How to Assess Electromagnetic Radiation from Mobile Phone Towers

By Dr Rajiv Kumar Singh

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At the boundary of the reactive near-field zone, a transition region exists wherein the radiating field, referred to as reactive-radiating near-field, begins to be important compared to the reactive component. Radial limit of the radiating near-field is 2D2 /λ.
The region between reactive near-field and far-field region, wherein radiation field predominates, is referred to as Fresnel zone. This region exists only if maximum dimension D of the antenna is large as compared to wavelength λ. It is noteworthy to mention that radiation field predominates here, and propagation of EM waves is not a plane wave propagation. Electric (E) and magnetic (H) fields are locally normal, and ratio of E/H ≈ Z0 (intrinsic impedance of free space=377Ω).

Field region beyond radiating near-field (distance≥2D2 /λ or larger value between 3λ and 2D2/λ) is radiating far-field, where field pattern is essentially independent of the distance from the antenna, and radiated power density is constant. In this region, electric (E) and magnetic (H) fields are perpendicular to each other, and the wave propagates as plane wave.

What to measure

What is to be measured {E, H, power density (S) or specific absorption ratio (SAR)} depends on where (reactive or radiating field) the observer is, and on field impedance. Specific absorption ratio is the time derivative of the incremental energy absorbed by (dissipated in) an incremental mass contained in a volume element of a given mass density. It is expressed in units of watts per kilogram (W/kg).

In reactive near-field, it is required to measure both E and H components, or evaluate SAR. Determining SAR instead of field measurement is preferable at the positions located very close to the antenna. In reactive-radiating near-field, if no information on field impedance is available, it is suggested to measure both E and H fields. If information on field impedance is available, it is possible to measure only one field component. If E/H>120π (high-impedance EMF), measurement of E component is required. If E/H<120π (low-impedance EMF), measurement of H component will suffice.

Now, in radiating near-field, it is suggested to measure only E component with impedance taken equal to free space impedance (Z0). In radiating far-field region, measurement of either E or H field component can be done to determine the equivalent power density. However, measurement of E field component is preferred. In far-field region, equivalent power density is obtained from the measured field by calculating, that is, by taking consideration of, Poynting vector.

Some technical considerations

Time averaging. Since mobile communication systems are highly time dependent, it is important to consider time averaging of the concerned fields. In this regard, ICNIRP has mentioned that RMS values of the fields are to be averaged over any six-minute period below 10GHz and over 68/ 1.05-minute period for frequencies exceeding 10GHz (where f is frequency in GHz). In cases of expected time variability of the source, measurements are to be done for an extended period of time. In cases of channel variability, that is, load variability, measurements are to be conducted during time of peak usage.

Multiple sources

In situations where there are multiple sources of radiations, the effect of multiple sources radiating EM waves at different frequencies is considered in a weighted sum. Each individual source is pro-rated according to the limit applicable to its frequency.

In most cases, a typical transmitting station contains many transmitting systems operating on many frequencies. In this scenario, all operating frequencies must be considered in a weighted sum, where each individual source is pre-rated according to the limit applicable to its frequency.

For simultaneous exposure to fields at different frequencies of interest (GSM, CDMA, HSDPA, WiMAX), compliance with the exposure limits is evaluated by considering field strengths due to all the frequencies of interest.

Spatial averaging

Strength of EM fields of an antenna is highly dependent on the distance/position from the antenna. Near close vicinity of the antenna, these fields are appreciably high. So, spatial averaging is required to get an exact estimation of field strength. Moreover, field strength may also vary with the spatial position due to the effect of reflection, shadowing and scattering about adjacent conducting surfaces. Fig. 3 clarifies such a situation where reflected fields add more values to the fields that may lead to over-estimation of the field during measurements.

Multi-path reflection of waves
Fig. 3: Multi-path reflection of waves

Multi-path reflections can create non-uniform field distribution. Therefore to assess the correct exposure levels, an averaging process is required. Field values should be determined at several points (p) as shown in Fig. 4. Basically, three points are recommended (Fig. 4a), but if higher accuracy is required, the number can be increased to six (Fig. 4b), nine (Fig. 4c), 20 (Fig. 4d) and so on. If there are multiple sources, measurement is performed by dividing the entire area into small grids of about one-square-metre each and taking measurements at individual grid points.

Measurement points for spatial averaging
Fig. 4: Measurement points for spatial averaging
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