Eastern Airlines blamed a second-quarter net loss of $15.9 million on a 73 percent increase in jet fuel costs and a 29-day strike in June that halted the carrier's Mexican operations.

The loss announced yesterday compares with net income of $26 million (99 cents a share) for the same period a year ago, which in turn was partially the result of a 58-day strike by United States Airlines workers and the grounding of Mcdonnell Douglas DC10 jets flown by several of Eastern's competitors, the airline reported.

The Miami-based airline reported first-half loss of $12.5 million, compared with profits of $39.3 million ($1.47) a year earlier.

Operating revenues for the second quarter were $850.8 million, up from $726.2 million a year ago. Operating expenses rose from $684 million a year ago to $863.5 million. Six-month operating revenues were $1.705 billion, up from $1.388 billion and operating expenses increased from $1.322 billion to $1.7 billion.

Announcing the profits dip at a board meeting in Hartford, Conn., Eastern Chairman and President Frank Borman said fuel costs climbed, despite a dip in second-quarter fuel consumption from last year. For the six months in 1980 the company used only 2 percent more fuel but the cost was up 90 percent, he said.

"The industry's traffic softened noticeably in response to a recessionary trend in the economy," Borman said, adding that about 56 percent of Eastern passagers flew on discount tickets in the first six months, compared with 41 percent last year.

American Broadcast Cos. Inc. reported second-quarter earnings were down 5.6 percent from the same period a year ago, while earnings in the first half were off 5.2 percent.

But in a joint statement, ABC Chairman Leonard H. Goldenson and ABC President Elton H. Rule, said "we continue to believe, however, that 1980 will be the second best year in the company's history." The company owns the ABC Television Network and other broadcast, entertainment and publishing interests. It had record earnings in 1979.

ABC said it earned $53.78 million ($1.91 a share) on revenues of $578.5 million in the second quarter ended June 28, compared with earnings of $56.96 million ($2.03) on revenues of $501.8 million in the same period a year ago.

First-half earnings were $78.1 million ($2.77) on revenues of $1.1 billion, compared with earnings of $82.4 million ($2.94) on revenues of $953.4 million a year earlier.

"In the second quarter, our television network, television stations, radio and publishing divisions all achieved record revenues," said Goldenson and Ruled in a joint statement issued in New York.

"The ABC Television Network's profits were approximately equal to those of the prior-year period despite the impact of greater programing costs. Our owned television stations division generated slightly higher profits, though the local advertising market remained soft during the quarter.ABC Radio reported flat profits during the period."

Specific figures were not disclosed on earnings and revenues from the broadcasting operations. In 1979, however, ABC's broadcasting operations accounted for 87 percent of the company's revenues and 97 percent of its operating income.

In addition to operating television and radio networks, ABC owns five television stations and 14 radio stations.

Goldenson and Rule said the ABC Publishing Division realized higher profits, mostly from the inclusion of a full quarter's operations of the Chilton Co.

They said over-all second-quarter profits were reduced, in part, by the start-up costs of ABC Video Enterprises and ABC Motion Pictures and by final operating losses of a subsidiary, the Travel Network.

Polaroid Corp. reported second-quarter net income yesterday of $15.06 million (46 cents a share), compared with $13.26 million (40 cents) in the second quarter of 1979.

The company, a leader in the domestic photographic industry through its quick-developing films and cameras, said net income for the first half was $32.50 million (99 cents), compared with $30.37 million (92 cents) a year ago. Worldwide sales for the first half were $625.52 million, compared with $586.68 million in the same 1979 period, the company said.

United Technologies Corp. had a 27 percent profit surge in the second quarter, compared with its year-earlier results, with net income reaching a record $100.8 million ($1.89) as sales soared 66 percent to $2.98 billion, the company said Monday.

First-half net income was $192.8 million, an increase of 28 percent over a year earlier. Sales climbed to $5.88 billion, an increase of 64 percent over the same period last year.

UTC, which is based in Hartford, Conn., manufactures aircraft and rocket engines, industrial gas turbines, elevators and escalators, automotive products and related products.

Officials of Knight-Ridder Newspapers Inc. say a reduction in advertising linage contributed to a 6.3 percent drop in earnings during the second quarter of 1980.

The newspaper group said Monday its second-quarter earnings were 74 cents a share, compared with 79 cents in the same period of 1979. For the first six months of 1980, earnings were $1.33 a share, compared with $1.29 in the first half of last year.

Knight-Ridder -- which owns 34 daily newspapers, three television stations a computer data-gathering group and one-third interest in a paper mill -- said newspaper ad linage, particularly for classified ads, dropped 4 percent.

Operating revenues for the quarter were $274.6 million, compared with $248.1 million in 1979, a gain of 10.7 percent. Total operating costs were $229.8 million, compared with $200.7 million, an increase of 14 1/2 percent. However, 1980 figures include recent acquisitions.