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I claim:
    1. In an instrument for detecting and measuring directional RF voltage wave signals on a coaxial transmission line, said instrument including a coaxial line section adapted to be inserted in said transmission line and having a tubular conductive metal body with a central axis and a transverse opening, and a coaxial center conductor extending axially through said body and electrically insulated therefrom, an inductive pick-up coil located in said opening and adapted for rotation about an axis normal to the axis of said metal body for sensing and measuring the RF voltage wave on said transmission line, D'Arsonval meter movement means for indicating on a meter scale the magnitude of said voltage wave on said transmission line, circuit means operatively connected between said meter movement means and said inductive pick-up coil for translating said inductive voltage sensed by said pick-up coil into a voltage signal transmitted to said meter movement means and an enclosure for said instrument, the improvement wherein said tubular conductive metal body is of rectangular cross section and is formed of a solid length of conductive metal with an axial bore extending therethrough and wherein said D'Arsonval meter movement means, said circuit means and said enclosure are all mounted on and supported solely by said conductive metal body whereby said line section alone serves as the base and support for said entire instrument independently of said enclosure.