It is known that both the Wheeler and reflection methods are accurate and simple techniques for measuring radiation efficiency of small antennas. In this paper, the radiation efficiencies based on both methods are simulated by use of the method of moments with wire-gird model. To clarify the frequency shift between simulated and measured efficiencies based on the reflection method, we pay attention to dips observed in the frequency response of the efficiency based on the Wheeler method. Some simulated results are shown to confirm our proposed techniques for avoiding the dips of the efficiency based on the reflection method.