This subject line may be a bit off topic for this list, however safe RF exposure limits is some what of a pet peeve with me so I'll chime in any way. Although I can agree with the math and over all fact of the calculations below I have to disagree with the assumption that FCC guidelines on safe RF emissions near a persons body are adequate. There is nothing to say that even 150mW is truly safe when pressed up against human tissue. I'm not trying to dismiss what the FCC says is safe, I'm just saying that as time goes on we learn more about the effects of RF against human tissue. Also keep in mind that the FCC has to consider what impact it would have on the cellular industry if they lowered the safe limit. It has been proven that RF heats human body tissue. The effects are accumulative over time just like defrosting food in a microwave on low power will eventually cook the food if you run it long enough. Obviously a microwave has considerably more power than your cell phone, but you get the idea. Also, what is safe for one person may not be safe for another. Just like some people have a higher tolerance to certain cancer causing substances (like smoking) some people may have higher (or lower) tolerances to RF. I predict that eventually cell phones of today will be found unsafe to use so close to the human body. Until then I plan use best efforts to lower my exposure by limiting my talk time, and using my hands-free head set - I urge my friends to do the same. Jay Gustafson > > The first of the two sources that you have linked below seems to contain > misleading, inaccurate information. My first tip as an RF Engineering > Student was the claim that you can "increase the effiency of your > antenna". They claim that "The increase gain can be from 3dB (100% > increase in signal strength) to 9dB (400% increase)." > > The first part of this statement is accurate: 3dB increase in signal > strength, in general, is *about* double the gain. (3dB = 10 * > Log10(1.99526) and 1.99526 is about 2, or twice the gain.) The second > part, however, is inaccurate. 9dB = 10* Log10(7.94328) or about a gain of > 8, not 400% or 4. This means that a one-watt signal would effectively be 8 > watts, not 4 watts. This means that our friend mis-calculated something or > didn't get his calculator out at all when creating the web page. > > Perhaps the most tell-tale sign that something is amiss when I am reading > web sites on the internet is when I see statements like: > > "Without getting into the physics of why it does not work,..." > > Whether the device does or does not work doesn't seem more apparent then > the accusations on the web page, and the tests done by the author seem to > lack any technical detail. > > The second link is interesting, as it comes from the Federal Trade > Comission. I always looked to the FTC for consumer advice on products, > until now. The FTC claims that: > > "According to the FTC, there is no scientific proof that the so-called > shields significantly reduce exposure from electromagnetic emissions." > > Perhaps this is true, given the current-day consumer-sold RF shields. But > that is not to say that we can't create RF shields. The real problem is > that consumers want small size, and don't want ugly antennas getting in > their way of cell phone usage. > > Do cell-phones cause cancer? Dr. Dean-Edell last night on the radio > dispelled this common belief. There is no scientific evidence either way. > Perhaps my RF engineering professor put it best: > > In the early days of cell phones in Europe, heavy users of early 1 and > 5-watt cellphones complained of getting headaches after using the phones > for an extended period of time. After some experimentation by the > goverment, Europe set it's limit at 1 Watt for hand-held phones with > radiators next to your head. In the USA, the FCC set that limit at .5 > watts. As a result, most of your cellphones radiate 450 mW = .450 Watts. > > The FCC also sets the safe RF exposure limits for humans. According to the > assignment that we recently handed in, located at: > > http://www.ece.umn.edu/class/ee4601/4601F02Ass3.pdf > > (see problem 3) The FCC says the safe exposure limits at 2.2 Ghz in a > 24-hour period are 2mW/cm^2. Let's take my cellphone, a voicestream phone, > for example: > > I have a 1.9 Ghz phone, like most other digital phones. The Professor > estimated that about 1/3 of the power from the cellphone is disappeated > into my head as I use it. That's 450 mW /3 = 150 mW. The antenna on my > phone spans the entire length of the phone casing, but I will discount > that for now and only talk about the external antenna. The external > antenna is 3cm tall by about 1 cm wide. This is 3cm*1cm = 3cm^2. > > Roughly, I am disappating 150mW/3cm^2 or 50mW/cm^2 into my head. If I can > assume that the FCC's guidelines for 2.2 Ghz are close to that of 1.9 Ghz > (since they are close in frequency) then I have exceeded the 2mW/cm^2 > exposure limit if I use my cellphone for a 24-hour period. > > But then again, the batteries on my cellphone only last 2 hours max. So, I > am exposed to 50mW/cm^2 of power for 2 hours, where the safe limit is > based on a 24-hour period. If I only use my phone 2 hours a day before the > battery dies I believe I will come just above the daily exposure > guideline. > > I apologise now for getting all technical and stuff here, but I have seen > some things go by on the list as of recent that are completely off-base > and I haven't had time to reply to all of them until now.