Assessment of safety distances of GSM base station antennas operating at 900 and 1800 MHz for occupational exposure

Objective: The safety distances of base station antennas can be obtained either by assessment of the electromagnetic fields and comparison with the reference levels or by assessment of the Specific Absorption Rate (SAR) and comparison with the basic restrictions [1].The objective of this paper is to determine and to compare the safety distances based on the electromagnetic fields for two different types of GSM base station antennas operating in a different frequency band.Furthermore, we will make a comparison of the safety distances based on the electromagnetic fields and based on the SAR using simulations and measurements for one type of antenna.Methods: We investigate GSM base station antennas of the type Kathrein 736863 at 947.5 MHz and K739495 at 1862.5 MHz, respectively.At these frequencies, the antennas radiate maximally.We determine the safety distances based on the electromagnetic fields in front of the antennas, noted as D X (X = E or H) because they will be most restrictive in this case.The measurements are performed with a network analyzer (Rohde & Schwarz ZVR) and are compared with FDTD simulations.We use a robot to position the measurement probes.The measurements are performed with a spatial grid of 2 cm, smaller than λ/10 at 947.5 and 1862.5 MHz.We determine the safety distances for the fields in three ways: by determination of the maximum field (worst-case situation) values X max (X = E or H) in xyplanes in front of the antenna, by determination of X plane , the field value averaged in xy-planes with dimensions 70x40 cm 2 , and by determination of X vol (X = E or H), the field value averaged in a volume of 70x40x20 cm 3 .Next, we determine the safety distances based on the SAR for the K736863 antenna, noted as D SAR .For the SAR assessment we use a rectangular box phantom (dimensions 80x50x20 cm 3 ) [2].We position the rectangular box phantom at distances 1 mm to 20 cm from the K736863 antenna.We use a SAR probe of the type ESD3DV1, delivered and calibrated for the DASY3 system of Schmid & Partner Engineering AG.Finally, we determine the difference of the safety distances based on the electric field and based on the SAR i.e., Δ = D X -D SAR [m] (X = E or H).Using Δ we can determine the input power from which on the electromagnetic fields will deliver the largest safety distances.Results: Fig. 1 and 2 show that the electric field values of the K739495 antenna at 1862.5 MHz are higher than those of the 947.5 MHz K736863 base station antenna for the investigated positions.This was expected because the gain of 1800 MHz base station antennas (around 18 dBi) is generally higher than the gain of 900 MHz base station antennas (around 15 dBi).The gain of the K736863 antenna is 15.5 dBi, while the gain of the K739495 antenna is 18 dBi.On the other hand the reference levels are higher at 1862.5 MHz than at 947.5 MHz [1] (129.47 V/m and 0.35 A/m at 1862.5 MHz against 92.34 V/m and 0.25 A/m at 947.5 MHz for occupational exposure).There is a very good agreement between measurements and simulations (Fig. 1 and Fig. 2).Table 1 compares the safety distances for the electric and magnetic field of both antennas.The safety distances of the K739495 antenna at 1862.5 MHz are of the same order as those of the K736863 antenna at 947.5 MHz.For averaging in a plane and volume, the safety distances of the 1862.5 MHz antenna are lower than those of the 947.5 MHz antenna due to the considerably higher reference levels at 1862.5 MHz.