Washington — Pretending to be a Soviet eavesdropper, I peer into the purple mists of northern Virginia, searching for the Pentagon. I am standing on the roof of an apartment on upper Wisconsin Avenue, one of the highest spots in Washington. Next door, the spindly legged frame of the new Soviet embassy is just emerging from morning shadows.
From this vantage point, two things about the half-finished embassy quickly become apparent: 1. The Soviets will have a great view. 2. Their antennas will pick up more than HBO and ''This Week with David Brinkley.''
To the south, next to the gray ribbon of I-395, the Pentagon is clearly visible. To the east is American Telephone & Telegraph's (AT&T) Arlington switching station, which sends an electronic beam of phone calls shooting right over the Soviet site. A few blocks north are the towers of the Naval Security Station and Western Union's Tenleytown microwave relay.
The prospect of foreign aerials in the midst of this electronic interchange illustrates a dilemma of modern telecommunications. Whiz-bang technology makes the United States system the best in the world, say telecommunication experts - but that same technology makes it relatively easy to intercept messages.
The USSR, from trawlers, trucks, and rooftops, has been listening in on our phone calls for years, say US officials with access to intelligence information.
The National Security Agency, the US government's secretive electronic intelligence arm, scans an unknown amount of US messages headed overseas, according to court records and civil liberties advocates.
Furthermore, it may be perfectly legal for anyone to eavesdrop on computer communications. Experts worry that wiretap law may cover only human speech, leaving the ''beedle-de-beep'' of computer talk unprotected.
In general, it is the demise of the wire which has made these activities possible. Once, phone calls traveled only paths of copper; today many are shot across country by microwave. Microwave beams can be a third of a mile across, and to catch them, all an eavesdropper must do is hoist small dish antennas in their path. If the beam is an AT&T trunk line, sophisticated computer analysis is then required to unravel it.
Overseas messages bounced off satellites are even easier to grab. With a good dish, satellite traffic can be stolen from anywhere in the US, from ships offshore, ''even from Cuba,'' wryly notes a former White House communications official.
Wireless phones are vulnerable, too. If you have a cheap model, neighbors may be able to hear parts of your conversation on AM radio. Last December, police in Woonsocket, R.I., used this eavesdrop technique to snare a 19-member drug ring.
All this doesn't mean there are lots of little guys out there listening to your calls. For the most part, only nations indulge in extensive electronic eavesdropping.
The Soviet Union is the most notorious example. Their buildings - from the old Embassy on 16th Street here, to the United Nations Mission on 67th Street in New York, to the West Coast consulate on top of a San Francisco hill - are topped with forests of antennas. From these rooftops and elsewhere, the USSR has been listening in on US phone calls for at least a decade, say government and academic sources.
They are probably after more than military secrets. ''Department of Defense (communications) will be encrypted,'' says an official who worked on the issue for the Carter administration. ''The problem is that sensitive private-sector information is vulnerable.''
Conversations pulled from the sky have likely helped the Soviets in grain-contract negotiations, for instance, says this source. Their Glen Cove, N.Y., weekend lodge is well-positioned to listen in on Long Island's defense industries. The San Francisco consulate is thought to hide equipment trained on Silicon Valley.
Defensive measures have been taken since the eavesdropping was first discovered. US government communications have been rerouted underground; important defense contractors have been outfitted with government scramblers. AT&T now beams most microwaves in a way that is much more difficult to unravel, says Willis Ware, a Rand Corporation communications expert.
But the USSR, at the same time, has been updating its interception gadgets. Overall, ''the situation is more or less the same,'' claims a congressional aide with access to intelligence information.
Other nations probably have electronic eavesdropping equipment in the US, though not on the same scale as the Russians. The US itself, however, has high-tech ears that put the Soviets to shame.
The National Security Agency (NSA), the US electronic intelligence arm, has six times the number of employees of the Central Intelligence Agency (CIA), according to congressional estimates, and has giant antennas from suburban Washington to Pine Gap, Australia.
Most of these ears are trained on other nations, straining to pick up chatter between Soviet pilots or data from Chinese missiles. But some are turned inward to monitor US phone calls and telegrams headed for other nations.
NSA dishes in Sugar Grove, W.Va., can eavesdrop on a nearby COMSAT post that handles half of all US international satellite communications, says James Bamford in his book ''Puzzle Palace.'' NSA installations in Maine, Washington State, and California have similar purposes, he claims.
The NSA sweeps up vast numbers of messages headed overseas from the US, according to records from a 1982 court case on use of the agency's intelligence. High-speed computers then rifle through this raw data at leisure. When they stumble across a keyword (''Khomeini,'' perhaps) that means the message might be useful, it is printed out for further study. Other communications are discarded.
A US appeals court judge, in the context of the '82 suit, did not find this activity illegal. But some congressional aides and civil libertarians feel the NSA impinges on citizens' constitutional rights, as the agency's methods inadvertently filter millions of innocent messages.
''The intrusion is no less serious because it's so quick, or because no trace is left, or because no human is involved,'' says David Watters, an electrical engineer and former consultant to the CIA.
The NSA's power has been abused in the past: Between 1945 and 1975, under ''Operation Shamrock,'' the agency was given copies of almost all telegrams sent overseas. During the Vietnam war era, the NSA listened to conversations of Jane Fonda, Dr. Benjamin Spock, and others on a ''watch list'' of 1,650 protesters.
Today, ''the NSA does not target the communications of US citizens,'' a former NSA director, Vice-Adm. Bobby Inman, said in an interview.
''The provisions are also in place to suppress any potential for saving a 'watchlist' of knowledge that's incidentally acquired,'' he added. ''There is not, in fact, a danger of Big Brother turning to listen to the communications of its citizens.''
There may be a danger in the US, however, of unwanted ears listening in on the communications of computers. The 1968 Crime Control Act, which governs non-national-security wiretaps, prohibits ''aural acquisition'' of telecommunications. In other words, it's illegal to intercept a communication you can hear and understand.
But as anybody who's ever listened to computer ''speech'' knows, you can hear it - but you can't understand it.
Ron Plesser, counsel to the 1977 Privacy Protection Commission, says that means that it may be perfectly legal to eavesdrop on computers. ''It's a real issue,'' he says, ''although I don't think it's that hard to fix.''
This glitch is a good example of how quick-footed innovation often outflanks efforts to control and protect it.
''These technological advances happen so quickly that the normal process our government and society uses for adjusting to change doesn't have time to take effect,'' says Arthur Bushkin, a former Commerce Department information policy official.