Vswr meter circuit using variable rf attenuator

Claim 1

In an instrument for detecting and measuring directional RF voltage wave signals on a coaxial transmission line and including a coaxial line section adapted to be inserted in said transmission line, an inductive pick-up coil mounted for rotation about an axis normal to the axis of said line section for selectively sensing and measuring the magnitude of both the forward and reflected RF voltage standing waves on said transmission line, D'Arsonval meter movement means, and circuit means operatively connected between said meter movement means and said inductive pick-up coil, said circuit means including a rectifier diode and a scale expanding potentiometer for setting the meter pointer to a fixed index point on the meter scale when said inductive pick-up coil is rotated to a position to sense the forward RF voltage wave on said transmission line whereby when said inductive pick-up coil is thereafter rotated approximately 180° to a position to sense the reflected RF voltage wave on said transmission line, said pointer may indicate the voltage standing wave ratio on a respective meter scale, the improvement wherein said circuit means comprises: a variable RF attenuator electrically connected between said diode and said inductive pick-up coil, said attenuator including a series resistor, a shunt resistor and said scale expanding potentiometer connected in series with said shunt resistor, whereby the deflection of said D'Arsonval meter movement means is relatively undistorted throughout the range of induced RF voltage levels sensed by said pick-up coil.