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 Φ-shaped balun is proposed to address the issue. Both the current and electric field have provided the physical mechanism of the Φ-shaped balun in the elimination of the gain anomaly. The Φ-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 Φ-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 Φ-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.