Ultra-wideband planar LPDA antenna with mode converter balun

A new balun used for feeding a planar log-periodic dipole array antenna (PLPDA) is proposed. This balun, which is based on mode conversion, converts gradually the transmission mode from quasi-TEM (coaxial cable) to full TEM (balanced line), with an accessible excitation current property (ECP), which is not achievable in classical feeding techniques (infinite or self balun). To validate this approach, an ultra-wideband (UWB) PLPDA antenna was designed with these balun structures for operating in the L and S bands (1-4 GHz) and the results were compared in terms of reflection coefficient and radiation pattern. A PLPDA antenna with a mode converter balun was fabricated. Results show that the proposed balun is a low length (10 mm) frequency-independent (4.5 GHz based on |S₁₁| ≤ -10 dB) device useful for feeding the UWB PLPDA antenna. Moreover, antenna´s ECP can be modified, which is necessary for phase centre variation compensation.