Design and Rapid Optimization Procedure of a Compact Broadband Microstrip Diplexer for VHF/UHF Band

In this paper, a design procedure of a compact broadband microstrip diplexer working on the Very High Frequency (VHF)/Ultrahigh Frequency (UHF) band is proposed. The diplexer consists of a fourth-order Butterworth Low-Pass Filter (LPF) and a sixth-order elliptic High-Pass Filter (HPF). The two filters are located on two substrates with a common ground. A microstrip-to-coaxial structure is utilized to transmit the power flow between the two substrates. For miniaturization, the utilized ideal capacitors and inductors are realized by the commercial lumped-element capacitors and the curved microstrip line sections, respectively. All the middle design and rapid optimization processes from the schematic simulation by Advanced Design System (ADS) to the circuit simulation by CST STUDIO SUITE (CST) are given in the article. A prototype is fabricated and measured. The testing results are consistent with the simulation. The diplexer can operate at 225–775 and 880–2500 MHz. The insertion losses are 0.8 ± 0.3 and 0.9 ± 0.4 dB in the two working bandwidths. The isolation bandwidth of −20 dB level is as small as 105 MHz. The roll-off is also sharp that the related bandwidths corresponding to the attenuation level −3 to −20d B are 90 and 30 MHz to the high and low bands, respectively. Besides, the return loss and isolation are larger than 10 and 20 dB approximately. The diplexer has a volume of 0.065 λ_g × 0.156 λ_g @775 MHz, which can be utilized to feed the miniaturized broadband VHF/UHF antenna system.