Journal of Research of the National Bureau of Standards
An improved formula is derived for accurately computing the near-zone magnetic field of a small circular transmitting loop antenna. Such a field can serve as a reference standard for calibrating field-strength meters employing small receiving loop antennas in the frequency range 30 Hz to 30 MHz. This formula includes correction terms for frequency (due to the finite time of propagation), as well as corrections for the finite radii of both the transmitting and receiving loops. Other formulas appearing in the literature often fail to include such corrections which can result in errors of up to 20 percent and more in computing standard-field values. The NBS formula is derived by expanding the integrand of the retarded vector potential into an infinite series of spherical Hankel functions of increasing order. The resulting series expression is in error by less than 0.2 percent, is rapidly converging and simple to use without recourse to a table of functions or a computer.
J. Res. Nat. Bur. Stand. Sec. C: Eng. Inst., Vol. 71C, No. 4, p. 319
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