Coaxial Low-Pass Filter Design and Manufacture

In this study, a low-pass filter with coaxial transmission lines is designed and produced. The objective for designing a coaxial low-pass filter was to utilize both low-loss transmission and high breakdown voltage capability for high power microwave applications to enable the suppression of the second harmonic of the 4.4-6 GHz C-Band (C-Band was selected as a proof of concept) signals with minimal insertion loss. Microwave and 3D electromagnetic simulators were used in the filter design. The design was started by establishing an 11 th order Chebyshev low-pass filter prototype giving 0.05 dB ripple. Based on this prototype, a filter circuit consisting of capacitors and inductors as lumped elements was designed. The low and high impedances to be selected for the stepped impedance method, which is the basis of the transition to coaxial transmission lines, were decided and the most suitable impedances for manufacturing was determined. The filter using these impedances and designed with stepped impedance lines was transformed into the filter in which coaxial transmission lines are used. Coaxial transmission lines in the low-pass filter circuit consist of conductors with an insulating material in between. While the insulating material is selected as air in the high impedance sections of the transmission lines, it is PTFE (Polytetrafluoroethylene) with an insulating constant of 2.1 in the low impedance sections. The designed filter was manufactured. The basic properties of the manufactured filter were characterized by using RF measuring devices and compared with the designed filter. The measurements results proved that the frequency responses of the designed filter and the produced filter were very close to each other. Namely, a low-pass filter with a cutoff frequency of 6 GHz, a sharp suppression of about 45 dB at the stopband of 9 GHz and a return loss of 20 dB was designed and produced by using coaxial transmission lines. The novelty in our work can be stated that the power handling capability is increased by two techniques. The first one is that the highest transmission line radius preventing the higher order mode excitations is used to decrease the conductor loss. Teflon is covered around the low impedance lines to enhance the dielectric breakdown capability. Teflon inclusion is also used to decrease the practical low impedance level that improves the filter characteristics.