solid line with crosses. Boxes indicate the result with two-port Ε½ . measurements obtained by Eq. 9 . Diamonds denote the slotted line data.
Agreement between two-port results and circuit simulator results is excellent. The difficulty of slotted line measurements is visible in the rough curvature of the path of the measurement results on the Smith chart. The agreement between the slotted line and two-port method results is fair: the track of the results on the chart is very similar, but the start and end points do not coincide. However, the slotted line measurement supports the validity of the other methods.
Interestingly, also, one-port measurements show some agreement with the two-port and circuit simulator results. This is due to the fact that the diagonal entries dominate in the 4 = 4 S-parameter matrix of this QFH antenna. However, this phenomenon should not be generalized to different antenna configurations.
Conclusions
Different methods for measuring the input impedance of a quadrifilar helix antenna were presented. Shortcomings of the known method, the slotted line measurements, were discussed, and new and efficient analytical equations for the input impedance were derived.
For a normally excited symmetric QFH, knowledge of S-parameters at only two ports is required. Hence, very fast and efficient input impedance determinations become possible with normal network analyzers. The utilization of circuit simulators for the input impedance determination was also reviewed for cases where the impedances of the excitations do not coincide. With the presented schemes, the development of QFH antennas in practical applications will be enhanced considerably.
Results obtained with different methods were presented, showing fair agreement between slotted line results and twoport or circuit simulator results. However, the slotted line results corroborated the new impedance derivations. Agreement between circuit simulator results and two-port measurements was excellent.