Sheath wave barrier

#1. Field of the Invention

#The invention is directed to a sheath wave barrier (also sometimes referred to as a sheath wave trap), preferably for employment in a feeder of a local antenna of a magnetic resonance apparatus having a winding composed ,of a coaxial line.

#2. Description of the Prior Art

#The single FIGURE is a side view, partly in section, of a sheath wave barrier constructed in accordance with the principles of the present invention.

#In a partial sectional view, FIG. 1 shows a side view of the sheath wave barrier of the invention. It is especially adapted for insertion into a feeder 2, that is unbalanced to ground, of a local antenna (not shown), that is balanced to ground, in a diagnostic magnetic resonance apparatus having an operating frequency of approximately 60 MHz. The feeder 2 is composed of an insulated coaxial line having a diameter of approximately 1 mm that merges into a winding 4. The winding 4 is implemented in the form of a spiral (when viewed from above or below) in two layers or plies 6 that each have nine turns around an axis. The winding direction is the same in both layers or plies 6. The two layers or plies 6 of the winding 4 are separated from one another by a thin, round insulating grommet 8 so that the turns are respectively fixed in each layer or ply. The connection from the first layer or ply 6 to the second layer or ply 6 ensues at the inner, smallest turn 9 of the winding 4.

#Respective, circular printed circuit boards 10 immediately adjoin the turns of the winding 4 in the axial direction at both the top and bottom of the winding 4. The printed circuit boards 10 are laminated on one side with a copper layer 12, both of these layers 12 facing away from the winding 4 and facing outwardly. The thickness of the printed circuit boards 10 essentially defines the axial spacing a at which the copper layer 12 is disposed from the winding 4. The spacing a is set such that, first, the shielding effect of the copper layer 12 prevents external fields from being coupled into the winding 4. Second, the spacing a is also not excessively small since the quality of the sheath wave barrier would otherwise decrease greatly because of currents induced in the copper layer 12.

#A central through opening 14 serves the purpose of accepting a plastic screw 16 that, together with a plastic nut 18, serves as a clamp element that compresses and fixes the printed circuit boards 10 and the intervening winding 4, including the insulating grommet 8.