In this paper, we show that the orbital angular momentum (OAM) communication performance with a circular loop antenna array can be drastically improved by exploiting the port azimuth effect at the 5-GHz band. The received signal and interference powers are analytically derived with generalized Z-matrices and the perturbation method for short-range OAM communication. The resulting formulas show that the interference power can be drastically suppressed by selecting the proper combination of port azimuths. We also explain the mechanism behind the reduction in interference power. For the obtained port azimuth combination, the simulated and measured transmission isolations at 1cm are better than 24.0 and 23.6dB at 5.3GHz, respectively. Furthermore, to estimate performance in 2×2 MIMO communication, constellations for 64-QAM are estimated. Measured EVMs are less than 3% where signals are clearly discriminated without any signal processing. For long-range OAM communication using paraboloids, the optimum port azimuth combination is estimated by monitoring the current distribution. For the obtained combination of the port azimuths, simulated and measured transmission isolations at 125cm are better than 15.7 and 12.0dB at 5.3GHz, respectively. The measured isolation for short and long ranges are improved by 9.2 and 4.5dB, respectively, compared with the data for the combination of the identical port azimuth.