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RF Exposure and Limits

Background

Recent developments in the electronics industry have led to the widespread use of radio frequency (RF) devices in various areas, including telecommunication, radio and television broadcasting, radar, industrial processing, medical applications and consumer products.

Electromagnetic fields extend over large areas when generated for communication, broadcasting and radar devices, but generally spread only over small areas when used in industrial, medical and consumer devices.

Reflection and scattering of electromagnetic waves and simultaneous RF emissions by more than one source frequently results in a complex condition known as “multi-path” propagation and spatially non-uniform fields.

Although there are very powerful RF sources in use for broadcasting, Radar and other industrial uses, most telecommunications applications involve very low power in comparison. Most fixed place wireless systems are not adjacent to a user and thus very low fields intensity results.


Guidelines

Several documents can be found that discuss guidelines for limiting RF and microwave exposure.

Generally, each government has its own recommendations (FCC, Industry and Health Canada, ETSI etc.). A major independent source of guidelines is the IEEE which works with governments.

In a field where technology is advancing rapidly and where unexpected and unique problems may occur, these regulations and guidelines cannot cover all possible situations and blind adherence to rules cannot substitute for the exercise of sound judgement.

Over the years tests have been performed on biological organisms, including humans, animals and cell systems. In most cases, the recommendations made by governments are several magnitudes lower than the threshold for damage.


Considerations

To determine whether the maximum exposure levels and durations are exceeded, full consideration shall be given to such factors as:

  1. Occupancy of areas;
  2. Actual duration of exposure and time averaging (including ON/OFF times of the RF generators, direction of the beam, duty factors, sweep times, etc.);
  3. Spatial characteristics of exposure, i.e., whole body or parts thereof;
  4. Uniformity of the exposure field, i.e., spatial averaging.
  5. Power levels and distance from the radiator In certain instances and over a specific frequency range, higher exposure levels are permitted for short durations.

Exposure to the public is potentially 24 hours a day for 7 days a week, compared with 8 hours a day, 5 days a week for RF and microwave exposed workers.


Definitions

Maximum Permissible Exposure limits are defined in terms of power density (units of milliwatts per centimeter squared: mW/cm2), electric field strength (units of volts per meter: V/m) and magnetic field strength (units of amperes per meter: A/m).

Antenna Surface: The maximum power density directly in front of an antenna (e.g., at the antenna surface) can be approximated by the following equation:

S=4P/A

Where:
P = power input to the antenna (in appropriate units, e.g., mW)
A= physical area of the aperture antenna

Far-Field Region: The power density in the far-field or Fraunhofer region of the antenna pattern decreases inversely as the square of the distance. The power density in the far-field region of the radiation pattern can be estimated by the general equation:

S = PG/π4R2

Where:
S = power density (in appropriate units, e.g. mW/cm2)
P = power input to the antenna (in appropriate units, e.g., mW)
G = power gain of the antenna in the direction of interest relative to an isotropic radiator
R = distance to the center of radiation of the antenna (appropriate units, e.g., cm)


Health Canada

Exposure Limits for Persons Not Classed As RF and Microwave Exposed Workers (Including the General Public) (Health Canada – Safety Code 6)

Frequency  (MHz)
Power Density
(W/m2 )
Averaging Time
(min)
300-1500
f / 30
6
1500-15000
50
6
15000-150000
50
616 000 / f 1.2

A power density of 10 W/m2 is equivalent to 1 mW/cm2


FCC

Evaluating Compliance with FCC Guidelines for Human Exposure to Radio frequency Electromagnetic Fields (OET Bulletin 65 Edition 97-01)

Frequency Range (MHz)
Power Density (S) (mW/cm2 )
Averaging Time
(min)
300-1500
f / 1500
30
1500-100,000
1.0
30

A power density of 10 W/m2 is equivalent to 1 mW/cm2


ETSI

Evaluating Compliance with ETSI Guidelines for Human Exposure to Radio frequency Electromagnetic Fields (Official Journal of the European Communities, EN REC519)

Frequency Range (MHz)
Whole body average SAR (W/kg)
Localised SAR (head & trunk) (W/kg)
Localised SAR (limbs) (W/kg)
Power Density, S
(W/m2 )
10MHz-10GHz
0,08
2
4

 

10GHz-300GHz

 

 

 

10

A power density of 10 W/m2 is equivalent to 1 mW/cm2


EION Calculations

Calculations are made with a 1.85 Meter parabolic dish, all other antennas are calculated to have less exposure

Sample Calculation is made for the Libra Plus Product Line (5.725-5.850 GHz)@ 126 mW (21 dBm) output power.

Transmitter Type Classification is Mobile as opposed to Portable. This means the transmitter is at least 20 cm away from Human body.

Far Field (Large Antenna):
S = PG/4pR2
= (126 mW)(5623)/4p (39.7m)2
=0 .036 W/m2 @ 39.7 Meters

Here P = Output power G = Gain Antenna S = Power density

Antenna Surface (Large Antenna):
S = 4P/A
= 4 (126 mW)/1.85 m2
= 0.188 W/ m2

Recall: FCC and ETSI Guidelines for Human Exposure to Radio frequency Electromagnetic Fields (OET Bulletin 65) Limit is 10W/m2 or 1 mW/cm2


Is Wireless Safe?

The FCC, ETSI and Health Canada RF standards have been developed by experts in science, medicine, engineering, public health and other fields The standards establish levels for safe human exposure to RF energy. These safety levels have substantial built-in margins of protection against any known harmful effects. EION Wireless products are designed, manufactured and tested such that they operate within regional and internationally recognized safety standards. Wireless technology has been around for the past sixty years and during this time studies have been performed to assure the public of the safety of this technology. The established judgment of expert panels, government agencies, standards bodies and public health authorities around the world is that radio signals from wireless devices are safe.

 

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