Compact Broadband Wilkinson Balun Using Metamaterial Transmission Lines

Recently, various structures such as zeroth order antennas, dual band branch line couplers, compact ring hybrids, broadband baluns, and so on, have been introduced by using metamaterial transmission lines (MTM TLs) of the composite right/left handed (CRLH) or the LC loaded (LCL) lines [1-2]. In general, most of passive circuits in RF/microwave systems are designed by using -90 or -180 phase shifting lines. The phase shifter using MTM TLs introduced in Ref. [3] is applicable for these circuits to achieve the size reduction and/or the bandwidth extension [4-5]. A broadband balun was introduced in Ref. [4]. The balun is comprised a Wilkinson divider, followed by a +90 and -90 MTM phase shifting lines along two output ports, respectively. Although the designed balun has broadband differential output phase bandwidth of 77 %, the size of the balun is increased when the operating frequency is lower than the designed frequency due to two quarter wavelength lines. In this letter, we propose a compact broadband Wilkinson balun using three MTM TLs. The size of the proposed balun is 70 mm  15 mm excluding the port areas and has no effect on the operating frequency.