Study On The Critical Frequency And The Cause Of Higher Order Modes In The GTEM Cell

Gigahertz Transverse Electromagnetic cell(GTEM cell),as an alternative test site to the open air test sites(OATS)and anechoic chamber,is widely used in the electromagnetic compatibility radiated sensitivity testing and radiated emission testing.However,the abrupt structure or the discontinuity within it can excite higher order modes,limiting its maximum working frequency and usable test volume.The objective of this study is to improve the performance of the GTEM cell by theoretical analysis in combination with the experimentation and simulation.On one hand,the effects of the hybrid termination parameters on the reflection coefficient of the critical frequency are investigated based on a 2.5 meter long GTEM cell.The structures of the GTEM cell are also optimized.On the other hand,the excitation sources of the higher order modes in the GTEM cell are explored,which may provide an important guidance for the manufacture and application of GTEM cell.A set of coaxial air line is designed according to the principle of the transmission/reflection method.The bakelite,teflon,and polymethyl methacrylate material are tested by this fixture and three methods to extract the permittivity of non-magnetic materials are compared.It is found that the permittivity calculated from the characteristic impedance method and the NRW method would be divergent on the corresponding frequency when the sample length is integral multiple times of the half-wavelength of the electromagnetic wave in the sample,while the result obtained from the propagation constant method is not affected by the sample thickness resonance.The origin of the multiple solutions for the propagation constant is investigated.It is shown that the imaginary part of the obtained propagation constant needs to be corrected when the sample length is odd multiple times of the half-wavelength of the electromagnetic wave in the sample.The corresponding frequency for this correction is also given.The permittivity of the pyramid foam absorbers commonly used in the GTEM cell is measured,and found that it is not uniform even for the same batch of samples.A simulation method based on the finite integral technique for the reflectivity of the absorbers is proposed,which is used to investigate the reflectivity of the electromagnetic compatibility absorbing material,such as ferrite tiles and pyramid foam absorbers.The results show that the reflectivity difference for the no-hole ferrite tiles between the simulation and the provided reference data is only 15 dB around the extreme requeney.For the ferrite tiles with a hole in the center,the reflectivity from the simulation is superior to the experiment result.But for the foams,the simulation result match the experiment result very well between 100 and 350 MHz,and the maximum difference from 350 to 600 MHz is only 10 dB.The effects of the size of the lumped absorber elements and the pyramid foam absorbers on the reflection coefficient of the critical frequency are investigated from the 3D simulation of the GTEM cell.The results show that either increasing the column or reducing the raw of the lumped absorber elements can reduce the reflection coefficient,and that the effects of the base height on reducing the reflection coefficient are more effective as compared to that of the pyramid height.One-order Debye's model for permittivity is used to investigate the effects of the electromagnetic parameters on the reflection coefficient.It is found that the reflection coefficient decreases with increasing zero-frequency permittivity.While increasing the relaxation time constant can increase the reflection coefficient of the high frequency.When measuring the permittivity of the pyramid foam absorbers,a revealed problem about how the uniformity of the electromagnetic parameters influences the reflection coefficient of the GTEM cell,is also investigated by both simulation and experimentation.It is found that,for the same batch of pyramid foam absorbers,mounting the foams with high permittivity around the septum is more effective to lower the reflection coefficient of the critical frequency.For a single foam,increasing the permittivity of the pyramid is superior to increasing that of the base to lower the reflection coefficient.The effects of the hybrid termination parameters on the critical frequency are investigated,which is used to optimize the parameters of the composite terminal.It is found that the reflection coefficient can reach its minimum when the total height of the foams is equal to the distance between the front-end of the lumped absorber elements and the back plate of the GTEM cell.It is also found that increasing the permittivity is not always effective on reducing the reflection coefficient.There exists an optimized range for the zero-frequency permittivity to satisfy the requirements of both the reflection coefficient and the impedance matching.The transverse and longitudinal components of the reference points along the longitudinal direction in the unloaded GTEM cell are calculated from simulation method.The results show that the decoupling is better on the reference point far from the hybrid termination,which indicates that the lumped absorber elements are the excitation source for the TM mode.By comparing the field strength of reference points within usable testing volume before and after loading the equipment under test(EUT),it is found that the variation of the normalized vertical electric field components is within ±3 dB for the unloaded GTEM cell,while the corresponding value is beyond ± 3 dB after loading the EUT.It is also found that the longitudinal magnetic field components of reference points increase.remarkably around the cutoff frequency of the TE10 mode after loading the EUT,indicating that the EUT is one important excitation source for TE modes.Further calculation demonstrates that the EUT introduce significant distortion to the electric field distribution in the usable test volume.The 5.5 meter long GTEM cell is tested experimentally.It is shown that the characteristic impedance and the VSWR satisfy the application requirements.By calibrating the field uniformity of the GTEM cell with IEC 61000-4-20,it is found that the electric field strength of the primary field component satisfy the uniformity of 0-6 dB;while the ratio of the secondary electric field components to the primary component does not satisfy the corresponding requirement.