Introduction:  The  use  of  mobile  communication  systems  has  dramatically  increased  over  the  past 
decade. Although many studies have been performed to determine the effect of radio frequency (RF) but 
less attention has been paid to the possible biological impact of exposure to extremely low frequency 
(ELF) components.  
The objective of this study is two folds. One is to design the equipments needed for the measurement of 
the ELF fields of two types of GSM900 mobile phone. Secondly, use a protocol suitable for an accurate 
assessment of the ELF fields. 
Materials  and Methods:  First  a  home-made  search  coil  was  provided  and  calibrated precisely  under 
several experiments. Using Fast Fourier Transform, the power spectrum density of the induced voltage in 
the search coil was analyzed and the amplitudes of 217 Hz and its harmonics were extracted and then the 
distribution of magnetic field in the back side of mobile phones was determined. 
Results: The values of B-field on the back side of the two kinds of GSM mobile phone were different. 
They  were  between  50  to  160  µT in  Nokia  3310  and  14  to  30  µT in  Nokia  8310.  Considering  the 
difference between the amplitudes of frequency components at 217 Hz and its harmonics in the two kinds 
of mobile phone, a range of magnetic flux density at different times in a five day period was measured. 
Discussion and Conclusion: These findings emphasize the need for considering the distribution of low 
frequency magnetic field from mobile phone when biological effects of magnetic fields are studied. To 
determine  the  intensity  windowing  effect,  one  must  consider  the  physical  characteristics  of  the 
fundamental  frequency  component  wave  (217  Hz)  and  its  harmonics  produced  by  the  mobile  phone 
similar to the one generated under a real situation.