Analysis and Calibration of Loop Probes for use in Measuring Interference Fields
NAVAL RESEARCH LAB WASHINGTON DC
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A small shielded loop antenna, to be used as a probe for indicating and measuring radiofrequency interference fields from electronic equipment, has been analyzed. The input impedance is similar to an equivalent shorted two wire, balanced, transmission line. Voltage induced by a field conforms, over the usable frequency range, to that calculated by equations applicable at low frequencies, but the output voltage varies because of the transmission line effect. A number of loop probe designs have been analyzed and found to have undesirable characteristic impedance discontinuities which make calculation of their characteristics almost impossible. A method of calibration, which uses another small shielded loop to establish a radiofrequency field of known characteristics, has been developed, and its accuracy and reliability are proved experimentally. A simplified loop probe design has been found to be the most satisfactory. Loop probes of the approximate dimensions of those analyzed have been shown to be usable for the desired application at frequencies below approximately 400 Mc only, both because of unsatisfactory response characteristics and because of difficulties of calibration at higher frequencies.
- Electricity and Magnetism
- Electrical and Electronic Equipment
- Test Facilities, Equipment and Methods