The ST-2A Surveillance Transmitter The ST-2A is one of the earlier models in a long series of equipment. The purpose of a surveillance transmitter (ST) is to transmit the sounds (conversations) from within a room to a person or recording device monitoring a receiver nearby. For example, an ST may be hidden in a hotel room before the targeted person arrives, and the opposition can set up equipment on an adjacent floor to monitor and record any conversations that take place.This device, marked “F-371 IndCoil”, is an audio wire tap, most likely intended for telephone lines. It is an inductive pickup, so does not require any direct connection to the signal wire. This makes it much more difficult to detect, and does not interfere with telephone operation in any way.
When clipped over the red wire on a traditional telephone line, and connected to a suitable preamp, it can efficiently monitor a conversation. Note that it is necessary to tap only one of the two signal wires – if both wires pass through the device, the signal is canceled. The tap includes a square, closed armature that is opened by pushing a spring-loaded button. Two sides of the square have fine wire coils wound on them, connected in series.
Pushing the black button on the right side causes the armature to open so that it can be clipped over a wire. The oblong aluminum portion is a separate impedance matching transformer, marked “3.2” (ohms) on the input, and “1200” on the output. It has a 1/8” plug on the input side, and a matching jack on the output side.
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