Attenuation of slablines with cylindrical centre conductors

One form of construction of interdigital or comb filters employs a slabline configuration in which the inner conductor is of cylindrical cross-section. The internal impedance level of these filters is usually taken to be 75 ¿, since this is the impedance for which a conventional coaxial line of fixed outer diameter has the lowest attenuation coefficient. The paper investigates the suitability of this choice in the configuration by calculating the attenuation coefficient of this type of line. It is found that a minimum attenuation coefficient is obtained for an impedance of about 60 ¿ which is about 55% of that of slablines using rectangular inner conductors at the 75 ¿ level.