Densitometry of Electromagnetic Field Exposure Due to Wi-Fi ... - piers

Progress In Electromagnetics Research Symposium Proceedings, Marrakesh, Morocco, Mar. 20–23, 2011 1385

Densitometry of Electromagnetic Field Exposure Due to Wi-Fi Frequency A. D. Usman1 , W. F. Wan Ahmad1 , M. Z. A. Ab Kadir1 , M. Mokhtar2 , and M. A. Zainal Abidin1 1

Centre of Excellence on Lightning Protection (CELP), Faculty of Engineering Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia 2 Wireless and Photonic Networks (WiPNET), Research Center of Excellence Faculty of Engineering, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia

Abstract— The world is undergoing electromagnetic revolution with many frequencies used for a variety of wireless devices. Recently, the emergence of Wi-Fi technologies deployed in schools, offices and other public places raised a lot of concern by the general public on the magnitude and safety of this exposure. To this effect, in situ measurements were conducted at various Wi-Fi access points at both academic and students’ residential areas in Universiti Putra Malaysia, Serdang, Selangor, Malaysia. An RF survey meter with tri-axis probe and a spectrum analyzer were used to carry out measurements at far field distance for 6 minutes average time. Various locations were investigated and it is found that the highest and the lowest readings obtained during the downloading and uploading activities were 5.63 V/m and 0.37 V/m, respectively. These readings are far below the ICNIRP reference guidelines for exposure to general public, i.e., 61 V/m for 2.46 GHz Wi-Fi operating frequency. 1. INTRODUCTION

Wireless Fidelity or popularly known as Wi-Fi is one of the new technologies that are widely used by both the young and the aged. This new technology enable users to communicate to internet from any place such as offices, homes, in space and any place without the hassles of plugging wires. Locations where users can connect to wireless networks are called Wi-Fi hotspots. The most popular Wi-Fi technology such as 803.11b operates at frequency range of 2.40 GHz to 2.4835 GHz. The network devices providing the Wi-Fi air interface between the broadband network and the user terminal is called Wi-Fi access point (Wi-Fi AP), which is a radio transceiver itself. The use of Wi-Fi services for private purposes is covered by an exemption order, but the provision of it to public in some countries is permissible under a class of license provided that the services do not cross the public streets or unleashed government lands. For these public purposes a fixed carrier license is required. Regulations were given by international bodies and scientific communities in terms of maximum exposure allowed [1–4]. However, even with much lower exposure, the energy emitted from radio frequency (RF) and microwave frequencies has unwanted side effects [5–9]. To date, there are no studies adequately done on effect of exposure to Wi-Fi. However, there are similar studies on the frequencies around the frequencies of operation of Wi-Fi resulting on complicating conclusions. Yet attempts were made by some countries to turn their cities to e-cities with Wi-Fi technologies thereby exposing the greater populace to radio frequency radiation (RFR). Few studies were performed on the RFR emitted by Wi-Fi hotspots as in [10–12], with none is done in Malaysia being one of the leading nation in South East Asia that employ the use of this technology in almost all facet of life, has motivated this research. Another factor is the public concern and the exposure of the members of staff of university community and students spending extensive amounts of time in buildings in which Wi-Fi networks has been established made us to investigate the RFR levels at both the Faculty of Engineering and the 10th college student hostel putting into consideration the activity levels, RFR emission from both the routers and the Wi-Fi APs during the time of measurements. A controlled environment was also used to carry out the experiment during the downloading and uploading activities to validate the practical measurements results. Note that both Faculty of Engineering and the 10th college student hostel are located at Universiti Putra Malaysia, Serdang, Selangor, Malaysia. 2. MATERIALS AND METHOD

Measurements were performed at 10 different points within the Faculty of Engineering and 10th college student hostel, following the experimental protocol as in [13]. The instruments used were the

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PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011

PMM8053A Broadband RF survey meter with a frequency range of 100 kHz to 3 GHz and electric field measurement range from 10 mV/m to 100V/m. The RF survey meter has a tri-axis probe. A laptop is used to retrieve the field strength information from the meter. A spectrum analyzer SA 9270A of frequency range 9 kHz to 2.9 GHz was used to determine the dominant operating Wi-Fi frequency. This meant for the dominant exposure level to be found. Having determined that, the RF survey meter operating in broadband is used to measure the field strength. Figure 1 shows the set-up of the measurement procedure at Wi-Fi APs 1 to 9, while Figure 2 illustrated the set up for a controlled experiment carried out in the 8th floor Laboratory at Wi-Fi AP 10. The far field approximate points were determined numerically using the far field equation as in (1), where r is the distance from observation point, rn is the distance in m, D is the maximum dimension of the antenna and λ is the operating wavelength. r > rn =

