The loop antenna as a probe

The properties of circular and square loop antennas as probes for measuring the magnetic field are investigated. The response of an electrically small loop to the magnetic and the electric components of a given electromagnetic field is determined theoretically for singly- and doubly-loaded loops in terms of suitably defined constants. A system error ratio is defined as a measure of the ability of a given probe to discriminate against the effects of the electric field. An experimental procedure for measuring loop sensitivities in the elliptically polarized near field of a quarter-wave monopole is described. Experimental and theoretical results are compared. It is concluded that very large errors are possible when a singly-loaded loop is used to measure magnetic fields unless its diameter is less than
0.01\lambda
. The doubly-loaded probe may be used with comparable accuracy when its diameter is as large as
0.15\lambda
.