Measurement jig using semi-rigid cable for differential impedance measurement by S-parameters

Recently, the S-parameter method using a vector network analyzer and a measurement jig instead of a balun was proposed whereby the differential input impedance of a balanced-fed antenna were obtained. As no balun is used, measuring impedance over a wide frequency bandwidth is now possible. To improve measurement accuracy of the S-parameter method, the dependence on thickness of the semi-rigid cable used for the measurement jig is obtained from the results of calculations using the FDTD method. Two types of jigs each made from semi-rigid cables of differing thickness were fabricated, and the input impedance of the dipole antenna calculated using the S-parameter method. The impedances were compared with those obtained from the available theories by King and calculation results of delta-gap-fed dipole antennas. Results from the S-parameter method agree with the available theories by King and the calculated results of the conventional model of a dipole antenna, over a range of about 6 GHz or less.