Numerical Analysis and Measurement of Electric-field Strength Inside Gtem Cell at Gsm Frequencies

Abstract— A miniaturized Gigahertz Transverse Electromagnetic (GTEM) Cell is designed and fabricated to generate uniform electric (E-) field, essential for studying the RF exposure effect on tissue equivalent liquids at Global System for Mobile (GSM) Communication frequencies (914MHz and 2.10GHz). The simulation procedure is discussed and its results are compared with measurement data. The E-field strength inside the GTEM cell is scanned using a microstrip based E-field probe and complete uncertainty evaluation procedure is discussed. Theoretical, simulated and measured E-field strength is reported with expanded uncertainty.

Keywords— E-field probe, GTEM cell, Microstrip, RF exposure, Uncertainty

Introduction

A Gigahertz Transverse Electromagnetic (GTEM) cell is fabricated to generate calculable uniform Electric (E-) field strength to study the effect of RF exposure on tissue-equivalent liquids [1]. The design of the GTEM cell is optimized for 914MHz and 2.10GHz, two most commonly used frequencies of communication in India. The GTEM cell design [2] is simulated on HFSS [3], which is based on well-known numerical method for electromagnetic problems - Finite Element Method (FEM). A dual-band microstrip E-field probe is used to measure the E-field strength. The performance of the probe is exhaustively characterized; details are given in [4]. This paper compiles the results of the measurements that are carried out for E-field strength. The matching condition and E-field strength at different positions and at different fed power inside GTEM cell is given. For validation of the measurement, another commercially available isotropic E-field probe is used and results are compared. The analytical value of E-field strength is calculated and reported.

Design of Gtem Cell

The designed prototype of the GTEM cell is shown in Fig. 1. Aluminum is used to fabricate the GTEM cell casing and copper is used for the inner conductor i.e. septum which is backed by Teflon coating. An N-type connector is used to provide the feeding to the cell. The connector pin is connected to the septum and the cell housing is grounded with the connector. The septum is supported with the upper sheet with microwave-transparent Teflon rods to avoid sagging with the running length in z-direction. Commercially available 4cm pyramidal microwave absorber is used for matching of the impedance and termination of the cell. The design parameters of the cell are schematically shown in Fig. 2 (a-b).

The use of copper backed by Teflon coating is suggested to suppress the generation of higher order modes due to the direct multiple reflections from the septum itself. The effect of septum is studied using the simulation results. The tapered shape of the GTEM cell ensures 50Ω characteristic impedance along the direction of propagation. It was understood from [1] that the design parameters are theoretically optimized for GSM band frequency ranges. Hence, it is found suitable for our purpose and the design is used to fabricate the cell for GSM and additionally Universal Mobile Telecommunications Systems (UMTS) band studies. As part of electromagnetic characterization of the GTEM cell, Voltage Standing Wave Ratio (VSWR) of the cell is measured at the frequencies of interest. The simulation of the design, Fig. 2 (c), is carried out beforehand on EM software-HFSS [2] to ensure the GTEM cell capabilities.

Theoretical Considerations

A. Analytical Approach

The E-field strength E inside the GTEM cell is calculable from septum height h, fed power P, characteristic impedance Z, modulation M and flatness F, given in [5] as

. (1)

However, the estimation of h can be made using the fact that electric field in the x-direction can be given by

(2)

where, V is the input voltage. It can hence be estimated that at h=0.119m and input power of 0dBm in 50W system, the electric field will be

(3)

B. Finite Element Method based Mathematical Model

The septum conductor when coated with material, such as Teflon, the potential and electric field changes due to the change in the effective relative permittivity inside the GTEM cell. The frequency dependence and effect of Teflon is studied using HFSS simulation. An observation plane is created at z=240mm from the input port. The simulation results on this plane are discussed in the following section.

Results and Discussion

The GTEM cell is characterized based on VSWR measurement along with the simulation and measurement results of electric field strength. The measured VSWR of the GTEM cell loaded with absorbing dielectric foam at 914MHz is 1.34±0.02 and at 2.10GHz the value is 1.14±0.02. The simulation gave the VSWR of the order of 1.15. The difference is accounted due to higher mismatch in the real structure than that of the ideal simulated one. The detailed measurement results of VSWR and reflection coefficient (Γ) are given in Table I.

TABLE I. IMPEDANCE MATCHING OF GTEM CELL

914 MHz 2.10 GHz

VSWR Γ VSWR Γ

GTEM cell terminated by microwave absorber 1.32 0.1379 1.14 0.0654

Microstrip E-field probe inserted 1.33 0.1416 1.14 0.0654

Standard Isotropic E-field probe inserted

1.34 0.1452 1.16 0.0740

Conclusion

A miniature GTEM cell is fabricated having length of 450mm. The variation in electric field strength due to the effect of Teflon coated septum conductor is analyzed in detail. A comprehensive numerical analysis and simulation is carried out for the construction of GTEM cell. The E-field strength inside the GTEM cell, at a distance of 240mm from the input port, is scanned using a microstrip E-field probe with an expanded uncertainty of ±0.80V/m at 914MHz and 2.10GHz. For the validation of the measurement, results are compared with commercially available isotropic E-field probe. A uniform E-field strength is found Emax=2.94V/m (theoretical E=2.86V/m) at center of the GTEM cell against +10dBm fed power. The GTEM cell is hence a suitable exposure system for studying the effect of RF radiation on different samples. The uniform electric field strength generated inside the calibrated GTEM cell ensures effective studies of exposure.