Abstract— In this paper, the effective parameters of a metallic layer of rectangular split rings placed on the top of a rectangular loop antenna in a lossy medium are investigated. The equivalent circuit parameters are analyzed; and their effect on the antenna radiation efficiency and gain is discussed. Investigations are conducted in a homogeneous lossy medium. It is found that decreasing each of the length and height of the inner split ring investigated in this paper by 2 mm decreases the transmission coefficient of the loop antenna by 23%. Also, it is found that increasing the strip width of the inner ring by 2 mm decreases the transmission coefficient by 38%. The effect of the equivalent circuit parameters of the split rings on the performance of a loop antenna in a lossy medium is linked to the antenna radiation efficiency and gain for the first time in literature. The results in this paper are very helpful to control the parameters of split rings to increase the power radiated from loop antennas in lossy media.