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A conventional 20-MeV drift tube linac (DTL) for the Proton Engineering Frontier Project (PEFP) has been developed as part of the low-energy section of the 100-MeV proton accelerator. The 20-MeV DTL consists of four tanks, and the RF power generated by a klystron is coupled to the DTL tanks through RF power couplers. By introducing analytical and numerical methods to correlate the coupling coefficient with the coupling hole size in a dumbbell shaped iris coupler, we optimized the coupling coefficient to minimize the required power. The results from the two approaches are in excellent agreement with each other and with the experimental measurements. The coupling coefficient is shown to be proportional to the third power of the coupling hole diameter.