Simply-Fed Four-Arm Spiral-Helix Antenna

An ultra-wideband four-arm spiral-helix antenna with simple beamformer is developed to have simultaneously good performance in time and frequency. Two microstrip feeds printed on the opposite side of the spiral with impedance following Klopfenstein taper are employed to eliminate the need for 180 ^° hybrids used in conventional beamformers. Ferrite beads placed around the coaxial cables connected between the ground and microstrip are utilized to choke unbalanced currents and prevent shorting the spiral aperture to the ground. Helix is used to improve the low-frequency performance and allow easy attachment of resistive loading to the ground plane. A metallic inset is placed inside the cavity to eliminate the destructive pattern interference and improve the high-frequency gain. To achieve low dispersion, the spiral is loosely wrapped. Simultaneously good time and frequency domain performance from 0.5 GHz to 3 GHz with VSWR<;2, nominal gain and radiation efficiency of 6 dBic and 80%, respectively, broadside axial ratio below 1.8 dB and fidelity factor of 93% over wide field of view are demonstrated in simulations and experiments.