EAVESDROPPING TRANSMITTERS HAVE ONE MAIN VULNERABILITY TO DETECTION--THE SIGNAL THEY RADIATE. THAT SIGNAL IS RELATIVELY STRONG AND ITS RADIATED ENERGY CAN INDICATE PRESENCE OF THE TRANSMITTER. WITH THE AVAILABILITY OF RADIO RECEIVERS THAT SEEK OUT AND LOCK TO THE FREQUENCY OF THE STRONGEST SIGNAL IN THE VICINITY IN A FRACTION OF A SECOND, AND WITHOUT THE NEED FOR MANUAL TUNING, THE QUESTION OF EAVESDROPPING SIGNAL STRENGTH VERSUS ORDINARY SIGNAL STRENGTHS BECOMES SIGNIFICANT. THIS ARTICLE DESCRIBES FACTORS THAT ENTER INTO THE PLANNING FOR EAVESDROPPING INCLUDING SELECTION OF TRANSMITTER POWER OUTPUT OUTLINING FACTORS AFFECTING THE STRENGTH OF SIGNALS FROM NEARBY BROADCASTING STATIONS. THESE THREE SYSTEMS ARE DESCRIBED: (1) A SYSTEM FOR DETECTING SIGNALS FROM R.F. TRANSMITTERS, (2) A SYSTEM FOR DETECTING SIGNALS FROM CARRIER CURRENT TRANSMITTERS ON THE A.C. POWER LINE, AND (3) A SYSTEM FOR DETECTING VOICE SIGNALS OR UNUSUAL CURRENT FLOW ON TELEPHONE LINES. ADDITIONALLY, THE ARTICLE POINTS OUT HOW A TECHNICIAN SURVEYING AN AREA FOR A HIDDEN TRANSMITTER CAN ESTIMATE WHAT HIS SEARCH PATTERN MUST BE AND WHETHER OR NOT UNATTENDED OPERATION OF AN AUTOMATIC SCANNING RECEIVER WOULD BE PRACTICAL AS AN EAVESDROPPING DEFENSE. ONLY R.F. DEVICES OPERATING IN THE VHF AND UHF RANGES ARE CONSIDERED. TABULAR DATA ARE INCLUDED. (MJM)