When the first issue of Broadcasting magazine went to press on Oct. 15, 1931, it aimed to be radio's trade paper, serving what it termed the "Fifth Estate." On the publication's 50th anniversary, it resurrected the tag line, "The News Magazine of the Fifth Estate," and last week added an explanatory line above the familiar lightening-bolt logo: "Radio-TV-Cable-Satellite."

As the journal continues to evolve, the staff of fewer than 50 churns out an average of 116 pages weekly--with as much attention to direct-broadcast satellites, teletext and interactive cable television as to radio. In addition to design alterations, last week also saw "evolutionary" corporate changes at Broadcasting Publications Inc., headquartered at 1735 DeSales St. NW. Three vice presidents were elected, a new bureau chief appointed and the retiring executive editor and bureau chief retained as consultants.

The magazine, known as the "bible of the industry" to 37,000 paid subscribers, is considered blatantly pro-industry by its critics. Chairman and editor Sol Taishoff, 77, says the editorial bias is frankly in favor of free enterprise. Taking that philosophy to heart, it sells for $55 for a year's subscription, with about 55 percent of each week's average of 116 pages devoted to advertising.

Lawrence Taishoff, 48, president and publisher, says advertising has doubled in the last five years. "It was generally thought a business publication couldn't exist in Washington," he said. "But, aside from the fact that the chairman of the board wanted to live here, it's worked because the communications system has improved and more and more our focus is here, whether the trend is to regulate or deregulate the industry."

In addition to the weekly magazine, the corporation owns a cold-type plant and publishes Broadcasting-Cablecasting Yearbook. Plans for a computerized "wired newsroom," are in the works, to keep pace with the industry it covers.

Broadcasting's competitors, including TV Digest, Television-Radio Age, Variety, Muitichannel News, CableVision, Advertising Age and various engineering magazines, have targeted sections of the industry audience. But Broadcasting editors believe they are alone in serving all facets of electronic communications.

In recent years, Broadcasting's management considered splintering off a cable publication. But managing editor Don West maintains "the electronic media are merging at such a rate, it's folly to try to split them up at this point." In fact, broadcasters own roughly 38 percent of the 4,500 cable systems in the country.

The corporation's staff changes as of March 1: Managing editor Donald V. West, director of sales and marketing David N. Whitcombe, and treasurer and business manager Irving C. Miller, all in Washington, become corporate vice presidents. Jay Rubin, senior correspondent and assistant bureau chief, was named New York bureau chief, succeeding Rufus Crater, who joined the staff in 1945 and now becomes senior editorial consultant, based in New York. Edwin H. James, on the staff since 1946 and executive editor in Washington since 1960, also becomes a senior editorial consultant.