Reconfigurable 1.2–3.16-GHz quad-channel diplexer with compact size, constant absolute bandwidth, and high isolation

This paper presents a varactor-tuned diplexer with four reconfigurable channel frequencies. To reduce size and enable flexible control of the two passbands at the same output, novel dual-mode resonators are proposed where the even- and odd-mode resonances are, respectively, applied to the higher and lower passbands and can be easily excited at different resonator locations. Moreover, the distributed coupling feed lines are used to generate transmission zeros in the passbands of the other output, leading to high isolation. Experimental results show that the four channels of the fabricated diplexer can be tuned from 1.2 to 1.47 GHz, 1.56 to 1.9 GHz, 2 to 2.45 GHz, and 2.55 to 3.16 GHz with constant absolute bandwidth, greater than 14.4 dB in return loss, and greater than 30 dB in isolation. The diplexer occupies a compact footprint of 0.13 λ
₀
× 0.17 λ
₀
, where λ
₀
is the free-space wavelength at 1.2 GHz.