With the rapid development of technologies such as 5G, the Internet of Things, unmanned systems, and artificial intelligence, as well as the construction and application of major infrastructure and advanced intelligent equipment and systems, electromagnetic environment adaptability and electromagnetic safety have become crucial factors for the reliable and secure operation of various intelligent equipment systems. In most cases, the installation of lumped filters requires significant space and weight, limiting their application. Optical fibers offer low-loss transmission for useful signals, while filter cables provide high suppression of interference signals while ensuring useful signal transmission. Filter cables based on defected conductor transmission structures, which can efficiently achieve distributed filtering functions, are expected to offer a new solution for complex electromagnetic environment effects and electromagnetic compatibility protection.
The evolution trajectory of filter cables can be traced back to the initial photonic bandgap (PBG) structures. The PBG structures evolved into traditional microstrip filters and defected ground structure (DGS) filters, which then transitioned to planar filtering transmission lines, developed into filtering cables utilizing defected conductor layers (DCL), and eventually transformed into filter cables with defected conductor transmission structures (DCTS) to address the radiation issues caused by DCL.
In our work, we initially etched a single six-slot-ring structure onto the inner conductor of the cable. Simulation experiments revealed that it exhibited low-pass filtering characteristics. By adjusting several parameters of the six-slot-ring structure, we were able to control the frequency point shift and correspondingly modify its stopband characteristics. By studying the current distribution and the equivalent circuit of the six-slot-ring structure, we found that the etched six-slot-ring structure on the inner conductor of the cable actually forms a complex low-pass filter network under excitation conditions, manifesting as a low-pass filter. Adjusting the structural parameters of the six-slot-ring altered the current distribution on the six-slot-ring structure, thereby affecting the parameters of the low-pass filter network and changing its filtering characteristics. In subsequent research, we cascaded multiple six-slot-ring structures and arranged them in a gradient symmetrical manner. The results showed that this cascading approach effectively connected multiple low-pass filter networks with different parameters in series, achieving excellent low-pass filtering performance. Consequently, by cascading 40 six-slot-ring defect structures (as shown in Figure 1), we achieved a passband below 6 GHz, with attenuation less than 0.38 dB within the passband, a stopband width of up to 17.3 GHz, and stopband attenuation exceeding 23 dB. The physical tests and simulation results were in excellent agreement (as shown in Figure 2).
Figure 1. The design of the suggested low-pass filter cable involves a defected conductor transmission structure (DCTS) which comprises of 40 square rings with varying lengths cascaded together, along with six gaps. a) The arrangement of the characteristic filter cable, illustrating the sequence of layers from the outermost to the innermost: shielding layer, transmission dielectric, DCTS, and inner cylinder dielectric.  b) 3D representation of the DCTS in the proposed filter cable, illustrating 40 cascaded six-slot-ring structures. c) DCTS with 40 cascaded structures consisting of six-slot-ring. d) Parameter diagram depicting two individual six-slot-ring structures within the DCTS.
Figure 2. S-parameters of the proposed low-pass filter cable with 40 six-slot-ring defected structures on the DCTS.
For more details, please refer to our recent paper published in Communications Engineering: Filter cable design with defected conductor transmission structures.