The system involves the use of a so-called Passive Element, or PE,
similar to the one used with the
Easy Chair Mark I listening device,
that is connected to an existing analogue telephone line inside
the target area. The PE is powered by a strong RF signal
injected into the telephone line from a tapping
point outside the building.
This done by means of the Wired Easy Chair,
shown in the image on the right. The WEC consists of a transmitter,
a receiver and a line matching unit or interface that also
acts as a duplexing unit for the transmitter and receiver.
The WEC was actually designed for using an existing analogue telephone
set, such as the
Ericsson Model 1951
and the Heemaf Model 1955,
to intercept a conversation in the target area. However, it could also be
used for the Rocking Chair (RC) project without any modifications
To activate the bug, the WEC injects a strong RF signal with a frequency
between 20 and 300 kHz directly into the telephone line. As this signal
is well outside the audible range, users of the line will not notice its
presence. Like the EC I,
the bug absorbes the RF energy and converts it into a DC voltage
that is used to power a small audio amplifier.
Any sound in the room is picked up by a microphone and amplified
in the amplifier, which causes modulated sidebands to be generated.
These sidebands are fed back to the diodes and eventually, through a
transformer, to the line.
The modulated reflected signal is picked up by the sensitive receiver
of the WEC unit, that is installed at a covert tapping point outside the
target building. In order to separate the weak reflected signal from the
strong activation signal (that operates on the same frequency), the WEC
contains an advanced duplexer that acts as a cancelling unit. After
demodulating the reflected signal, a high-quality reproduction of the original
intercepted audio will become available.
➤ More information about Wired Easy Chair (WEC)
The application of the RC is limited to those situations were a
regular (analogue) telephone line is present in the target area,
which is connected to the exchange of a Public Switched
Telephone Network (PSTN), somewhere outside of the target area.
Furthermore, it is necessary that a tapping point can be made
inconspicuously on the line between the target area and the exchange.
The Passive Element (PE) is now placed in the target area and is
connected in parallel to the telephone line. It will be
powered by the RF energy from the WEC unit that is placed at
the tapping point.
The diagram above shows how this works. The tapping point can be
made at an existing junction box, in a building where the cable
passes through, at the exchange, at an underground cable joint,
or by digging up the cable and cutting into it.
For situations where it was difficult to access and operate the WEC at
the tapping point unobtrusively, a remote control unit was developed.
The diagram above shows the additional remote control unit (RCU) at
the bottom left. It is connected to an extra telephone line that is
connected to a remote listening post (LP). The WEC unit can be
controlled via this telephone line which also delivers the intercepted
audio at the LP.
Below is the circuit diagram of the Passive Element (PE) that was used
in the Rocking Chair (RC) system. It is similar (but not identical) to the
PE of an Easy Chair (EC) system, such as the EC Mark V.
At the far right is the line interface by which the PE is connected to the
existing telephone line.
The interface has been dimensioned in such a way, that it provides minimum
loading on the line, making it very difficult to detect.
Two HG5004 diodes are used to convert the RF signal into a DC voltage
that is high enough to power the rest of the circuit. The actual PE is built
around three Philips OC44 transistors in a rather strange arrangement.
The first two transistors (T1 and T2) are used as pre-amplifiers, whilst the
third one is the final amplifier that feeds back the modulated sidebands
to the rectifier diodes, which will in turn pass the signal on to the
All transistor stages are couple via miniature Fortiphone 1 transformers.
A sensitive dynamic microphone, such as the
Shure MC-30, can be connected to
the coloured terminals at the left. A 1:10 transformer is provided to allow
the use of both high and low impedance microphones.
At the time, Fortiphone was a British manufacturer of heading aids, and
was able to supply the smallest possible components. Fortiphone transformers
were also used in the PEs of the the EC I
thru EC V.
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© Crypto Museum. Created: Thursday 30 March 2017. Last changed: Saturday, 01 April 2017 - 15:46 CET.