The Senate voted yesterday to authorize $1.1 billion for two years of antiterrorist security improvements at U.S. diplomatic facilities abroad, about half the amount President Reagan requested but still the largest U.S. overseas construction program ever launched.

The measure, approved on a voice vote after relatively amiable debate, has passed the House as a $4.4 billion program over five years. A House-Senate conference committee is expected to meet soon after the July 4 recess to work out a compromise.

Senate Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) said his committee had cut President Reagan's request and limited it to two years because even the reduced construction program is "much larger than anything the Department of State has undertaken before," and will need continued congressional scrutiny.

Lugar led committee action that eliminated many controversial embassy construction and renovation projects and slashed others in order, he said at the time, to avoid inevitable and possibly deeper cuts from critics during floor debate. They had charged -- and Lugar's staff reports confirmed -- that many of the original building plans were overpriced, unnecessary or poorly designed.

As a result, the final measure enjoyed bipartisan support. Sen. Claiborne Pell (D-R.I.), ranking minority member of the Foreign Relations Committee, noted in supporting the bill that the number of U.S. ambassadors killed in service abroad since World War II has been higher than the number of U.S. generals and admirals killed in military action.

"We must do everything possible to provide security for our diplomats abroad," Pell said.

The measure would provide $857.8 million for land purchase, construction and furnishings for 70 new embassies, and for renovation of 170 other facilities abroad.

It also would provide $243.1 million for salaries and expenses for communications and counterterrorism training for embassy personnel, perimeter guards and equipment; $7.2 million to expand Peace Corps recruiting; $5 million for rewarding terrorist informers; $4.8 million for antiterrorism aid in training and equipment for foreign governments, and $2.8 million for research and development.