There's an economic Catch 22 taking shape in the national mortgage market that you ought to know about if you're planning a home sale, purchase or refinancing this year.

The catch works like this: Mortgage rates are likely to drop by another full percentage point in the next 90 or 120 days, and could drop still further by fall.

An FHA rate of 10 1/2 percent by the last quarter of 1983--and a dip below 10 percent early next year--are even forecast by at least one nationally known economist.

That's the good news.

Now the catch: Forget those attractive rate drops if you see the national economy begin to bounce back to health from its current anemic state.

In other words, don't bank on significantly better mortgage rates if things start getting significantly better for the economy overall in the next 12 months. The quicker the recovery from recession, the less likely you'll be to see lower rates for home loans. That's the sobering consensus that emerges from discussions with top mortgage-market economists around the country.

The economists agree chances are good that the current 13 1/4 percent prevailing rate for a conventional home loan will slip to the low 12 percent range before summer. They agree that conditions for home buyers and sellers during the first half of this year are likely to be the most favorable since late 1979. And they agree that home starts and purchases of moderately priced houses and condominiums should be up substantially this spring over last year's levels.

But the economists also warn that home buyers and sellers who are trying to time their moves to catch the low point in the current interest-rate cycle face some tough calculations.

They suggest that the key to your timing should be your answer to one critical question: Are you bullish (optimistic) about the prospects for the national economy overall during the next six to nine months, or are you a bear (a pessimist)?

If you're the former--and you believe that the economy finally is going to turn around this year--then don't wait long before plunging into the market and locking up mortgage money. That's because even a mildly resurgent national economy will mean heavier demands for capital from business. Almost inevitably, that demand will exert upward pressure on rates. Mortgage money will be more expensive later in the year than it is now.

If you're bearish--and you think the recession can't be shaken off in 1983--then you should wait for long-term mortgage rates to slide two or more points below where they are today.

James Christian, chief economist for the U.S. League of Savings Institutions in Chicago, is an articulate exponent of the former view. He's mildly optimistic about the direction of the economy, and he doesn't expect mortgage rates this year to go below the 12 1/4 percent level he's forecasting for the spring.

Christian predicts that Congress and President Reagan will come to some political "accommodation" on federal spending and taxes that will lessen the likelihood of deficits ballooning beyond the $200 billion levels that have frightened Wall Street in recent weeks. He also expects to see a slow, steady recovery for the economy overall, led by housing sales and construction.

"I wouldn't put off a housing decision simply to wait for lower rates," he says. "The likelihood is that home prices will firm up or even rise during the year, and that would cancel out advantages of lower rates, even if they did stretch into the fall."

Another articulate economist--but one with different conclusions on rates--is Nat Eisenberg, president of the Houston-based Economic Advisory Services, a consulting firm.

Eisenberg was one of the few economists who accurately predicted the rapid decline in rates that occurred last fall. He still believes that rates will decline further "for essentially the same reasons I've been giving my clients for the past year: The economy is simply too weak and too deflationary to support such high rates. The economy won't really turn around until we've got mortgage rates of 10 percent or less. Only then will you see consumers come out of the woodwork, business expansions begin to take off, and rates begin to firm up again."

The magic 10 percent level is "within reach during the next year." Eisenberg maintains that rates will be at or near 10 1/2 percent by December and should crack back into single digits early in 1984.

"I guarantee it," says the fearless Eisenberg. "And you can print that."