WPKX-FM/AM (105.9/730), the determined underdog in the local country music circuit, has snagged exclusive Washington broadcast rights for the Farm Aid concert, the event organized by country stars to help American farmers. The time for the Sept. 22 concert from the University of Illinois at Champaign has not been announced yet, but Waylon Jennings, Willie Nelson, Johnny Cash, Kenny Rogers, Merle Haggard, Alabama and Loretta Lynn have signed up. A Tap Dancing Winner

Miriam Sellers, 64, a dance teacher for 53 years, a great-grandmother and a third-generation Washingtonian, won the first "Woman of the Year" contest of WWRC-AM (980). While the petite, white-haired contestant was cheered on by her family, including Garland Sellers, her husband of 47 years, Sellers tapped to "Thoroughly Modern Millie." Asked "What advice would you give to women under 40 today?" Sellers didn't hestitate: "You are what you eat . . . live a good, clean life, and when you get my age, do the same." Sellers, who will now help WRC promote its over-40 thrust and its nostalgia swing and jazz format, won a trip for two to Copenhagen and a $10,000 fur coat. The Mall and the Music

Loo Katz, spokesman for WAVA-FM (105.1), the station that broadcast and cosponsored the Fourth of July concert from the Mall this year, said this week the station is still interested in participating in next year's program. Last week the National Park Service announced it was planning a more patriotic and "family-oriented" program than this year's Beach Boys-led show. "We are keeping our connections wide open with the Park Service," said Katz. "Whatever they are going to do, we want to be at the top of the list." WASH's Eye on the Beltway

When the new sunrise show on WASH-FM (97.1) debuts Monday, host Arthur Crofton will be trying a magazine-type format to attract Washington's "upscale listeners." Crofton, 33, who was born in Sheffield, England, and has lived in the United States since he was 16, will incorporate music, fashion news and celebrity facts and gossip from London, Paris and Rome. "We want to be topical but in addition to what is happening in Washington, we want to focus on other capitals," said general manager Tom Durney. His approach to this international focus was inspired in part, he said, by trying "designer fashion" selling techniques to attract the local worldly, checkbook-heavy consumer. The station intends to hire a female cohost for the show and Crofton will be doing on-air auditions from 5 to 6 a.m. The news department at WASH, meanwhile, picked 30 listeners with car phones to help report traffic conditions and other news tips. Bob Schmidt already does the airborne traffic reports and Piper Anne works from a cruiser. "The incentive to the volunteers was the phone calls to our newsroom are free of charge over the Cellular One system," said news director Wally Hindes. News and Nostalgia

Beginning Sunday, big band WWDC-AM (1260) will switch its hourly news from the RKO Radio Network to the Cable News Network. Joan Carey will still do local news and Ernie Kaye sports. Muscular Dystrophy Events

As tie-ins with Jerry Lewis' annual telethon for the Muscular Dystrophy Association, several stations are planning fund-raising events. WAVA and WMJR-FM (107.7) will both participate in a "Hands-On Contest." Participants keep their hands on a truck or car as long as they can until only one remains or 72 hours pass. WAVA will be at Anton Toyota on Frederick Road in Rockville. WMJR will be at Anton Datsun-BMW on Liberia Avenue in Manassas tomorrow . . . KIX is a cosponsor of a fundraiser at Landover Mall from 11 to 6 p.m. Monday. John Bodnar, Paul Bottoms, Bill Celler and Chris Connors will join the festivities . . . Starting tonight at 5, WCLY-FM (95.5) will take pledges and song requests. Jeff Baker and David Burd will present the pledges received by Monday at about 1 p.m. on the telethon, aired on Channel 5, but pledging will continue until 9 p.m. . . . The young team that puts together "All the Way" for WKYS-FM (93.9) on Sundays at 11:30 p.m. will join the hosts of the telethon from 1 to 10 a.m. Monday. Sickle Cell Fundraisers

To raise money for the National Association for Sickle Cell Disease, WYCB-AM (1340), WUST-AM (1120) and WHUR-FM (96.3) will participate in two events tomorrow. WYCB will broadcast from 1 to 5 p.m. at Western Plaza, where the station will sell an album specially created by eight Polygram artists, including Stephanie Mills and the Bar Kays, for this benefit. There will also be a mobile health unit to test for sickle cell trait, and Alvin Jones of WHUR will be at the plaza. On the same afternoon, from 2 to 6 p.m., WUST will participate in a similar program, but will not do a remote, from the Eastover Shopping Center. Weekend Music

WDCU-FM (90.1) will kick off 96 hours of music for the holiday with gospel at 6 a.m. today and groove until 6 a.m. Tuesday, closing out with six hours of Miles Davis . . . WXTR-FM/AM (104.1/1560) is planning a three-hour Beatles special Sunday at 9 p.m. and will repeat its listener-selected countdown of favorite oldies from noon to 7 p.m. Monday . . . Sonny Melendrez of WQRA-FM (94.3) will host a three-hour program of summer music starting at 3 p.m. Monday . . . Fred Fiske will be preempted on WAMU-FM (88.5) Monday at 8 p.m. for a concert by Arlo Guthrie and Pete Seeger, recorded this summer at Wolf Trap. WETA-FM (90.9) airs the same concert tomorrow at 9 p.m.