (two in every break room).
As I sit here at my desk three and a half hours past the end of the normal
business day when there are only a couple dozen people in the building
working, my Fluke spectrum analyzer at my desk is measuring a noise floor of
between -61 and -52 dB. The Spectran several feet away with a different
antenna is measuring within two dB of the same. Channel utilization across
all WiFi channels is no less than 10% and as high as 40% right now. During
the middle of the day, the spectrum analyzers measure no less than -45dB
anywhere in the building complex and as high as -15dB on the main
call-center floor.
The nearest microwave oven to my desk is about 100 feet and they'll
typically kick in a signal strength of around -70dB. (these are calibrated
analyzer/antenna setups).
This is just in the 2.4 gHz ISM band. In the 5gHz ISM band I am seeing two
continuous-wave microwave systems which are point-to-point systems running
across town, of which my office location is directly in line with them.
Both of these are 100% duty-cycle systems and even though our windows are
RF attenuating (the nearest cell tower is four blocks away, but the moment I
go into the building the reception drops 2-3 bars), the signal strength as
measured is -22 dB.
The sad thing is that compared to most city centers, this is nothing. Last
trip I took to Chicago I brought the handheld spectrum analyzer with me and
the typical RF noisefloor in the 2.4gHz ISM band was about -45 dB down by
the board of trade out on the sidewalk.
As to the 1.9 gHz cellphone bands, you really do not want to know.
Seriously, you don't want to know....
> And yet, a properly installed and maintained microwave oven will leak
> no more than microwatts into the ambient, whereas a cell phone is
> pumping several watts directly into one's skull -- that's a 1,000,000
> to 1 difference in power levels!
>
Well, sorta. Depends on the cellphone brand/model as well as the mode.
Analog cell coverage, which is becoming rather "film like" (ie,
nonexistent) can utilize a lot more power output than most digital cellphone
systems which will automatically wilt the signal strength (wattage) of the
cellphone to allow the cellphone to extend the life of the battery as well
as reducing interference across multiple towers.
Wifi is between the two. You get no more than milliwatts exposure from
> wifi, even when sitting on your lap -- roughly 1,000 times less energy
> than that emitted by a cell phone. And yet, some people who use cell
> phones with abandon claim wifi somehow hurts them.
>
LOL, maybe YOUR WiFi equipment. :) I take mine right up to the legal
limit. (That's my story and I'm sticking with it--ahem, don't mind the
+18dB antennas...).
There's also the "square law" relationship to consider:
> electromagnetic radiation goes down as the square of distance. This
> means EM drops off fairly rapidly with distance. Stand 1cm from a
> microwave oven to get the same dose of EM as standing 31cm from a wifi
> hub, or standing 10 metres from an operating cell phone!
And don't forget the fact that the natural attenuation of 2.4gHz in the air
is more of a limiting factor than anything. I can put high-gain antennas on
radios for point to point or fixed-to-mobile installations, but this is
still not enough to "burn through trees".
Common sense is all too uncommon when it comes to this subject.
>
Agreed. I still use cellphones, but my desk headset remains wired.
You should see what a room full of Blackberrys will do to the RF space!
AG (Glowing) Schnozz
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