embodiments are secured to the rf feed through fittings and embedded in the filter element in the same manner as described hereinafter transmitting antenna 20 and a receiving antenna 22 extend through the wall of the vessel through the rf feed through fittings 180 and penetrate the filter medium each antenna includes a straight conductor portion 182 which passes through its associated fitting and a pair of laterally spaced arms 184 and 186 connected together by a semi circular portion 188 each arm is generally l shaped with a radial or transverse portion 190 and an axial portion 192 as shown in the drawings the axial portion penetrates the filter medium for illustration purposes for a filter containment vessel with a length and diameter of 30 5 cm the lateral spacing between antenna arms may be 40 mm the radial portion of the arms may be 90 mm the axial length of arm penetration into filter may be 120 mm the offset of the semi circular portion from filter containment wall may be 20 mm and the radius of curvature of semi circular portion may be 20 mm it will be obvious to those skilled in the an that there are variations on the above which might be better or equally good insofar as antenna performance is concerned it will also be understood that while two arms have been shown improved performance might be achieved by the addition of a third arm to the antenna by adding to the number and or modifying the position and or length of the various metallic antenna elements it should be understood that both the size and or the shape of the filter volume sampled by the antenna system may be modified to suit various measurement requirements fig8 plots the attenuation through the.