ssr@TUMTUM.CS.UMD.EDU (Dave Kucharczyk) (10/25/87)
Phone tap detectors: Most of the phone tap detectors that are commercially sold are just voltage drop detectors and completly worthless since they won't even pick up a lineman's "butt-set". The only way to be reasonably sure a line isn't tapped is to disconnect both ends and put an signal generator on one end and an oscilliscope on the other end and check the signal loss at several frequencies and then store these values for the next time a sweep is done. One could also use a sweep generator and a spectrum analyzer for a more complete and accurate record, but none of these methods are fool proof because humidity, temperature and other outside conditions can cause impedance changes that would cause one to suspect that an 'addition' has been made to the line. The best way would be to terminate one end with a purely resistive 600 ohm impedance (ie a 600 ohm carbon resistor) and use a time- domain reflectometer. This device sends out short pulses of RF and then listens for reflections and shows them on a screen (ie reflected power vs. time). The 600 ohm termination on the end of the line shows up as the largest peak and any impedance changes along the line (ie terminal connections,cable splices taps) show up as smaller peaks between the start of the trace and the termination peak. This also gives one a relative distance to the suspected tap. The phone company has automated testing apparatus that will give a nice printout of line impedance, capacitance etc. (that's how they know how many phones you have on your line, since every ringer adds impedance to the line) but I wouldn't bother to try to pry such info from them. Even with all this if a tap is well designed it is almost impossible to find. A very high resistance input with low capacitance/inductance would go unnoticed by any of these methods. (ie a FET input transistor amp has about 10 Meg input resistance with 2-10 pf input capacitance and would be totaly hidden to all but the most sensitive instruments.) ssr ssr