It is your classic example of the revolving door in action - a government attorney moving to private practice and then appearing before the agency for which he used to work.

Until October Roderick K. Porter worked for the Federal Communications Commission as an assistant to then-chairman Richard E. Wiley. Five months later, Porter surfaced in an FCC proceeding as an attorney for Coral Gables, Fla., radio station WYOR-FM, which was appealing a ruling made by the commission while Wiley was chairman.

Moreover, Porter refused to tell Administrative Law Judge Reuben Lozner what role, if any, he had in the case while he worked for the FCC.

The FCC yesterday ordered Porter to file a sworn statement within 15 days detailing any part he played in the case while he was an assistant to Wiley. If he does not, the FCC ruled, Porter will be considered in contempt and will be banned from participating in the case.

Porter declined comment until he has a chance to see the official order, which probably will not be released by the FCC until next week. He said he was not at the commission meeting for most of the discussion of his case because it started 30 minutes earlier than scheduled.

A draft opinion prepared by the FCC staff, which the full commission approved yesterday, stated that Lozner "has essentially taken notice of the fact that the commission's legal assistants play a vital role in the deliberative process of the commission. As advisers to the commissioners, the assistants make make recommendations regardingthe outcome of cases before the commission, and as a result they may have access to relevant confidential information.

"Since the possibility exists here . . . that Mr. Porter may have previously participated in this case as a commission employe, the extent of any such participation should not be left to conjecture," the draft opinion said.

The case provides one of the most graphic illustrations of a common practice among Washington lawyers - crossing the street from the government to private practice and specializing in the same areas handled while they were on the public payroll.

This practice has come under increasing attack lately. President Carter and consumer groups have decried it, and the D.C. Bar Association's boards of governors is considering stiff.

Under these proposed rules, not only Porter but his entire law firm as well - Fletcher, Heald, Kenehan & Hildreth - would be barred from appearing before the FCC without a special waiver on any case that Porter played a major role in while he worked for the commission.

Lozner saw Porter's participation in the case as a possible violation both of FCC rules and of a canon of the American Bar Association's code of professinal responsibility that says, "a lawyer should avoid even the appearance of professional impropriety."

"The fact that crucial commission decisions (in the WYOR-FM case) were made at a time when Mr. Porter was assistant to the chairman of the commission necessarily creates at the very least the suspicion that the record may be tainted," Lozner said.

Porter, in a statement before Lozner, took the position "that the signing of pleadings in a case and the filing of a notice of appearance in a commission proceeding by an attorney . . . constitutes a representation that there are no conflicts of interest matters which would bar the firm or the firm attorney from acting as counsel in the case."

The case involved a request by WYOR-FM to move its antenna to the top of a Miami building, which would give it more power. Two other Florida FM stations, WFTL in Fort Lauderdale and WWOG in Boca Raton, objected that the move would put them in an unfair competitive position.

They are represented by Lisa J. Steven of the firm of Koteen & Burt. Steven is also a former FCC employe, but she volunteered to Lozner that she had nothing to do with the case when she worked for the government.