D2 2λ

(1)

Note that the Wi-Fi APs used in Universiti Putra Malaysia campuses were Omnidirectional of two types, i.e., ceiling and wall mounted. The antenna were made from Cisco with operating frequency of 2.46 GHz, a gain of 5.2 dBi and approximate length of 15 cm. Hence, the far field was found to begin at 0.1 m. Therefore, electric field measurements were performed at 0.2 m, 0.4 m, and 1m distances. Equation (2) was also used to validate the field measurements at various distances, were E is the magnitude of the electric field strength in V/m, P is the power of the source in W , and r is the radial distance. µ ¶1 1 P 2 E= 377 × (2) 2r π The measurements from the controlled experiment were validated by estimating the polarization of the signals from the Wi-Fi router. This was achieved by repeatedly engaging the network with downloading and uploading activities while the antenna of the router was set at different angles of rotation. The orientation of the antenna with maximum readings was noted to indicate that the antenna was aligned with the polarization of the emission. The measurements were taken by keeping the Wi-Fi router and the computer at a fixed point of a reasonable distance apart. Note that the computer was used for downloading and uploading activities while measurements were taken with the RF survey meter at 0.2 m, 0.4 m, and 1 m distances each for 2 minutes. Then, the router and the RF survey meter were kept fixed and the laptop was moved at a distances of 0.2 m, 0.4 m and 1 m, respectively. In both cases the readings were taken during downloading and uploading activities.

Figure 1: Experimental Set-Up for RF measurements at Wi-Fi APs 1-9.

Figure 2: Top view of the experimental Set-Up for RF measurements at Wi-Fi AP 10.

3. RESULTS AND DISCUSSIONS

Based on the far field determination using (1) and (2), measurements were conducted at 0.2, 0.4 and 1 m distances at all the 10 Wi-Fi APs, where the results are as in Tables 1 to 11. At each location, the maximum, minimum and average values of the electric field magnitude are determined


against time. Also, their percentage differences with ICNIRP standard for general public exposure are calculated, i.e., 61 V/m for 2.46 GHz Wi-Fi operating frequency. At the Faculty of Engineering, it is found that the highest field strength of 5.16 V/m was obtained at Wi-Fi AP 3, situated between the two Lecture Halls 6 and 7. This corresponds to 8.5% of ICNIRP reference level for exposure to general public. The lowest readings of 0.37 V/m corresponding to 0.6% ICNIRP guidelines were obtained at Wi-Fi AP 1. At the student’s hostel, it is demonstrated that the highest reading is obtained at 4.51 V/m, i.e., at Wi-Fi AP 8, Wing C in 10th college which represents 7.4% of the ICNIRP reference level for exposure to general public. While, the lowest reading of 1.3 V/m at a distance of 1 m away from Wi-Fi AP 7, Wing B in 10th college was recorded where this is only 2.1% ICNIRP reference level. The controlled environment results obtained at Wi-Fi AP 10, 8th Floor Laboratory, Faculty of Table 1: Results obtained for Wi-Fi AP 1 at Cafeteria, Faculty of Engineering. Distance from APs to the RF Survey Meter, r (m) 0.2 0.4 1.0

Measured Electric Field Strength (V/m) Minimum Maximum Average 0.00 1.49 0.67 0.00 0.87 0.43 0.00 0.37 0.09

Table 2: Results obtained for Wi-Fi AP 2 in front of Lecture Hall 4, Faculty of Engineering. Distance from APs to the RF Survey Meter, r (m) 0.2 0.4 1.0


Table 3: Results obtained for Wi-Fi AP 3 between Lecture Halls 6 and 7, Faculty of Engineering. Distance from APs to the RF Survey Meter, r (m) 0.2 0.4 1.0


Table 4: Results obtained for Wi-Fi AP 4 behind the Conference Room, Faculty of Engineering. Distance from APs to the Measured Electric Field Strength (V/m) RF Survey Meter, r (m) Minimum Maximum Average 0.2 0.00 0.98 0.33 0.4 0.00 0.78 0.18 1.0* 0.00 1.35 0.35 ∗ Note that, the downloading and uploading activities were simulated at this distance at the time of measurement. Table 5: Results obtained for Wi-Fi AP 5 between Lecture Hall 8 and Research Division, Faculty of Engineering. Distance from APs to the RF Survey Meter, r (m) 0.2 0.4 1.0


