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An expression P=2RcVL rmsHa rms was theoretically derived for the ac power losses in a circular superconducting disk in a perpendicular ac applied magnetic field when the losses are measured by a planar circular pickup coil surrounding the disk. Here, Rc is the radius of the pickup coil, which is coplanar with the disk, and VL rms and Ha rms are rms values of the loss voltage and applied magnetic field. This method of measuring losses has an advantage over a susceptibility technique, since the coil need not be calibrated. However, this expression is valid only for large values of Rc relative to the radius R of the disk, since the dipole approximation was used in the derivation. We experimentally evaluated the minimum value of the diameter 2Rc for which this method provided approximately correct losses. A YBa2Cu3O7 disk with diameter 2R≈5.3 mm was used for the measurements. The electronically determined apparent losses at all field values decreased with increasing coil radius, and asymptotically approached a constant value for Rc/R>∼3.