Precise Power Flux Density Measurements at Base Stations

Wolfgang Müllner, Georg Neubauer, and Harald Haider

A new precision measurement method combines the advantage of a field probe--isotropic behavior--with that of frequency-selective measurement methods.

With the rapid increase in the use of mobile phones, public concern regarding the health and safety aspects of mobile telecommunications equipment has also increased. Some countries have established safety guidelines for protecting humans from radio-frequency exposure.

The basic exposure limits are commonly defined in terms of specific absorption rate (SAR). In practice, however, SAR is not accessible for routine evaluation of real-life exposure. Rather, it is determined primarily from expensive simulations, measurements in phantoms, and tissue measurements in laboratories. The only real-life quantities that can be measured somewhat easily are the free-field electric and magnetic field strengths. Standards, therefore, also provide the derived limits for electric and magnetic field strengths.

Standards

Derived limits are given in terms of power flux density S (W/m²) or in terms of electric field strength E (V/m) and magnetic field strength H (A/m). At operating frequencies of mobile communications base stations in far-field conditions, the measurement of the electric field strength is sufficient. A simple equation (Equation 1) describes the correlation to power flux density:

(1)

The limits are frequency dependent as shown in the example in Figure 1 for the International Commission on Non-Ionizing Radiation Protection (ICNIRP) limit. Because limits are frequency dependent, a frequency-selective measurement system is required to evaluate the results with the corresponding limit values. If the measurement system is not frequency selective, then the lowest limit within the measurement range must be chosen.

Figure 1. Frequency-dependent field-strength limit.

The European Community provided general guidelines in its Council Recommendation of July 1999.¹ ICNIRP published similar guidelines in April 1998.² Table I gives a sampling of the international and national field-strength limit values for the general public and continuous exposure.

Country Standard ELim 950 MHz ELim 1850 MHz International Council Recommendation 1999/519/EC 42 V/m 59 V/m International ICNIRP Guidelines, April 1998 42 V/m 59 V/m Austria ÖNORM S1120 49 V/m 61 V/m Belgium Belgisch Staatsblad F.2001-1365 21 V/m 30 V/m Germany 26. Deutsche Verordnung 42 V/m 59 V/m Italy Decreto n. 381, 1998 6 (20) V/m 6 (20) V/m The Netherlands Health Council 51 V/m 83 V/m Switzerland Verordnung 1999 4 V/m 6 V/m United States IEEE C95.1 49 V/m 68 V/m China Draft: National Quality Technology Monitoring Bureau 49 V/m 61 V/m Japan Radio-Radiation Protection Guidelines, 1990 49 V/m 61 V/m

Table I. A sampling of international and national field-strength limits for mobile communications frequencies.

In situations in which there is simultaneous exposure to fields of different frequencies, it is important to determine whether these exposures are cumulative in their effects. Such exposures should be examined separately for the effects of thermal and electrical stimulation, and the basic restrictions below should be met. The formula applies to relevant frequencies for thermal considerations under practical exposure situations:

(2)

where E^_i = the electric field strength at frequency i and E^_Lim = the electric field strength limit at frequency i.

The summation in Equation 2 assumes the worst-case conditions among the fields from multiple sources. As a result, typical exposure situations may in practice require less-restrictive exposure levels than indicated by Equation 2 for the reference levels. Country-specific standards are shown in Table II. Limits vary from country to country.

In Italy, for example, when a person is exposed to mobile phone emissions for more than 4 hours inside a building, a precaution limit must be used. The precaution limits are 6 V/m for the electrical field strength and 0.016 A/m for the magnetic field strength. The equivalent power flux density is 0.1 W/m² at frequencies greater than 3 MHz. The precaution limit at 950 MHz, as well as at 1.85 GHz, is 100 mW/m².

Switzerland issued its regulation in December 1999. In general, the ICNIRP limits are valid there. However, in sensitive locations such as residential areas and hospitals, compliance with emissions limits is required. This limit is 4 V/m (42.4 mW/m²) for base stations operating at around 900 MHz, 6 V/m (95.5 mW/m²) at 1.8 GHz, and 5 V/m (66.3 mW/m²) at stations operating at both frequencies.

Measurement Methods

Using a mobile communications base station as an example, this section describes the state-of-the-art methods for measuring field strength. The strengths and weaknesses of each method are provided. For this example, the field strength of a dual-band GSM base station (at 900 MHz and at 1.8 GHz) in an urban area must be measured at several locations. The distance from the antenna mast varies for each location. The relevant frequencies and the type of modulation are known. However, significant information is not available, including:

Measurement with Field Probe. In this example, measurement of the field strength is done with a field probe specified for 80 MHz to 40 GHz. Obtaining a measurement is simple, convenient, and fast. Due to the isotropic characteristic of the field probe, the unknown direction of maximum field and the unknown polarization are not important. In this case, the measurement is taken of the signal sum. The resulting measurement, however, raises the question as to whether the measurement is really the field strength generated by the base station.

It is important to note that a field probe is not designed to distinguish between emissions of different frequencies such as radio and TV broadcast stations, GSM mobile phones, or the base station to be measured. Therefore, the field probe provides no information as to whether the meter reading corresponds with the base station's emissions or with some other signal within the probe's measurement range. In fact, the reading will correspond with the strongest signal or the sum of several signals.

