24th

NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2007)

March 13-15, 2007

Faculty of Engineering, Ain shams Univ., Egypt C39 1

DESIGN AND IMPLEMENTATION OF A ROBUST SYSTEM TO MEASURE GSM

RADIATION USING MICROSTRIP ANTENNA ON A FOAM SUBSTRATE

Tarief Elshafiey, Omar Hosny, Judy Sirafim and Ramy Gamal

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

Email: telshafiey@gmail.com

Abstract ـــــ This work presents design, analysis and implementation of a robust system

to measure the power of the signal in the GSM range varying from 890 to 960 MHz. The

system is composed of microstrip antenna on a foam substrate, RF detector, low pass filter,

amplifier, analog to digital converter, and finally digital interface with a PC. As for the

antenna, we designed and implemented microstrip antenna with bandwidth of 8% using

foam as dielectric substrate. We used the system to measure the power of the signal in

different places at different times during the day. Using the system we assured that the

signal varies depending on the traffic at different times and the location with respect to the

base station.

I. Introduction

The frequency spectrum in the GSM application is considered the most crowded range from

the number of users’ point of view which reached 9 million users of mobiles in Egypt for

instance. In recent years there has been a developing awareness that EM fields produced by

everything ranging from power lines to mobile phones are implicated in a variety of illnesses

including cancer [1] and [2]. It has been proven that EM radiation has a lot of hazards depending

on the power and the frequency of the signal [3]. There are many ways listed in the literature

describing the measurements of the EM strength in the far-field such as measurements on CW

signals, modulation measurements and spectrum and network analyzers. Good references for

these methods can be found in [4] and [5]. Unfortunately, most of these methods are complicated

and not robust. The purpose of this work is to design, implement and analyze a simple, robust

and portable system to measure GSM radiation. This work presents an extension of our previous

work [6].

II. System Overview

The system is simply composed of a microstrip antenna with a central frequency of 925 MHz,

followed by the direct RF detector and a low pass filter (LPF) ended by the digital system

interface with the PC as detailed in Fig. 1.

24th

NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2007)

March 13-15, 2007

Faculty of Engineering, Ain shams Univ., Egypt C39 2

III. System Design

In this section we present the design of different phases of the work.

A. Phase (1) Antenna Design:

In antenna design and analysis we used the analytical method using transmission line model [7].

In this method, we calculated the width (W) and the length (L) provided the available dielectric

material with its height (h) and dielectric constant (εr).

We created a Matlab code to determine the dimensions of the microstrip antenna (L) and (W). In

this code we faced a difficulty to calculate the integral equation in a closed form hence we solved

it numerically using iteration method. We used the output values obtained from the Matlab code

as initial parameters for the numerical software which employs Method of Moments (Zeland

package) to optimize the design. First we started by optimizing the central frequency (925 MHz)

by varying the length of the patch around the value calculated from the Matlab code. The second

step is to minimize the return loss (S21) by varying the location of the feeding point along the

diagonal of the patch in order to ensure dual polarization.

The final step is to cover the required BW (890 to 960 MHz). Conventional microstrip antennas

cover approximately 1% BW whereas in our work we required the antenna to cover up to 8%

BW. To do so, we varied the height of the dielectric substrate until we reached the optimum one.

As a matter of fact the process is not that simple. Practically, the change in the length, width or

height has an effect on the return loss, central frequency and BW. So we have to repeat the

process until the required performance is obtained. Fig. 3 presents the final design which we

implemented practically and used in our work. Clearly demonstrated on the figure is the covered

BW (72 MHz) with a return loss of -22 dB centered at 925 MHz. Table (1) summarizes the

initial and optimum values of the design.

Table 1 Analytical versus Numerical results.

Analytical

results

Numerical

results

Length of the patch 13.394 cm 8 cm

Width of the patch 15.97 cm 12 cm

Height of the dielectric. 1.5 cm 1.5 cm

B. Phase (2) Detecting The Power Of The Received Signal:

We used Direct Detection method using RF diode [8]. A diode is used to convert a fraction of an

RF input signal to DC power which is proportional to the power of the input signal. One major

issue we considered during the design is to match the antenna connected to the transmission line

to the detector and next to the LPF and the rest of the circuit. One major problem we faced is the

variable diode resistance according to the forward current as shown in Fig. 4. We basically

considered a 50 ohm as a reasonable diode resistance.

C. Phase (3) Convert The Received Signal To Digital And Interface It With The PC.

24th

NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2007)

March 13-15, 2007

Faculty of Engineering, Ain shams Univ., Egypt C39 3

We used an analog to digital converter to convert the RF diode output analog signal to digital.

The signal then passed from the A/D through a microcontroller to a serial interface IC (HIN232)

which sent the data to the PC. We used visual studio.net C++ to write the program for the PC to

receive the signal, calculate the average and maximum values and to display the power measured

versus real time.

D. Phase (4) Make the system stand alone.

In order to make the system self power dependant we needed to have a constant mobile power

supply. We solved this problem using the 5 volts obtained from the USB port in the PC.

