The yen reversed course during the past two months after reaching its all-time low last November, increasing in value by 21 percent against the dollar--a shift that is expected to raise the price of Japanese goods here and possibly ease congressional tensions over trade.

The yen closed yesterday on the New York exchange at 229.85 yen to the dollar after having reached an 11-month high on Wednesday. The yen has increased by almost 50 points since falling to its low point of 278 on Nov. 1, leading an upward movement of most major currencies compared with the dollar.

The rising yen--which experts said had been undervalued by as much as 40 percent--will lessen the competitive price edge Japanese goods have had over American products and partially answer complaints by American businessmen that their companies are losing orders because of the unusually low value of the yen. This potential easing of protectionist pressures coincides with an important visit to Washington by the new Japanese prime minister, Yasuhiro Nakasone, scheduled in 10 days. Trade will be a key issue in his talks with President Reagan.

"A stronger yen," said Ann Parker Mills, a vice president of Irving Trust Co. specializing in international finance, "can defuse protectionism against Japanese goods."

Price cuts, however, are unlikely to show up immediately for American consumers as Japanese exporters are expected to take the extra costs out of the profits bulges built up during the yen's low period. Prices of Japanese cars, for instance, were neither cut when the yen was low nor increased as it has moved up, and Robert McElwaine, president of the American International Automobile Dealers Association, said they are not likely to change until the yen goes to 200 to the dollar.

"In the long run it will make U.S. goods cheaper. It is good from the point of view of stimulating exports and making American products more competitive," said Mills.

Mills called the sharp rise of the yen in such a short time "impressive," while Phillip H. Trezise, a senior fellow at Brookings Institution described it as "phenomenal."

"I don't think it is or should be the end of the movement. It's got further to go," added Lawrence J. Krause, a Brookings economist.

American economists and Japanese sources credited the stronger yen to lower interest rates in the United States, which makes investment here less attractive, and to the sharp deterioration in America's balance of payments with its trading partners during the third quarter. Greater American imports than exports tends to cheapen the value of the dollar, economists said.

Furthermore, they said, the feeling that the world's financial giants have decided to protect the international banking system and the end of the Mideast war helped draw capital out of its safe haven in U.S. dollars, driving their price down.

Until September, for instance, capital was flowing from Japan at a rate of $20 billion a year. By October, the outflow had dropped to a rate of $16 billion a year and in November it had decreased by about 80 percent--to an annual rate of $4 billion.

At the same time, the Tokyo Stock Exchange began coming to life, indicating an inflow of capital--especially, said Japanese sources, money from the Mideast that had been turned into dollars because of fears the Israeli invasion of Lebanon might spread.

With the exception of the English pound, other major currencies have also gained strength against the dollar, though none as much as the yen. And Mills predicts continuing weakening of the dollar during 1983.