Northern Virginia radio talk jockey Allan Prell is always on the same frequency-which is not necessarily true of his listeners, drawn from the ranks of the suburban lonely.

"How many hours a day do you spend thinking about sex?" Prell asks his calllers, most of them between 18-45 years old.

"Twenty minutes a day," the woman answered. "Every waking minute," according to one man. No match here.

"I think these questions tell a lot about a person," Prell said between phone calls that jam the WEEL (1310 AM) switchboard from 4 to 6 p.m. every Friday in response to Prell's show, "Dateline."

Nearly out of every four persons in Fairfax County, Alexandria and Arlingtron is either single, separated or divorced, according to statistics. Allan Prell is doing something about that.

"The show started a year ago as a joke," Prell said in a recent interview. "I wondered if it would be feasible to get people together on the air. There are an awful lot of lonely people out here. It's hard to meet people in this area. It's so sprawling and transient."

Callers asked to rate themselves (looks and personality) on a scale of 1 to 109

"Naturally, they're all nine's," the host said.

The minimum age is 18 ("cause I don't want a bunch of runny noses on my program") and each caller is assigned a number. Names and phone numbers are not revealed on the air.

"Only pink booties and blue booties should call in." Prell announced over the air one day. "Lavendar booties-and you know who you are-should not."

When asked to rate himself, the recently separated Prell said, "To the right woman, 10 plus. But who can account for taste?"

Which is the reason Allan Prell's "Dateline" is such a success.

"It's a blind-date type of thing," said Pete Dundas, a 26-year-old lab technician from Fairfax who has participated in the show three times, the first time on a dare. "You take a chance. It's like being in high school again."

Dundas said two of the three dates were "total turnoffs" but he hasn't given up on the idea.

"It's hard. She can rate herself as a 10 but turn up looking like yesterday's sneaker. But the more you do it, the better chance you have of meeting someone really nice. I think it's a novel way of doing that."

The calls, accompanied by a syrupy rendition of "Fly Me to the Moon," are sandwiched between op-40 rock music and slimming studio commercials: "Are you embarrassed because you can't DISCO?"

Prell, the 41-year-old, pudgy-faced program director at WEEL, is not known for his subtlety in prying loose his listeners' vital statistics, romantic fantasies and any other embarassing details permitted by the FCC.

"He has a way of bringing out people's innermost secrets," said one "Dateline" caller, Wen Little, a 36-year-old teacher and musician from Fairfax. "He's almost like an alcoholic beverage."

"There is a sadistic side to me," the host allowed. "I razz the hell out of them."

One recent example of Prell's persistence was the exchange with a woman caller who said she had never had a romantic experience.

?Are you too nice, little goody-tow-shoes . . . frigid and cold uncaring? Just because you're on the radio and thousands of people are laughing at you is no reason not to tell me. There must be be something-like smearing chocolate sauce all over each other."

The woman answered, "Some of the things that were romantic are things I wouldn't want to share with other people."

"That dirty, huh?" Prell dead-panned.

The dial-a-date hotline, which has produced two engagements, is, according to Prell, "more glamorous" than singles bars.But not everyone is satisfied with the results.

"I've called in twice, but it didn't work," said Linda Robbins, 29, separated mother of two who works for the National Institutes of Health. "They got my telephone number wrong. And besides, most of them sound like real duds."

As for Prell, he's still single. "I'm probably in the best position in Northern Virginia to find a woman," he said recently. "I still haven't found one yet."

Wen Little put it this way: "I'm still looking for the perfect woman. I'm just as likely to find her on the radio as anywhere else."