Design and Prototyping of New Flexible Stripline based Transmission Lines as Alternatives to Semi-Rigid Coaxial Cables

We present the design, assembly, and prototyping of new multi-channel flexible printed circuit board stripline based transmission lines for transmitting RF signals. Stirpline transmission lines have been used for decades and consist of a narrow center conductor that is surrounded by a dielectric substrate and then sandwitched between two grounding planes. Incorporating several striplines in the same substrate using flexible materials may be used as alternatives to industry standard stainless steel semi-rigid coaxial cables which are frequently used as RF transmission lines in a variety of devices. This is often the case for astronomy instruments that have multiple pixels working together to form a focal plane array. Larger arrays require many coax transmission line cables which can increase the footprint of a device, as well as add to the total heat load of an instrument. We have developed a single flexible circuit ribbon that can replace up to 8 individual coaxial cables. The flex circuits were designed in CST Studio Suite where transmission characteristics were simulated and subsequently optimized by adjusting the circuit dimensions. The flexible circuits make use of a novel design wherein the top and bottom stripline grounding layers are reduced to 0.015” wide strips rather than a full grounding plane. This minimization of copper grounding material increases flexibility, while decreasing heat load, all without introducing significant loss of signal. The circuits will be used in an upcoming NASA Class-D Balloon mission: GUSTO[1]. Specifically for GUSTO the circuits will need to transmit over an IF bandwidth from 0.3 - 5 GHz while one end of the ribbon is held at 20 K, and the other end is at 300 K. In this temperature configuration, a 20” length of flex circuit has a loss of 5.77 dB at 5.00 GHz which is roughly the equivalent of a stainless steel coaxial cable with an outer diameter of 0.085”. In this same configuration the flex circuit loss is 9.67 dB at 10 GHz. Due to its reduced grounding layer the flex ribbon generates 75% less heat than 0.085” diameter coax. The flexible ribbon is also made of durable Kapton material which makes the circuit considerably more malleable than semi rigid coax. The full bandwidth of the flex ribbon is 0 - 10 GHz. The flex circuits have been tested at cryogenic temperatures for multiple cool-down and warmup cycles without any signs of degradation. They are capable of delivering the comparable transmission characteristics as coaxial cables while offering improvements on versatility, and thermal conductivity.