junction of the v arms 8 signal processing circuits 22 are coupled to the ported coaxial cable 18 by way of a conventional coaxial cable 24 fig2 a and 2b illustrate two alternate ways of disposing the ported cable 18 within the coil 16 these figures are purposely not drawn to scale for the sake of clarity of the individual elements in fig2 a the ported coaxial cable is held at the longitudinal axis of coil 16 by several support rods 26 which are attached to coil 16 while in fig2 b the ported cable 18 rests at the bottom inner surface of coil 16 fig2 a depicts several conventional barbs 25 while fig2 b depicts a form of barbed projection 27 known as barbed tape electromagnetic theory states and practice shows that if the spacing between the individual turns of coil 16 is about a tenth of a wavelength or less they will reflect any rf energy that strikes them and contain this energy within the interior volume of coil 16 in effect the coil acts like a solid metal tube to rf energy in a practical embodiment of the present invention the coil 16 may have a turn spacing of 4.