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PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011 Table 6: Results obtained for Wi-Fi AP 6 at Wing A of 10th college student hostel. Distance from APs to the Measured Electric Field Strength (V/m) RF Survey Meter, r (m) Minimum Maximum Average 0.2 0.00 4.21 1.19 0.4 0.00 1.75 0.34 # 1.0 0.00 1.36 0.47 $ 1.0 0.00 2.45 1.27 # Note that, the measurement is done at empty room 310. $ Note that, the measurement is done at room 309 during two students are inside the room, and downloading and uploading activities are available at that time. Table 7: Results obtained for Wi-Fi AP 7 at Wing B of 10th college student hostel. Distance from APs to the RF Survey Meter, r (m) 0.2 0.4 1.0


Table 8: Results obtained for Wi-Fi AP 8 at Wing C 10th of 10th college student hostel. Distance from APs to the RF Survey Meter, r (m) 0.2 0.4 1.0


Table 9: Results obtained for Wi-Fi AP 9 at Wing D of 10th college student hostel. Distance from APs to the RF Survey Meter, r (m) 0.2 0.4 1.0


Table 10: Results obtained for Wi-Fi AP 10 at 8th Floor Laboratory when in NON-ACTIVE mode at Faculty of Engineering. Distance from APs to the RF Survey Meter, r (m) 0.2 0.4 1.0


Table 11: Results obtained for Wi-Fi AP 10 at 8th Floor Laboratory when in ACTIVE mode at Faculty of Engineering. Distance from APs to the RF Survey Meter, r (m) 0.2 0.4 1.0



Engineering while keeping the computer and the Wi-Fi router at constant position and moving the RF survey meter away from the router are as listed in Tables Xa and Xb for non-active and active modes, respectively. Note that there is no downloading and uploading activity during the nonactive mode and vice versa during the active mode. The highest electric field strength is obtained at 0.2 m, i.e., 5.63 V/m, and this field strength decreases as one move away from the AP with low readings of 1.13 V/m found at 1 m, corresponding to 5.9%, and 1.3% respectively, of the ICNIRP guidelines for public exposure. While keeping the Wi-Fi router and RF survey meter at 1 m apart (r = 1) and moving the computer, it is found that the highest electric field strength is obtained at 0.2 m, i.e., 4.09 V/m, and again the field strength decreases to 0.54 V/m at 1 m away where these are 6.7% and 0.1% respectively, of the ICNIRP guidelines for exposure to general public at 2.46 GHz operating frequency of Wi-Fi technology. The electric field magnitudes measured in this work are found to be much lower than the ICNIRP standard general public exposure where the calculations are done in a fraction of the standard and presented in percentage values. These regardless the maximum, minimum and average values of the measured electric field magnitudes. The proximity of the Wi-Fi APs and the Wi-Fi routers to the point of downloading and uploading activities should be as far as possible as results obtained demonstrated that, the magnitude of the electric field generated at these APs are highly depending on the downloading and uploading activities levels as well as the distance one person is from an AP. It was also found that the electromagnetic field strength was higher adjacent to the Wi-Fi router rather than near to the computer, as been illustrated by a controlled experiment at WiFi AP 10. This is due to the higher duty cycle at the base station as it serves as the hub in communications with many computers. These results are comparable to the findings in [10, 11] and will no doubt help in various campaign carried out about the fears of citing Wi-Fi hotspots in schools and residential areas. 4. CONCLUSION

Wi-Fi RF exposure assessment was done at Faculty of Engineering and 10th college student hostel, where both are located in Universiti Putra Malaysia, Serdang, Selangor, Malaysia. The frequency of operation of the Wi-Fi is 2.46 GHz which belongs to the IEEE 802.11g unlicensed frequency. The measured results obtained demonstrated that the highest readings to be 5.63 V/m and the lowest was 0.37 V/m. These readings were however much below the 61 V/m ICNIRP reference guidelines for general public. However care should be taken as these APs are very close to the students rooms at about 1.5 m away and the exposure are for 24 hours a day. While, in the case of academic areas people are only exposed to about 8 hours per day where the ICNIRP guidelines adopted by Malaysian government are meant to safeguard against short term thermal effect only. Generally, the electric field intensity is highly depending on the downloading and uploading activity levels, and the distance one person is from the Wi-Fi APs and the Wi-Fi routers. The extent of the exposure at any point was found to be a function of both the field emission and the distance of the transmitters. REFERENCES

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