A circuit for processing radio frequency signals that includes an adjustable transmission line stub (104). The adjustable transmission line stub (104) has an input (106) at one end, an electrical length and a termination (112). The circuit also includes a signal return conductor (124) and at least one fluid conduit (114) extending from the transmission line stub (104) to the signal return conductor (124). A fluid control system (150) is provided for selectively moving a conductive fluid (126) from a first position to a second position. The fluid control system is responsive to a control signal (174) for selectively moving the conductive fluid (126) between the first and second position. The fluid control system (150) can include a pump (154, 158, 156) for moving the conductive fluid (126) between the first position and the second position.