San Antonio-based broadcaster Clear Channel Communications Monday posted the most successful second quarter in its 21-year history, according to president and chief executive L. Lowry Mays.After-tax cash flow — which the company considers its most important barometer — increased 36% to $ 6.3 million, or 52 cents per share, from $ 4.6 million (39 cents), in the same period last year. Second-quarter net income increased 27% to $ 2.1 million, or 18 cents per share, from $ 1.7 million (14 cents). Clear Channel, which operates 31 radio stations and seven television stations in 18 markets, said gross broadcasting revenue increased 41% to $ 31.5 million, due primarily to improvement in existing station operations and the addition of several stations acquired since 1992. Station operating income before depreciation and amortization increased 28% to $ 9.9 million. “The acquisitions completed over the last 12 months have created a stronger foundation for growth,” Mays said in a prepared statement. He said Clear Channel will continue to take advantage of the FCC’s recently changed radio duopoly rules and will continue to evaluate acquisition opportunities in both radio and television. For the first six months of 1993, after-tax cash flow increased 54% to $ 10.1 million, or 85 cents per share, from the year before. Net income increased 15% to $ 2.3 million, or 19 cents per share, from $ 2 million (17 cents) in the year-earlier period. Gross broadcasting revenue for the six months increased to $ 55.6 million from $ 38.1 million in 1992.