There are frequently misperceptions as to how safe or harmful radio frequency (RF) transmissions can be. This is complicated by the fact that RF is invisible and for many people is a total unknown. For example, I know people who would be surprised to learn that a cell phone is, in fact, a radio and that toys with small DC motors and microwave ovens even radiate RF at all.
Being in the business of providing devices that radiate Ultra Wideband (UWB) energy, we frequently have to address concerns about transmit power. In fact, I don’t even like using “radiate”, “Ultra Wideband” and “energy” in the same sentence since for some it conjures up images of some sort of death ray. So I wanted to offer a few reasons why UWB is safe.
This is not a UWB device.
First, it is important to note that UWB technology went through an exhaustive evaluation process before the FCC defined rules that govern its legal use in the US. I personally was involved in that effort for nearly 5 years. The industrial products Time Domain makes today are compliant with those rules.
Second, UWB transmits at about the same frequency as other common emitters but at much lower power levels. The following table illustrates the transmit power of several technologies:
Transmit Power (Watts)
|Allowed leakage from a MicroWave oven||1.00000 Watt|
|Typical mobile phone transmit power||0.25000 Watts up to 1 Watt|
|Class 1 Bluetooth device (100 m range)||0.10000 Watts|
|Class 2 Bluetooth device (10 m range)||0.00250 Watts|
|Sunlight reflecting from the head of a pin (on a sunny day)||0.00100 Watts|
|UWB device||0.00005 Watts|
Transmit power of several RF emitters (Note 1)
As much as I like numbers, I find that it’s hard to think in orders of magnitude. So in the above table, if we equate transmit power to height, then the allowed leakage from a microwave oven is the height of the Empire State Building, mobile phone transmissions are like the height of the Saturn V rocket (our local icon), the Class 2 low power Bluetooth is like the height of a meter stick, and our UWB transmissions are like the height of a nickel (US 5 cent coin, lying flat). Imagine yourself walking down the streets of NYC near the towering Empire State Building and bending down to pick up a coin on the sidewalk and then looking up, that’s the difference between our low power UWB emissions and a leaky microwave oven (which are located in every home, apartment, office, and most hotel rooms, etc).
Third, the other parameter to consider is the degree to which transmitted energy is concentrated. One metric is the amount of energy transmitted over the operational band of the device or power/MHz. For example, a device that radiated all of its energy at one frequency (like a laser) might be more of a concern than a device whose transmissions were extremely diffuse. The following table presents this measure for the previously described devices.
Power Density (Watts/MHz)
Allowed leakage from a MicroWave oven: 1 Watt/100 MHz (Note 2) 0.01000000 Typical mobile phone transmit power: 0.25 Watts/5 MHz (Note 3) 0.05000000 Class 1 Bluetooth device (100 m range): 1 Watt/1 MHz (Note 4) 1.00000000 Class 2 Bluetooth device (10 m range): 2.5 mW/1 MHz 0.00250000 UWB device – max limit 0.00000008
Power density of several RF emitters
Time Domain UWB modules transmit 31,000 times less power density than the Low power Bluetooth – or – 125,000 times lower power density than what a microwave oven can leak.
Additionally, OSHA and FCC standards (Notes 6 & 7) exist for safe RF exposure for humans (non-ionizing radiation). Time Domain UWB emissions are well over a thousand times less than these safety limits.
Lastly, there is also another class of radiators that should be mentioned. All items in the universe radiate energy either because they generate energy internally or because they absorb and reradiate energy that falls on them. This is called black body radiation. In the course of metabolizing food, people radiate energy. They also absorb energy from the sun, campfires, car heaters,etc., and then reradiate that energy. Some of this energy is radiated in the same band as the RF transmitters we have previously discussed. An analysis was performed (see Note 5) to compare the energy that people radiate with the transmission limits defined by the US FCC and Europe. What may be surprising is that a single person standing next to a receiver will radiate as much as a UWB device located ~10m away, or that a group of people can radiate as much as a UWB device at the same distance. In the GPS bands, a single person holding a GPS receiver can have as much black body radiation as a FCC compliant UWB device ~1m away.
While this last example is both cheeky and geeky, the important point is that UWB devices barely radiate at all (certainly compared to other devices that are ubiquitous and we commonly use) and when they do radiate they spread what little energy they have as much as is possible.
Note 1: http://en.wikipedia.org/wiki/DBm
Note 2: http://www.ko4bb.com/Test_Equipment/Microwave_oven_leakage/
Note 3: http://books.google.com/books?id=J4cvP7NuvncC&pg=PA42&lpg=PA42&dq=occupied+bandwidth+of+UMTS/3g&source=bl&ots=tCUPpnbF4q&sig=pGYje0QTDDXowxf_3wndy3PAJik&hl=en&sa=X&ei=KvU0UczaMZDc8wSS2ICgBA&sqi=2&ved=0CC0Q6AEwAA#v=onepage&q=occupied%20bandwidth%20of%20UMTS%2F3g&f=false
Note 4: http://cp.literature.agilent.com/litweb/pdf/5989-4204EN.pdf
Note 5: http://www.timederivative.com/2005-04-032rX-UWB&black-body-radiation.pdf
Note 6: http://www.osha.gov/SLTC/radiofrequencyradiation/
Note 7: FCC CFR 47 Paragraph 1.1310; ANSI/IEEE C.95.1-1992; DOD Instruction 6055.11