A Broadband Balun with Tunable Phase-Shifting Function for Low-Cost Phased Array

A broadband balun with tunable phase-shifting function is presented in this article. The proposed structure is based on a typical tapered balun loaded with several extra varactors. The proposed balun operates similar to a loaded line (LL) phase shifter, in which the varactors are added between the microstrip line and ground plane. It can provide balanced feed for broadband antennas and, at the same time, change the phase of the balanced feed in a cost-efficient way. It is found that range of the tunable phase shift relies on the number and variable capacitance of the varactors. To simplify the design procedure and explore the optimum performance, characteristics of a uniform microstrip line loaded with lumped capacitors are studied. It is found that gap between the capacitors and length of the microstrip line has relatively large effects on the impedance matching characteristics. According to this observation, the proposed balun is designed by applying the LL structure to a tapered balun. Within the frequency range of 0.8-2 GHz, the proposed balun loaded with two varactors achieves a tunable phase shift ranging from 17° to 61°. If the number of varactors increases to four, the range of tunable phase shift is from 41° to 161°. Finally, potential usage of the new structure for low-cost phased array is briefly discussed.