Metromedia Broadcasting Corp., the giant communications company that owns WTTG-TV (Channel 5) and WASH-FM in Washington, broke new ground on Wall Street last week when it raised $1.3 billion by selling high-yield, low-rated securities known as "junk bonds."

The offering was both unusual and creative, according to financial experts, who believe other companies may attempt similar offerings. The $1.3 billion, the most ever raised through junk bonds, came from a broad group of institutional investors, including pension funds and insurance companies, according to investment bankers at Drexel Burnham Lambert Inc., which arranged the transaction. Drexel is the leader in marketing these high-yield, high-risk securities.

Junk bonds, the name given to these high-yield, unrated bonds by Wall Street investment bankers and traders, have become popular recently as a new means of raising capital. They are popular among investors who like the high yields and are willing to accept increased risk. Some of the Metromedia junk bonds will carry yields as high as 17 percent before they reach maturity.

Metromedia, which was a public company before investors borrowed $1.2 billion from a group of banks earlier this year to take the company private in a leveraged buyout, raised enough through the offering to pay off its bank borrowings. As a result, Metromedia has greater financial flexibility since, for example, it no longer has to abide by the covenants and restrictions that were conditions of the bank borrowings.

Leveraged buyouts have become increasingly popular in the last two years, and experts believe other companies that have used that technique to go private will try to copy Metromedia. In a leveraged buyout, investors borrow the money to purchase a company, pledging the company's assets as collateral, with the expectation of paying off the loans using cash generated by the company's operations.

"There are tremendous advantages for us in doing this," said Leonard Pack, Metromedia's associate general counsel. "First and foremost, the agreement with the banks would have required us to sell a major television station by June 30, 1985, so that we could repay about $200 million of the borrowings. That would have forced us to break up our crown jewel, which is a group of seven television stations in seven of the top 10 markets. Having paid off the bank debt, the requirement to sell off the station has gone away.

"A second advantage is that a large part of the bond offering consists of zero coupon notes and until they start maturing in 1988, there is no cash interest payable. Also, since one of the notes we issued starts out bearing a lower rate of interest and later yields a higher rate, our cash interest cost has been significantly reduced for the first five or so years." (Zero-coupon notes don't pay any interest and sell at a substantial discount from face value. Investors get their gains from the difference between the discounted purchase price and the value of the bonds at maturity.)

Metromedia also benefits because the junk bonds are backed by Metromedia's television and radio stations only, while the bank borrowings were backed by all of the company's assets. These other assets include an outdoor advertising business, an entertainment subsidiary that owns the Ice Capades and the Harlem Globetrotters, and a telecommunications company that is involved in radio paging and cellular telephones. Metromedia is a partner in Cellular One, a company offering celluar telephone service in the Washington and Baltimore area.

"These publicly offered junk bonds represent a new source of refinancing leveraged buyouts," said Leslie M. Corley, a partner at Kelso & Co., a New York firm specializing in leveraged buyouts.

One of the reasons the unusually large junk bond offering was successful, according to Leon Black, a Drexel Burnham managing director, is that institutional investors were comfortable investing in them. The reason is that the company has a sensational group of television properties, Black said, including television stations in New York, Los Angeles, Chicago, Boston, Houston, Dallas and Washington.

The offering was also creative because it included a variety of different types of securities, with lots of "bells and whistles," as they are called on Wall Street. These include variable rates of interest tied to the company's performance and were important in the offering's success because they stimulated demand from a broad group of institutional investors. CAPTION: Picture, Metromedia's WTTG-TV and WASH-FM are in this building at Wisconsin Ave. and Harrison St. N.W. BY JAMES M. THRESHER -- The Washington Post; Chart, Metromedia junk bond offering: $1.3 billion. Metromedia 1983 revenue: $343.7 million. Metromedia assets as of 9/30/84: $1.45 billion.