Ralph Nader's proposed that all U.S. radio and television stations be required by law to turn over a half hour of prime time each day to the public for its own use didn't exactly rock the broadcasting industry yesterday. It had about as much effect as a letter arriving in a network mailroom with 5 cents postage due.

The idea, put forth by Nader at hearings of the House Communications Subcommittee - now considering changes in basic broadcast law - may sound farther out than "Alien," at first encounter, but then, the Family Hour was a daffy scheme, too, and that one was adopted before you could say, "Leave It to Beaver," which is just what they should have done with it.

And the plan, however patently unworkable, arises from a legitimate impulse - to recoup a little particle of the "public" airwaves for the actual use of the public. The politics of access will be one of the red-hot issues of the '80s and the communications revolution ahead. One problem, though, is deciding exactly who "the public" is and what its interests are. Of course, that's never been a toughie for Nader.

One virtuous aspect of the proposal is that it would guarantee exposure on television and radio of communications issues that are going to become more and more important as we tumble into the Age of Ultra-media.

If television is Main Street, USA, it is at the moment without a public park, a streetcorner for peaceable assembly, or a town hall. Nader told the subcommittee that his "Audience Net-work" organization, which anyone could join for a $5 fee, would not only prepare TV shows but also "represent the interests of its members before the FCC, the courts and the Congress itself - wherever broadcasting policy is being made."

The night before his testimony, Nader said, "The First Amendment guarantees the right of Free Speech to the people, not to the owners of television stations. Well, this is the people" - meaning his Audience Network. "It will give the people access through the same media whose managers and owners now tell them they're giving them what they want."

Nader was told that giving away a half hour a night of prime time television would represent enormous losses in revenue to the networks and stations. Nader laughed and said, "That's too bad.Whose property is it originally?"

In fact, an industry spokesman said from New York yesterday, the total loss in sales of commercial time to the networks alone would amount to about $9 million a week or $468 million a year.

Spokesmen for ABC and NBC declined comment on the proposal, but at CBS. Vice President Gene Mater had himself a chuckle and then said, "The concept of access of that kind goes back a number of years. It's not a new idea, but it is a silly a discredited one. It's a fantastic proposal."

Mater said one problem would that the scheme would result in "really terrible programming of the worst sort. It would just be an excuse for each person to go on the air and grind his own ax. We're certainly not interested in that type of programming. It serves no purpose."

Another CBS spokesman said the plan would work a hardship on station owners, since they are by law accountable for what goes out on their channels, yet they would also by law be enjoined from interfering in productions of the Audience Network. Nader said before testifying that "Audience Inc." would simply operate as a "sublicensee" and therefore take the responsibility on itself.

"It's an interesting thing to mull over," Nader said of his proposal, made on behalf of his group, the National Citizens Committee for Broadcasting (NCCB). "It will also be interesting to see the response. The networks will be straining to find the hypothetical unthinkables, but it's really a very difficult thing to attack."

Nader says the proposal would help make the opposing forces in communications more like equals, "because now what you have are the media monopolies on one side, and the unorganized, disenfranchised viewers on the other." He wants to organize and enfranchise them, and though the Audience Network is clearly not the way to do it, there are bound to be more such proposals and they may not all be, as committee chairman Lionel Van Deerlin (D-Calif.) yesterday characterized this one, "rather hastily conceived."

Van Deerlin, whose rewrite of the Communications Act of 1934 has met with obstreperous opposition both from the broadcasting industry and from citizen's advocacy groups like Nader's, said he though the Audience Network scheme belonged in the same bag with Nader's ineffectual "FANS" organization - "the one with which he was going to reform sports."

It isn't totally worthless as an idea, Van Deerlin added. "It's an interesting proposal, but I can hardly see it forming the basis for regulation chartered by Congress."

Van Deerlin was asked if there really is a national coalition developing of communications activist groups. "I don't know" he said. "When they begin to pick up the unions, that's impressive, although a guy will come in here and say, 'I speak for 225,000 men,' and I'm not sure he even speaks for his wife. But I'm not downplaying it, you understand, and this sort of thing could cause enough of our members to want to go slow."

Rep. Al Swift (D-Wash.), a member of the subcommittee wanted to question Nader about his amazing cure-all on Wednesday but was called to the House floor for a vote. Yesterday Swift said he thought the proposal deserves serious consideration but, "I question whether there is any program, including a presidential address, that should automatically blank out the airwaves on everything."

Actually Nader hasn't suggested that his Audience Network show be on every station simultaneously; the exact air time would be negotiable with the station, and viewer groups in any city could elect to take a series of short messages instead of a solid half hour if they wanted.

But Swift had another objection. "These people who say they want public access not filtered through station management keep going at it so that the management they want to be free of is also responsible for building up an audience for them," he said. "That doesn't seem fair. There's nothing in the First Amendment that says you have a right to an audience."

Swift said he would characterize the Nader plan as "an idea that needs a lot of thought, rather than a well-honed concept."

What gaudy plots like Nader's do accomplish, however, is to draw more attention to communications issues that are going to get increasingly crucial and pervasive as the technologies involved go forth and multiply. "Contributing to a dialogue" may be a terrible '70s cliche, but at the very least, Nader has made another step in that direction, though still not so colorful or violent a step that the television networks deem the all-important rewrite controversies worth covering and reporting to the people they most concern.

As for the rewrite itself, an embattled Van Deerlin now concedes that not only is there no chance of passage during this term, but it is unlikely the bill will get through in a package next session either, since a presidential election will be in the way.

"If this were the morning line in Las Vegas," Van Deerlin said, "I'm afraid I'd put the rewrite right along-side the San Diego Padres - for this Congress, anyway."That gives Nader and others who claim, legitimately or not, to have the public interest at heart more time to wake the town and tell the people about matters that will be affecting everybody's lives in the decade ahead. The only problem is, Nader is going to have a helluva time getting, even for himself, the access he wants for everybody. He can't ride down Main Street, U.S.A., in a bandwagon, because Main Street, U.S.A., is owned by people he opposes. CAPTION: Picture 1, Ralph Nader; by Frank Johnston