S Parameter Extraction Approach to the Reduction of Dipole

Modern test equipment used for antenna measurements, such as vector network analyzers (VNA), generally provide unbalanced coaxial measurement ports. This presents a problem when performing measurements on devices such as dipole antennas, which present a balanced input. A balun must be used to interface the dipole to the VNA. Since there is no straightforward way to include the balun in the standard VNA calibration procedure, the calibration, and thus the measurement is referenced to the unbalanced input of the balun. However, it is desired to measure the antenna response at its input terminals, which are on the output side of the balun. Therefore, it is necessary to account for the balun response in VNA measurements of dipole antennas. Methods based upon scattering parameter, or S-parameter analysis are discussed here for this purpose. First the reduction of measurements from a single dipole to determine input impedance is considered. Then the reduction of measurements from a pair of dipoles to determine the coupling between them is considered. A typical test setup used to measure the input impedance of a dipole antenna is shown in Figure 1. A VNA is connected to the unbalanced side of a balun. The balun is assumed to have coaxial connectors on all three ports. Short adapter cables are used to connect the ports on the balanced side of the balun to the dipole radiating elements. It is assumed that these adapter cables each consist of a coaxial connector, in order to mate to the balun ports, and a short bare conductor, in order to mate to the dipole elements. It is desired to determine the complex reflection coefficient at the antenna terminals, from which antenna input impedance can be determined. However, the VNA complex reflection coefficient measurement is referenced to the unbalanced side of the balun. In general, if the S-parameters for the cascade of components between the balun input and the antenna terminals are known, it is possible to determine the reflection coefficient at the antenna terminals, Γ ant , from the VNA measured reflection coefficient, Γ meas. A standard three port model for the balun is shown in Figure 2(a) where the balun S-parameters are denoted as S. All ports are assumed to be referenced to a common characteristic impedance of Z 0. Analysis is greatly simplified by converting the S-parameters corresponding to the three port model, S , to those corresponding to an equivalent …