A new microstrip tri-mode balun bandpass filter (BPF) with good filtertype and balun-type functions is presented. To realise the tri-mode BPF performance, the multimode stepped-impedance resonator is introduced and parallel coupled to both the balanced and the unbalanced ports. To achieve 180°phase difference, two types of transition structures, i.e. the microstrip-coplanar waveguide-microstrip transition and the microstrip-coplanar stripline-microstrip transition, are adopted for the two balanced output ports design, respectively. A balun BPF operating at 2.6 GHz is designed and fabricated to validate the feasibility of the new approach. Results indicate that the proposed balun BPF exhibits not only high selectivity with two transmission zeros locating at 1.7 and 3.7 GHz but also good balance performance with 0.5 dB magnitude imbalance and 5°phase imbalance between the balanced outputs.Introduction: The ever increasing demand for compact and low-cost wireless communication systems leads to the requirement for high the integration of microwave circuits especially for the passive components. The development of a balun bandpass filter (BPF) that can properly integrate both the functions of balun and the BPF turns out to be a necessary way to reduce the size and cost of systems. The balun BPF, in other words, is a multi-function embedded passive component, as it can not only convert an input unbalanced single-ended signal into its balanced differential one but also make the desired signals to be transmitted smoothly over a certain frequency band while blocking other undesired signals. Over the past few years, much effort has been focused on this area [1][2][3][4][5].To reduce circuit dimensions, a balun BPF using the low-temperature co-fired ceramic technology was proposed in [1], yet the multilayer process increases the complexity and the cost of the circuit. By combining the topology of the coupled-line and the stepped-impedance resonator (SIR), a good performance microstrip balun filter was introduced in [2]. On the other side, microstrip dual-mode patch resonators have been receiving much attention to design a balun BPF with dual-mode response [3,4]. Moreover, to further reduce circuit size and the improve harmonic suppression performance of the dual-mode balun filter, a dual-mode patch resonator with capacitive loading was proposed in [5]. The dual-mode balun filters mentioned above are all designed based on the loop-type resonator. However, to the best of our knowledge, there is no report on the design of a microstrip multimode balun BPF by using the widely used open-type microstrip multimode resonator.The primary motivation of this Letter is to design a new wideband tri-mode balun BPF with an open-type microstrip multimode resonator. By utilising the first three modes of the multimode SIR resonator and the electric filed distribution characteristic of the transition structures, a new design method of a microstrip tri-mode planar balun BPF is presented. For demonstration, a tri-mode balun BPF is designed, fabr...

A new ultra-wideband balun and its application for balanced antenna are presented in this paper. To realize the balun-type function, two different types of microstrip to slotline transition forms are adopted for the unbalanced and balanced outputs of the balun. Further, an ultra-wideband balun and its integration with a Vivaldi antenna are designed and fabricated to validate the feasibility of the new approach. Results indicated that the proposed balun and the antenna can operate from 3.6 to 10.7 GHz. For the proposed new balun, it exhibits a good balanced performance of within 0.5 dB magnitude imbalance and less than 6°phase imbalance between the two balanced outputs. Meanwhile, the antenna reveals good unidirectional radiation patterns.