The circular loop source is calculated by the method ofinscribed polygon inner the circle or numerical calculation. The former uses decomposed polygon in each side with electric dipole, then stack all the electric dipole response; the latter adopts the method of digital filter to calculate the integral expression directly. There is some error between above methods and exact solutions in transient electromagnetic response values.This paper compares the two kinds of method. First, the equivalence

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... of inscribed polygon is analyzedand the number of edges of the polygon, the proportioncoefficient of transceiver distance and the length of the dipole are debated. Second, the contrast research in homogeneous half space with the two methodsis carried on. Finally, layered medium is inversed by Artificial Bee Colony (ABC) algorithm and studying the fitting error. The results show that if inscribed polygonhas few numbers of edges, the response error and fitting error calculated by the method ofinscribed polygon inner the circle are higher than numerical calculation method in any conditions. But when the edge of inscribed polygon reach a certain number, the calculation accuracy is better than the numerical calculation methodunder a suitable parameter combination. Keywords-circular loopsource; inscribed polygon inner the circle; parameter combination; numerical calculation; inversion of layered medium