Communications Satellite Corp. (Comsat) reported yesterday a 37 percent increase in profits during the second quarter of 1979. Net income was up to $10.6 million ($1.32 a share) from $7.7 million (97 cents) during the same period last year.

However, second-quarter income declined by $1.3 million (17 cents) from the first quarter of 1979 due to Comsat's. 15 percent rate reduction in May for international services and the planned transfer of Puerto Rico service from Intelsat to a U.S. domestic system.

Comsat president Jopseh V. Charyk told the New York Society of Security Analysts yesterday that the corporation plans to enter new marketing areas where rates are not regulated by the Federal Communications Commission and where the profit potential is therefore increased.

The Washington firm will do this both through internal development and acquistions. Comsat plans to invest $200 million in new activities, he said.

Comsat's most innovative program, according to Charyk, is Satellite Business Systems, designed to provide business and other large users of communications requirements with sophisticated private networks.

SBS is a joint venture along with subsidiaries of IBM and Aetna Life & Casualty. Charyk said he hopes the system will be established by 1981 and will become profitable by the 1980s.

Comsat also is expanding its technical services division which includes managements, planning, construction and operation of satellite systems. Among its current customers are an Arab regional satellite system and the U.S. Postal Service, which is developing electronic delivery of international mail via satellite.

Comsat also recently made its first acquistion, Environmental Research & Technology, Inc. of Concord, Mass. ERT collects and transmits to customers data on air and water quality.

D.C. National Bank, the city's eighth largest bank, announced an 83 percent increase in earnings for the first half of 1979. Net income rose to $564,473 ($2.75 a share) from $308,795 ($1.54 a share) in the same period in 1978.

Chairman S. Greenhoot Fischer attributed the rise to substantial growth in both deposits and loans. The average balances increased 23 and 33 percent, respectively over the same period last year.

National Savings and Trust Co. reported a 15.6 percent rise in net income for the first half of 1979. The bank earned $2.2 million ($4.77 a share compared with $1.9 million ($4.13 a share) in the 1978 period.

The regular quarterly dividend of $1 per share on common stock was declared, payable Aug. 1 to stock-holders of record as of July 20.