The system operates at 375 MHz and works on the principle that,
when using a good directional antenna, a fraction of the RF signal
reaches the Target Area (TA), which is just enough to power a simple
and low-power covert listening device, known as the
Passive Element (PE).
Comparable to the instrument of a field-strength indicator.
At the PE, a microphone picks up any sound in the room and sends it
to a small amplifier that is powered by the energy picked up by its
antenna. The amplified signal causes a load and, hence, a change in
the antenna's reflected RF energy.
The changes in absorbed or reflected energy at the PE antenna,
can be picked up by a sensitive receiver in the Listening Post (LP),
which operates on the same frequency. This means that the TX and RX
antennas at the listening post have to be separated properly in order
to reduce spillover. 2 The received (weak) signal is mixed
with a fraction of the transmitter's signal, to recover the
audio signal from the PE. This principle is known as autodyne
or synchronous detection. Today it is known as direct conversion.
Note that for this to work, both radio
signals have to be in-phase.
The first working version of the EC I was ready in the second half
of 1955 and was demonstrated to the CIA in October of that year.
It opened the door to future developments, as the CIA ordered six complete
sets right away and awarded the NRP with a long-term contract for further
The first complete EC I sets were delivered to the CIA during the course
of 1956, and marked the beginning of a long line of
Easy Chair equipment and other bugging devices.
A year after the the introduction of the EC I,
it was succeeded by the EC Mk II
which offered two-way communication, and a year after that by the
EC Mk III
which had an improved range and an improved enclosure.
In this context, passive means that the device doesn't need its
own local power source, but that instead it is powered by a strong RF
signal aimed at it. It does however contain active components such
as transistors. This is different from the
Russian resonant cavity microphone,
which did not contain any active parts.
In a radio system, spillover is the excessively strong RF signal
from a transmitter that is leaking into the input of the receiver,
where it can potentially overload or even damage the receiver.
The image below shows the Passive Element (PE) of the Easy Chair I,
as it was installed at the Target Area (TA). Two isolated silver plated
2 mm thick wires are used to form an open dipole antenna. At the center of
the dipole (i.e. at the feed point) is a small perspex 1 box that contains
the detector diode, also known as the crystal, plus a few passive components.
At the bottom of the box are two screw terminals to which the red/blue
wires from the amplifier are connected.
At the other end of the wires is a 3-stage amplifier, that is
fed by the energy that is picked up by the antenna. Audio from a
small hearing aid microphone is amplified and causes
a load (actually a mismatch) to the antenna. Think of the entire
system from LP to PE, as a long transmission line. The amplified
microphone signal effectively amplitude modulates (AM) the silicon crystal,
causing small changes in the amount of incident energy that is absorbed,
or reflected back to the LP.
The image above shows three design variants of the PE. At the far right is
the 3-stage amplifier with integrated microphone described above.
It is housed inside
a milled-out perspex box that can be opened if necessary. At the left is
the same device but without the microphone, potted in black expoxy, so that its contents are no longer immediately visible. At the centre is the same variant
potted in semi-transparent epoxy. This was probably a prototype. The potted
versions are connected to the microphone and the detector by means of
standard hearing aid cables.
Note that the CS2A crystal is a very
sensitive part. When in transit, it has to be protected against excessively
strong RF signals by
twisting a wire around the antenna elements.
The wire causes a capacitive coupling between the
two antenna arms, which shorts-out the RF energy.
Furthermore, the antenna wires were usually bent during transit, as shown
in the image above. When installing the device, the (black) antenna wires were
unfolded and the (red) protective wire was removed.
Perspex is one of the trade names of Polymethyl Methacrylate (PMMA), also
known as acrylic or acrylic glass. It is also known by other
trade names, such as Plexiglass, Lucite and Acrylite.
The diagram below explains how the system works. At the left is the
Listening Post (LP), which consists of a powerful transmitter and a
each connected to a directive antenna with a
high gain. The antennas have to be separated properly in order to
avoid overloading of the receiver.
At the right is the Target Area (TA) in which a concealed
Passive Element (PE) is present.
The TA is typically located between 50 and 100 metres from the LP,
and it is important that any obstructions are avoided whenever possible.
The antenna of the transmitter at the LP is aimed at the PE, where
a fraction of its energy reaches the PE's dipole antenna. This energy
is rectified and powers a microphone amplifier which
in turn loads the antenna. This results in changes in the amount
of energy that is absorbed or reflected by the dipole antenna.
The resulting Amplitude Modulation (AM) is subsequently picked up
and demodulated by the sensitive receiver at the LP.
Development of the EC I started in 1954, about two years after the Americans
had discovered a novel listening device
in the study of the American Ambassador
in Moscow. As it was initially unclear what the operating principe of the
bug was, it was nicknamed The Thing
and various US agencies investigated
it. After the main FBI-led investigation was completed in December 1952,
the CIA started its own research project under the
CIA engineers built a number of replicas of The Thing, to see if the
technology could somehow be useful to them.
During the course of 1953, the CIA contacted the
Dutch Security Service (BVD)
with the request to put them in contact with a high-tech scientific laboratory
to research and possibly duplicate The Thing.
