2025, Vol. 7, Issue 2, Part A

This paper presents a new realization of reflectionless and semi-reflectionless diplexer design aimed at signal reflections suppression as well as enhancement the performance of RF front-end systems. Design a traditional diplexer with two second-order BPFs (band-pass filter), which are resonant at different frequencies. This base design was compact and efficient, using half-wavelength structure realized by inward-coupled open-loop resonators as the BPFs.
An Absorptive Stub was added to improve impedance matching of the diplexer and bottom plateinkle loss at the input port. At the junction of the two filters, we connected a stub with Step Impedance Resonator (SIR) structure loaded with a resistor (R1), called Filter3. It served, as its name implies, to take up those direct signals that would otherwise reflect back to the source and make the diplexer work fully reflectionless. The prototype was printed using a low-cost FR4 dielectric material with an ?r value of 4.6 and the thickness of the substrate was set as 1.5 mm. The size of the fabricated prototype was about 50.1 mm × 41.2 mm with SMA connectors connected to all ports. Excellent agreement with simulated results was observed in the case of measurements performed using an N9917A network analyzer. In more detail, over a frequency interval from 1.45 GHz to 2.8 GHz, the input matching bandwidth of the diplexer was wideband (S11 < 0 dB) with insertion losses of -5dB and ?6dB at frequencies centered at 1.72GHz and 2.0 GHz respectively. Moreover, an equivalent qusi-reflectionless modified semi-a period of reflection for the absorptive stub is replaced by band stop filter-shaped resonator (BSF) loaded with resistor R2. This version achieved some reflection suppression but still could not reach the full broadband reflectionless performance of the original design. The results encourage the benefit of providing absorptive structures in order to realize efficient and reflectionless diplexers for contemporary RF applications.