New Coaxial RF-DC Ammeter

A unique new type of RF ammeter is described. The ammeter combines broad frequency coverage with broad current range and is designed for use in a 50-ohm coaxial transmission line. The ammeter consists of an elliptic-cylinder silver reflector of infrared energy with a cylindrical thick-film heater along one focus and a heat-sensing thermopile along the other focus. Advantages over older ammeter designs include a shielded construction with inherent electrical isolation between the RF and dc circuits, a large dc output (approximately 1-100 mV) corresponding to a decade dynamic range (0.25-2.5 amperes), and a frequency range from dc to about 1 GHz. This coaxial ammeter is believed to be the first to have very low standing-wave ratio (SWR) (1.03) in a 50-ohm transmission line. Ammeters of lower current range can be built, but at a penalty of increased SWR due to the larger heater resistance required.