2007 Loughborough Antennas and Propagation Conference

DESIGN, IMPLEMENTATION AND ANALYSIS OF A ROBUST SYSTEM TOMEASURE GSM RADIATION

Tarief Elshafiley, Omar Hosny, Judy Sirafim and Ramy Gamal

October Universityfor Modern Sciences and Arts, 14Amer St., El-Dokki, Cairo, Egypt

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Abstract- This work presents design, analysis and implementation of a robustsystem to measure the power of the signal in the GSM range varying from 890 to 960MHz. The system is composed of antenna, RF detector, low pass filter, amplifier, analogto digital converter, and finally digital interface with a PC. As for the antenna, wedesigned and implemented microstrip antenna with bandwidth of 8% using foam asdielectric substrate. We used the system to measure the power of the signal in differentplaces at different times during the day. Using the system we assured that the signalvaries depending on the traffic at different times and the location with respect to thebase station.

I. Introduction

The frequency spectrum in the GSM application is considered the most crowded range fromthe number of users' point of view which reached 9 million users of mobiles in Egypt forinstance. In recent years there has been a developing awareness that EM fields produced byeverything ranging from power lines to mobile phones are implicated in a variety of illnessesincluding cancer [1] and [2]. Researchers are now trying to identify the various types ofeffects that may lead to health risks to humans. It has been proven that EM radiation has a lotof hazards depending on the power and the frequency of the signal [3]. There are many wayslisted in the literature describing the measurements of the EM strength in the far-field such asmeasurements on CW signals, modulation measurements and spectrum and networkanalyzers. Good references for these methods can be found in [4] and [5]. Unfortunately,most of these methods are complicated and not robust.

II. System Design

The system is simply composed of a microstrip antenna with a central frequency of 925MHz, followed by the direct RF detector and a low pass filter (LPF) ended by the digitalsystem interface with the PC as detailed in Fig. 1. In this section we present the design ofdifferent phases of the work.

A. Phase (1) Antenna Design:

We used the output values obtained from the Matlab code that we wrote as initial parametersfor the numerical software which employs Method of Moments (Zeland package) to optimizethe design. Conventional microstrip antennas cover approximately 1% BW whereas in ourwork we required the antenna to cover up to 8% BW. Fig. 2 presents the final design which

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we implemented practically and used in our work. Clearly demonstrated on the figure is thecovered BW (72 MHz) with a return loss of-22 dB centered at 925 MHz.

Fig. 1 Block diagram of the overall system

Fig. 2 Final Antenna Design

B. Phase (2) Detecting the Power of the Received Signal:

We used Direct Detection method using RF diode. A diode is used to convert a fraction of anRF input signal to DC power which is proportional to the power of the input signal. Onemajor issue we considered during the design is to match the antenna connected to thetransmission line to the detector and next to the LPF and the rest of the circuit. One majorproblem we faced is the variable diode resistance according to the forward current. Webasically considered a 50 ohm as a reasonable diode resistance.

C. Phase (3) Make the system stand alone.

In order to make the system self power dependant we needed to have a constant mobilepower supply. We solved this problem using the 5 volts obtained from the USB port in thePC.

III. System Implementation

In order to reach to 8% BW we were obliged to use a thick dielectric substrate which causes asurface wave that reduces the antenna efficiency. To solve this dilemma, we have to use amaterial having a smaller dielectric constant. The foam with a dielectric constant of 1.06 isthe best choice [6]. One major problem using foam as a dielectric substrate is the etching ofthe conducting patch and the ground plane. Again we solved this problem using cupper sheetsfixed on the top and bottom of the foam using glue as shown in Fig. 3. As For the RF diodedetector, which is a surface mount component, we used a dielectric substrate to connect thediode taking in consideration the width of the microstrip line to ensure matching.

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2007 Loughborough Antennas and Propagation Conference

Fig. 3 The microstrip antenna using foam as dielectric substrate and copper sheet asconducting patch and ground plane

Fig. 4 Testing the antenna on spectrum analyzer to measure the GSM reverse and forwardchannels

Fig. 5 Car kit used to test the microstrip antenna.

Fig.6 All modules assembled together not including the antenna

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2007 Loughborough Antennas and Propagation Conference

IV. Results, Conclusion and Recommendation

To ensure a system works as designed, first, we first tested the Antenna separately usingtwo methods: first using the spectrum analyzer. We connected the antenna to the spectrumanalyzer and were able to see the GSM band in both forward and reverse channels as shownin Fig. 4. Second we built a car kit as shown in Fig. 5 and connected the antenna to it whilethe internal antenna of the mobile phone is totally blocked using a metallic shielded box. Wewere able to communicate with the mobile using our antenna. The prototype of the work asshown in Fig. 6 is designed, analyzed, built, tested, and successfully operated to measure thepower of GSM signal within the range 890-960 MHz and display it in real time as shown inFig. 7. As a matter of fact we were not able to calibrate the system since we didn't find aproper equipment to measure the GSM radiation. The idea of using foam as a dielectricmaterial in building the microstrip antenna with 8% bandwidth is unique in the GSMtechnology. The technique used for power detection using RF diode is simple and efficient.Interfacing the system with the PC and monitoring the power of the signal in real time isinnovative. Using spectrum analyzer, we clearly demonstrated the frequency hopping conceptand the forward and reverse channels used in mobile communication. For future work and forimprovements of the system, we recommend calibrating the system and scaling the verticalaxis. This system could be used to measure the power for GSM1800 and UMTS and not justGSM900. The antenna is the only module needs to be redesigned at the new frequencies andbandwidths.

Fig. 7 The measured power versus real time captured from a PC display

V. References

1. Riadh E. Y. Habash. Electromagnetic Fields and Radiation, Human bioeffects andsafety, NY: Marcel Dekker inc., 2002.

2. Paul F. Wacker and Ronald R. Bowman, "Quantifying Hazardous ElectromagneticFields Scientific Basis and Practical Considerations", IEEE Transactions onMicrowave Theory and Techniques, Vol. MTT-19, No.2, February 1971, pp.178-187.

3. Mann, S.M., T. G. Allen, R. P. Blackwell, and A. J. Lowe, Exposure to radio wavesnear mobile phones base stations, National radiation protection board. UK, 2000.

4. IEEE Recommended factor for the measurement of potentially hazardouselectromagnetic fields-RF and microwave, IEEE standard C-95.3, Institute of Electricand Electronic Engineers, New York, NY, 1991.

5. Ian Hickman, Practical RF Handbook, New Delhi: Newnes, 2002.6. A.F.A.Ayoub, "Analysis of Rectangular Microstrip Antennas with Air Substrates", J

ofElectromagn. Waves and Appl., Vol. 17. No.12, 2003, pp.1755-1766.

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