Gain Anomaly Mitigation in Coaxial-Fed Log- Periodic Dipole Array Using Wideband Φ-Shaped Balun

Log-periodic dipole array (LPDA) is always plagued by the gain anomaly together with pattern deflection caused by unbalanced coaxial feeding-an issue without a universal solution. In this letter, a novel Phi-shaped balun is proposed to address the issue. Both the current and electric field have provided the physical mechanism of the Phi-shaped balun in the elimination of the gain anomaly. The Phi-shaped balun not only performs the unbalanced to balanced conversion but also performs a wideband impedance transformation. Moreover, owing to the compact planar configuration, the proposed Phi-shaped balun is easy to integrate into the RF circuit without compromising the antenna performance. Finally, the proposed prototype is fabricated and measured for demonstration. The measured results demonstrate that Phi-shaped balun eliminates the unbalanced current caused by the coaxial feeding, thus the LPDA exhibits stable radiation performance with an average gain of 7 dBi, without pattern deflection and gain anomaly within the operating frequency range of 0.9-2 GHz.