Channel 4 announced yesterday that Fred Thomas will relinquish his co-anchor duties on the 11 o'clock weeknight news after April 8 as he moves fulltime into public affairs programming for the station.

Thomas said yesterday that after eight years as a late-night anchorman first at Channel 7, and since 1975 at Channel 4, he is "very pleased to take a breather from the daily anchoring grind."

Thomas said that two years ago, when he left Channel 7, he had decided not to anchor anymore but it was only recently that Channel 4 could make adjustments in its staff to permit the change.

"I haven't ruled myself out of doing anchor work again sometime" said Thomas, "it's mainly that it will give me a lot more time with my family, which is very important to me at this time."

Currently, Thomas co-anchors the 11 o'clock news with Jackson Bain. No replacement, if any, for Thomas was named yesterday by the station.

Thomas is expected to develop a series of public affairs documentaries under his assignment, in addition to remaining as a co-host of WRCTV's "Sunday" Sunday morning show and producer and moderator of the Sunday afternoon "One on One" talk show."

Thomas also wil continue as host of "Knowledge," which is produced alternately by the five NBC-owned TV stations for early-morning weekday audiences.

Channel 7 yesterday announced it has decided to keep "7:30 Live," its early evening magazine show, alive at least through next fall.

Citing the latest Nielsen book - for the month of February - WMAL general manager Tom Cookerly said a 57 per cent rise in audience since the shore's October debut was a key factor in the decision to retain the program.

In the latest ratings book, "7:30 Live" averaged 162,000 households (up from 65,000 last fall), compared to 168,000 for WRC, 134,000 for WTOP, 85,000 for WDCA and a whopping 302,000 for Channel 5's syndicated "Brady Bunch" reruns in that week-night time slot.

Cookerly said the magazine show was apparently growing in popularity with its target 18-to-49 adult audience due to its live nightly format.

WOL radio (1450) will broadcast a tribute to the late Julius Hobson Sr. Saturday during a 12-hour memorial fund-raising drive honoring the widely known civil rights activist who died Wednesday after a long illness.

The goal of the phone-in drive is $10,000 which will go to the American Cancer Society. The campaign begins at 9 a.m. The special telephone number for pledges will be 537-2000.