A T-resonator technique for aperture plane admittance parameters of electrically short RF monopoles

[摘要] This thesis focuses on the coaxial-microstrip T-resonator measurement technique todetermine the aperture plane capacitance and radiation resistance of electrically shortmonopoles above a finite ground plane at Radio Frequencies. The determination ofthese impedance parameters is of interest as they are needed in the Théveninequivalent circuit of the monopole to establish the relationship between the monopole,the electromagnetic field within which it is immersed, and its surrounding mediumproperties. Electromagnetically short monopoles are used in various applications suchas in permittivity and biomedical measurement techniques and near-field metrology.The coaxial-microstrip device allows the measurement of transmission coefficientnulls when connected to the network analyser. By measuring the frequency shift intransmission null points and analysing the results, the capacitance of short monopolesof various lengths can be obtained. The radiation resistance of the monopole can alsobe determined by comparing the depth of transmission coefficient nulls points atvarious resonant frequencies.Monopoles ranging from flush-mounted to various sizes of protruding centreconductors, both bare and insulated, are investigated in the frequencies ranging from45 MHz to 3 GHz. The experimental results compare well with FEKO numericalpredictions at the ranges where this technique is applicable. The technique is howevernot applicable near and beyond the resonant frequency of the monopole where it isconsidered to be electrically long.Measurements to obtain monopole impedance parameters were conducted byconnecting the device to the HP 8510C Network Analyser. To enhance accuracy, theNetwork Analyser was calibrated using a high quality calibration kit with preciselydefined standards. The technique used was found to accurately yield capacitances inthe range of 0.0211 to 1 pF and radiation resistances in the range of 0.9245 to 12.1Ohms. These results were obtained for monopoles of length 0.0062 < h/l < 0.24.Results from literature, viz. W.R Scott, R.W.P. King and L.J. Cooper, show radiationresistance in the ranges: 2.5 to 35 Ohms (0.079 < h/l < 0.22), 1.26 to 74.08 Ohms (0.026 < h/l < 0.28) and 5.59 to 74.98 Ohms (0.1 < 0.28) for electrically shortmonopoles. The results of this work are compared with those in the literature andshow variations of less than 6%.A technique capable of accurately measuring capacitance and radiation resistance ofthe monopole at various lengths of protruding centre conductor has been developed.The capacitance for a flush probe compares very well (agreement is less than 6%)when compared with those obtained by the previous researchers. The technique iscapable of yielding accurate results radio frequencies in the challenging low radiationresistance range around 1 Ohm.

[发布机构] Stellenbosch University