IV. System Implementation

In order to reach to 8% BW we were obliged to use a thick dielectric substrate which causes a

surface wave that reduces the antenna efficiency. To solve this dilemma, we have to use a

material having a smaller dielectric constant. The foam with a dielectric constant of 1.06 is the

best choice [9]. One major problem using foam as a dielectric substrate is the etching of the

conducting patch and the ground plane. Again we solved this problem using cupper sheets fixed

on the top and bottom of the foam using glue as shown in Fig. 5.

As For the RF diode detector, which is a surface mount component, we used a dielectric

substrate to connect the diode taking in consideration the width of the microstrip line to ensure

matching. The diode mount is shown in Fig. 6.

V. System Testing

To ensure a system works as designed first it has to undergo a certain procedure. This

procedure states testing each module separately for functionality then integrating the module in

the system then check for the functionality of the whole system together.

Antenna: We first tested the Antenna separately using two methods: first using the spectrum

analyzer. We connected the antenna to the spectrum analyzer and were able to see the GSM band

in both forward and reverse channels as shown in Fig. 7.

Second we built a car kit as shown in Fig. 8 and connected the antenna to it while the internal

antenna of the mobile phone is totally blocked using a metallic shielded box. We were able to

communicate with the mobile using our antenna.

Direct detector: We connected the function generator as an input to the diode, selected the same

frequency range as GSM ones and detected its DC output on the oscilloscope. Then we

connected the antenna to the input of the diode and we have seen the variation of the DC level on

the oscilloscope since the antenna is receiving variable signals.

LPF: We tested its function properly in the required frequency range using a function generator

and oscilloscope.

A/D and interfacing with PC: We tested them together by introducing an analog signal to the

A/D and reading the output using a multi-meter then we tested the output of the microcontroller

by connecting the output to LED in order to view the values and the change corresponding to the

input variations. Then we connected all together and viewed the equivalent digital values on the

PC.

The system integrated: We integrated all modules together as shown in Fig. 9.

24th

NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2007)

March 13-15, 2007

Faculty of Engineering, Ain shams Univ., Egypt C39 4

VI. Results and Conclusion

The prototype of the work is designed, analyzed, built, tested, successfully operating to

measure the power of GSM signal within the range 890–960 MHz and display it in real time as

shown in Fig. 10. As a matter of fact we were not able to calibrate the system since we didn’t

find a place to borrow an accurate device to measure the GSM radiation. The idea of using foam

as a dielectric material in building the microstrip antenna with 8% bandwidth is unique in the

GSM technology. 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 is

innovative. Using spectrum analyzer, we clearly demonstrated the frequency hopping concept

and the forward and reverse channels used in mobile communication.

VII. Recommendation For Future Work

We recommend the following improvements and enhancements for future work:

� Instead of measuring the power, we recommend measuring the SAR (Specific absorption

rate calculated in watts per kilogram of human body) level.

� Using Shottky diode instead of the PIN diode and compare the results.

� Calibrate the system and scale the vertical axis.

� Use the calibrated system to measure the power of GSM signal at different places and

different time, and determine the peak power level.

References

1. Riadh E. Y. Habash. Electromagnetic Fields and Radiation, Human bioeffects and safety,

NY: Marcel Dekker inc., 2002.

2. Paul F. Wacker and Ronald R. Bowman, “Quantifying Hazardous Electromagnetic Fields

Scientific Basis and Practical Considerations”, IEEE Transactions on Microwave 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 waves near

mobile phones base stations, National radiation protection board. UK, 2000.

4. IEEE Recommended factor for the measurement of potentially hazardous

electromagnetic fields-RF and microwave, IEEE standard C-95.3, Institute of Electric

and Electronic Engineers, New York, NY, 1991.

5. Ian Hickman, Practical RF Handbook, New Delhi: Newnes, 2002.

6. Tarief M. F. Elshafiey, Omar Hosny, Judy Sirafim, and Ramy Gamal, "Design,

Implementation and Analysis of a Robust System to Measure GSM Radiation" LAPC

Proceeding, LAPC2007, to be published in Apr. 2007.

7. Constantine Balanis, Antenna Theory Analysis and Design, New York, NY, John Willey

and Sons Inc., 1997.

8. David Pozar, Microwave Engineering, New York, NY, John Wiley and Sons, Inc., 1999.

9. A.F.A.Ayoub, “Analysis of Rectangular Microstrip Antennas with Air Substrates”, J. of

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

24th

NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2007)

March 13-15, 2007

Faculty of Engineering, Ain shams Univ., Egypt C39 5

Fig. 1 Block diagram of the overall system

Fig. 2 Simple microstrip patch antenna

Fig. 3 Final Antenna Design

Interface

A/D RF

Diode

LPF

Antenn

PC

BBWW == 7722 MMHHzz

24th

NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2007)

March 13-15, 2007

Faculty of Engineering, Ain shams Univ., Egypt C39 6

Fig. 4 Diode resistance versus forward current

Fig. 5 The microstrip antenna

Fig. 6 RF diode mounted on dielectric substrate

24th

NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2007)

March 13-15, 2007

Faculty of Engineering, Ain shams Univ., Egypt C39 7

Fig. 7 Testing the antenna on spectrum analyzer

Fig. 8 Car kit used to test the antenna.

Fig.9 All modules assembled together

24th

NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2007)

March 13-15, 2007

Faculty of Engineering, Ain shams Univ., Egypt C39 8

Fig. 10 The measured power versus real time