UPDATE 1-U.S. FCC chief Kevin Martin leaving Jan. 20

(Adds details on departure, background on his tenure)

By Kim Dixon

WASHINGTON, Jan 15 (Reuters) - Kevin Martin, a Republican who chaired the U.S. Federal Communications Commission for four years, said on Thursday he would be leaving the agency when the new administration takes over.

Martin will become a fellow at the Aspen Institute in Washington, a conservative think tank. His departure is effective Tuesday, the day U.S. President-elect Barack Obama will be sworn in.

His exit will leave the normally five-member commission with one Republican and two Democrats for a time as lawmakers face a long list of new appointments to confirm.

Obama is expected to name technology executive Julius Genachowski, a former FCC staffer, to the top FCC spot, Democratic sources have confirmed.

Martin could have stayed on as a regular member of the FCC but the chairman has much more power than other commissioners, setting the agenda and controlling a large staff.

His tenure was marked by aggressive attempts to get cable television companies to sell channels separately instead of requiring consumers to buy big packages of programming.

Under Martin, the FCC had its most lucrative spectrum auction ever in 2008, raising $19.6 billion from airwaves won mainly by AT&T Inc (T.N) and Verizon Communications (VZ.N).