Epsilon-near-zero transmission in quasi-TEM microstrip circuits

In this paper, we show for the first time that epsilon-near-zero (ENZ) transmission is possible in conventional quasi-TEM microstrip circuits without converting the propagating mode into a TE₁₀ mode and without the use of substrate integrated waveguides. Complementary split-ring resonators are used to obtain near-zero permittivity in the stopband of a conventional low pass filter with open-ended stubs. In this way, a new passband is created, characterized by low losses and 3 dB bandwidth of around 9 %, thus demonstrating the potential of this approach for the design of entirely new class of bandpass filters. The propagation in the new passband is analyzed in detail. To demonstrate the validity of the approach, ENZ microstrip structure is fabricated and measured.