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A near-zone magnetic field generated by a small transmitting loop antenna has been used for the calibration of a radio receiver and a field intensity meter. In the calibration, the transmitting and receiving loops are sometimes located in a coplane with each other. In such a case, a theory to obtain magnetic-field strength is derived by using an infinite series. The errors due to treating the infinite series as a finite series and due to simplifying frequency terms are estimated. The applicable regions of the theory are shown as functions of loop spacing, loop radii, and wavelength. The differences between theoretical values and measured values were within ±0.1 dB at loop spacings of 25 to 60 cm for frequencies from 10 to 30 MHz.