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The quality factor is analyzed for the spherical modes which represent fields of an antenna in an electrically lossy medium. Like the previous approach in free space, the stored energy in the evanescent field is represented by subtracting the energy associated with the radiation field from the total field energy. Instead of evaluating the stored energy over the whole space, simple equations on Q using the radiation efficiency and the medium property are derived using the concept of power dissipation in a lossy medium and the relation between Q of TM and TE modes. The theory is verified by the comparison of Q's evaluated from the impedance bandwidths of TM and TE modes and the proposed equations in a lossy medium.