Novel ultra-compact 868 MHz/5GHz lowpass–bandpass diplexer for IoT

In this paper, we present a novel lowpass–bandpass diplexer designed to operate at frequencies of 868 MHz and 5 GHz. This diplexer is designed to overcome the limitations identified in previous studies. Notably, it achieves a maximum group delay of – 0.77 ns in the second passband and 1.37 ns in the first passband. Moreover, this diplexer has an ultra-compact size of just 0.007 λg². This very compact size is obtained because the lowpass and bandpass cells have shared sections, which verifies a novelty of our proposed diplexer. This compactness, along with the operating frequencies, is beneficial in Internet of Things environments. The proposed diplexer can suppress the 1st up to the 6th harmonic. In comparison with the previous lowpass–bandpass diplexer mentioned in this work, it shows the best harmonic suppression. The introduced diplexer is analyzed mathematically to find the behavior of the basic structure. To validate the proposed design, this diplexer has been fabricated and measured. The measurement results confirm the accuracy of simulations and the effectiveness of the design methodology.