Compact microstrip feed networks for low-impedance quadrifillar helix antennas

Abstract

Quadrifilar helix antennas (QHA) are widely used in various applications. QHA performance strongly depends on its feed network parameters. A proper network should equally divide power from input port between output ports and provide quadrature phases (0, 90, 180 and 270°) within antenna frequency band. Two microstrip feed networks are presented in the report. Both networks are based on Wilkinson power splitter and have similar shape — a 39 mm diameter circle. First one is designed for a small QHA with a low input impedance (12Ohm). It consists of planar two-port impedance-transforming Wilkinson splitters. Each splitter reduces output impedance by a factor of 2. Three splitters are cascaded to form a 4-port network. Proper phasing is achieved by means of microstrip delay lines. To equalize output signal amplitudes the splitters have been made non-symmetrical. Bandwidth of the network is much higher than required for a narrow-band (2–3%) QHA. Insertion loss is 1 dB higher than that of ideal network. Relatively high return loss (− 16 dB) results from interference with second stage dividers. Another drawback is high reflection from via holes which inevitably act as π-type filters. The second feed network is designed for a larger QHA with input impedance 28 Ohms. It is an impedance-transforming 4-port Wilkinson divider. Like in the first design, microstrip delay lines provide proper phasing. Network bandwidth and transmission losses are similar to the first one, but has the advantage of lower return losses due to absence of interference with second stage splitter. Simpler design allowed placing almost everything in the top plane of PCB except for splitter resistors. In a matched case, no current flows through these resistors, so the negative effects of via holes are negligible. Reducing complexity and demands for impedance transformation improved performance of this network compared to the first one. Return loss is lower than −25 dB within antenna operation range. Insertion loss is 0.7 dB higher than in case of ideal network and slightly lower than return loss of the first network.