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

Published: 2021-06-17 06:32:52
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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.
A Gigahertz Transverse Electromagnetic (GTEM) cell is an alternative of anechoic chamber for EMI and EMC testing while doing high frequency measurements. This paper reports the fabrication of a miniature GTEM cell for generating known and uniform Electric (E-) field strength to study the effect of RF exposure on sample under test [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 microstrip E-field probe working on the above mentioned frequencies is used to scan the E-field strength inside the cell. The performance of the probe is exhaustively characterized; details are given in [4]. The matching condition and E-field strength inside the GTEM cell at different positions with varied input power is given and discussed. The E-field strength measurement results obtained using indigenous probe are compared with the results of another commercial isotropic E-field probe. The analytical and simulated analysis of E-field strength is also presented.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 simulated 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.
Finite Element Method based Mathematical Model
The septum conductor when coated with material, such as Teflon, the potential and E-field strength 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. An observation plane is created at z=240mm from the input port. The simulated results on this plane are discussed in the following section.
The GTEM cell is characterized based on VSWR measurement along with the simulation and measurement results of E-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.
The E-field strength is measured inside the GTEM cell for fed power ranging from P =-10 to 10 dBm in the test area where h=98mm and distance from the input port is 240mm. For a 50Ω system, the measured values of the E-field strength are compared with the theoretical value calculated using (1), taking M=3.24 and F=2. The results are also compared with the measurement carried out using an isotropic E-field probe, R&S TSEMF-B2, shown in Fig. 3. Fig. 4 shows the simulated results of HFSS in form of contour plot for E-field strength at a plane on z=240mm. It is found that instead of a copper septum, if the sheet of copper is backed by Teflon, then larger area of uniform electric field can be generated. The simulations agree well with experimental measurements of E-field strength envisage a highly uniform field distribution that closely approximates a TEM wave. The uniform distribution also justifies the recommendation of one-third area given in [7]. The results are also in agreement with analytical values in (2) and (3).
Simulation results of E-field strength at z=240mm, h=98mm, 0dBm power for (a) f=914MHz and (b) f=2.10GHz. The white boundary shows the measured results in that area. Teflon backed conductor is used as septum for suppressing the higher order modes required to increase the area of uniform electric field. The uncertainty evaluation is carried out considered all the parameters as given in IEEE standard 1309-2013 [8] and CCEM.RF-K20 [9]. The computed expanded uncertainty for the reported measurement is ±0.80V/m.
A miniature GTEM cell is fabricated having length of 450mm. The variation in E-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 commercial isotropic E-field probe. A uniform E-field strength is found with maximum strength of Emax=2.94V/m against the analytical value of E=2.86V/m at center of the GTEM cell for a fed power of +10dBm. 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 in the calibrated GTEM cell ensures effective studies of exposure.

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