Standard field strength meter

Abstract

Abstract A hollow conductive sphere having a narrow insulated gap along an equator is suspended in a radio frequency electromagnetic field with the plane of the gap perpendicular to the electric field vector. The sphere halves are electrically connected together by a series resonant LC circuit disposed in the sphere. The resonant circuit is tuned to the frequency of the field. The field impinging on the sphere causes a current to flow across the equator of the sphere, via the resonant circuit. The field strength can accurately be related to the resonant current by an analysis of the scattering of the field by the sphere. To measure the current, the sphere contains a circuit for detecting the voltage developed across the inductance of the LC circuit and for energizing a miniature incandescent lamp when the detected voltage exceeds a threshold value. The light from the lamp is visible through a small diameter tube disposed through the sphere wall (the tube is a waveguide below cutoff for the impinging field). The sphere thus provides an optical indication when the field exceeds a threshold or standard field value. The sphere is calibrated by adjusting the threshold circuit to cause the lamp to light when the appropriate radio frequency current is passed through the resonant circuit. The calibration current is applied to the sphere by means of two hollow conductors of the same diameter as the sphere which are connected to the two sphere halves near the equator containing the insulated gap. The two hollow conductors form the inner conductor of a coaxial line. The calibration current applied to the coaxial line thus flows across the gap just as the induced current does when the sphere is in the field being tested.

Inventors

• 👤 Robert A Lawton
• 👤 Charles Mckay Allred

Bibliographic Details