A field probe can be sensitive even to out-of-band signals. The frequency range in the probe's specification is not necessarily correlated with its actual sensitivity range. Furthermore, the lowest limit of 2000 mW/m² at 80 MHz must be applied for the entire frequency range because the meter reading cannot be correlated to a specific frequency. In this case, the measurement result could severely overestimate the hazard.

The calibration factor of the probe is usually valid for sinusoidal signals. The waveform of the measured signal or signals is unknown because the duty cycle (which is responsible for the waveform) of many mobile communications signals depends on the load (number of simultaneously connected people). The load is variable and generally unknown, and, therefore, additional errors can result.

In addition, the calibration factor of the probe is generally not constant over the entire frequency range. A frequency-specific application of the calibration factor is not possible. The sensitivity of the probe is poor: 0.1 V/m at the lowest; typically, diode probes have a lowest sensitivity of 0.5 V/m.

For this kind of measurement, the use of a field probe is not suitable and should be avoided. A field probe can be used only when results are confirmed by additional frequency-selective measurements that the signal of interest is much stronger than the other signals at the measurement location.

Measurement with Directive Antenna. Another measurement method uses a system that consists of an antenna plus a frequency-selective receiver or a spectrum analyzer. Currently available antennas that cover the frequency range of mobile communications are either directive or narrowband. Using typical directive antennas--log periodic or horn antennas--measurements are taken in either a sweeping mode or at discrete frequencies with a certain bandwidth of the receiver. For each reading, the measurement frequency is known, and the frequency-dependent antenna factor can be applied. In addition, the appropriate frequency-dependent limit values can be applied. For this method, out-of-band signals play no role as long as the receiver is well designed. The modulation and duty cycle of the signal are also not important as long as the measurement is done with a maximum-hold scan for a specified time period.

Disadvantages of this method result from the type of antenna used. Because of the directivity of the antenna, the measurement must be repeated at numerous antenna orientations to get an overview. The number of orientations required depends on the directivity. For example, a beam width of 45° requires a minimum of 16 directions (45° steps in three axes) for each of the two polarizations. Depending on the directivity of the antenna, additional scanning in the expected direction must also be done for the maximum exposure. It is important to remember that different frequencies might require a fine scanning in different directions. This can be a very time-consuming measurement method. However, the frequency-selective measurement with the directive antenna provides a precision measurement when it is done carefully.

New Measurement Method. A new method has been developed based on the frequency-selective measurement method with a receiver or spectrum analyzer. This new method uses a broadband omnidirectional receive antenna. The precision biconical antenna (PCD 8250) covers the frequency range of 80 MHz to 2.5 GHz continuously. The directional characteristic of this antenna is similar to that of an elementary dipole. Therefore, the effective field strength can be obtained from three voltage measurements with orthogonal orientation (e.g., x-, y-, and z-axis) of the antenna: U^_x, U^_y, and U^_z [V] are measured. The field strengths are calculated in linear quantities:

(3)

The effective field strength E^_eff [V/m] is calculated as follows:

(4)

where AF is the antenna factor in linear quantities [1/m]. All contributions (U, AF) and, therefore, E^_eff are frequency dependent.

The measurements in three orthogonal directions are done with a single antenna (see Figure 3). Therefore, the only problem that remains is that the readings do not happen at the same time. To avoid measurement errors due to rapidly changing signals, sufficiently long measurement times with maximum-hold acquisition must be chosen for each direction.

Figure 3. Three orthogonal E-field components obtained by multiplying the voltage readings with the antenna factor and the effective field strength Eeff according to Equation 4 in logarithmic quantities (20 log10(E)) as a function of frequency.

The new method is called Add3D, which stands for addition of three-dimensional field components (see Equation 4). In this method, the measurement procedure is simple and time efficient because it is controlled by software (see Figure 4). The power flux density can be calculated from the effective field strength with Equation 1 (see Figure 2). An operator positions the antenna in the three different orientations. For each orientation, the software sets the receiver bandwidth and the frequency range. It also stores the measured data. Measurements can be done in the frequency range of interest, and the appropriate limit values can be applied. As with the directive-antenna method, out-of-band signals play no role as long as the receiver is well designed.

Figure 2. Power flux density S in mW/m2 as a function of frequency; S = Eeff2/377.

Figure 4. Screen shot of the CalStan/W 4.1 measurement software.

With one set of measurements (three directions), the effective field strengths of all neighboring base stations (operating at different frequencies) can be determined. By doing so, much time is saved when mapping the field distribution.

Conclusion

Both international and national standards provide derived limits in terms of power flux density. The limits are frequency dependent, and, therefore, a frequency-dependent measurement system is essential. This new measurement method, Add3D, combines the isotropic behavior of a field probe with frequency-selective measurements with directive antennas. Based on the frequency-selective measurement method with a receiver or spectrum analyzer, it uses a broadband omnidirectional receive antenna to achieve three voltage measurements in orthogonal orientation.

References

1. 1999/519/EC, "Council Recommendation of 12 July 1999 on the limitation of exposure of the general public to electromagnetic fields (0 Hz to 300 GHz)," Official Journal of the European Communities (Luxembourg: EUR-OP, 1999).

2. "ICNIRP Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic, and Electromagnetic Fields (up to 300 GHz)," Health Physics 74, no. 4 (1998): 494­522.

Wolfgang Müllner (head of the business unit), Georg Neubauer (project manager), and Harald Haider (project manager) are with ARC Seibersdorf Research GmbH (Seibersdorf, Austria). They can be reached at emc@arcs.ac.at.