Abstract: An apparatus for detecting the accumulation of particulate material on a filter medium formed of dielectric material and disposed in a chamber is provided. The apparatus is operable for generating and transmitting an RF signal through the filter medium and for monitoring the transmission loss of the signal through at least a portion of the filter medium so as to provide an indication of the content of particulate material accumulated on the filter medium. The apparatus includes an input antenna for transmitting the signal and having at least one antenna element extending longitudinally of the chamber and disposed within the filter medium; and an output antenna for receiving the signal transmitted by the input antenna and having at least one antenna element extending longitudinally of the chamber and disposed within the medium in parallel, spaced apart, axially overlapping relation with respect to the input antenna element.