It was thought that the Philips corporation
might be interested, but Philips declined,
stating that they were not interested in small research.
The director of the BVD, Louis Einthoven, then turned to his wartime friend
Joop van Dijk who had just opened the
Dutch Radar Laboratory (NRP)
in Noordwijk to bring the country up to speed with the latest
After several meetings at the BVD, van Dijk agreed to work for the CIA
and introduced the latter to his staff in Noordwijk during the course
of 1954. In July 1955, the CIA gave the NRP a replica of
along with a full scientific description
and a suitable countermeasures receiver .
Initially, the NRP would only do scientific research for the CIA,
partly based on the wishes of the CIA and partly on their own initiative.
It was the intention to produce any equipment in the US. More often than
not however, production of the actual bugging devices, activation transmitters
and receivers, was left to the NRP.
Due to the highly secret nature of the work, most of it took place
in the evening hours, separate from the regular work
on radar technology. The main goals were to establish
the Thing's operating
principle, and to develop useful systems based on it.
Although the NRP was unable to produce a working copy of
The Thing1 ,
they came up with good alternatives that produced similar results, and that
could be built right away with the transistor technology that had
just become available. After a successful demonstration in October 1955 at the
Contracting Group of the CIA in Washington, the latter awarded the NRP
an order for six complete EC Mark I sets — plus a long-term contract
for further research and development.
Ten years later, in 1965 the NRP was able to demonstrate a mature and
robust version of the
resonant cavity microphone (The Thing),
but by that time the CIA had lost interest in that type of bugging device.
The Passive Element (PE) of the Easy Chair Mark I (EC I) consists of two units:
the antenna with the crystal detector, and a separate unit with the microphone and
the amplifier. Several constructions and variations were tried and suggested,
leading to two major versions: a dipole and a monopole solution, both of which
are further described below. Remember that at this time the antenna was placed
horizontally (i.e. horizontal polarisation), as it was believed that this made
the signal less sensitive to (vertically) walking people in the bugged room,
at the price of making it directional.
The block diagram above shows how the EC I works. At the top is the horizontally
placed dipole that picks up part of the strong RF signal from the Listening
Post (LP). The signal is rectified with a CS2A diode
and the resulting DC voltage is used to feed the electronic circuit
at the bottom.
The electronic circuit is a 3-stage amplifier, that amplifies the sound that is
picked up by the microphone. In the last stage, the amplified signal is used
to cause load on the power lines and, hence, cause a load or
mis-match on the antenna. As a result, this will cause variations in
the amount of energy that is aborbed or reflected by the
antenna. At the LP, the (weak) reflected signal is mixed with (part of) the
original activation signal, in order to retrieve the PE's audio.
In the most ideal situation, an open dipole was used, with the detector diode
at the feeding point and two chokes blocking the RF energy from the DC output
lines. Two design variations are known, both of which are shown below.
The first one is embedded in a wooden rod that is open at the centre.
The two dipole elements are made of insulated silver-plated wire.
Insulation is necessary to reduce the dielectric effects of the wood somewhat.
Two soldering tags are available at the bottom
for connection to the mic/amplifier, which may be placed up to 10 metres
Note that the dipole is fore-shortened in order to compensate for the
dielectric effects of the insulation and the wooden rod, but that it is
longer than expected in order to match the impedance of the crystal and
also to compensate for the stray capacitance of the crystal.
The more popular second variant is shown below. It consists of a small
at the centre, from which two isolated wires extend sideways.
These two wires form a dipole in free space and are therefore longer than with
the previous variant (A).
The advantage of this model is that it can be embedded more easily into a
concealment, such as a piece of furniture, as the dipole elements may be bent
in order to fit, without affecting the antenna's properties too much.
In this variant, the detector and the two RF chokes are housed in a transparent
cabinet which is
milled out of a solid piece of perspex.
The antenna is placed horizontally, and the microphone / amplifier is either
mounted directly to the contact pins of the detector, or via a wire pair that
from the centre of the antenna.
The wire pair may be up to 10 metres long.
A third variant – shown below – was offered as an alternative to the two-element
dipole above. It can be seen as an off-centre-fed dipole of which
the amplifier's enclosure forms one arm. The other arm is a silver-plated rod
that can be removed in order to protect the crystal.
The advantage of this model is that it can be placed horizontally as well as
vertically, but its performance is less good than that of
the (A) and (B) variants. With the later
Easy Chair Mark 3 (EC III),
the amplifier was embedded in one of the arms of the dipole antenna.
It is unlikely that the Model C variant was actually used, as it does not
appear in the final EC I technical manual .
With the EC I, and also with the later EC II,
the microphone and the amplifier were always located in a separate plastic
enclosure, except for the Model C variant,
which was an all-in-one solution in a metal case (but was probably
never used in practice because of its inferior performance).
For the A
and B variants, there were two options:
an amplifier with a separate microphone, and one with
a built-in microphone as shown here.
In both cases, the amplifier was connected to the antenna/detector
by means of a two-wire cable that had to
run perpendicular from the
centre of the antenna. This means that in most situations,
the antenna had to be placed horizontally, which
limited the number of possible concealments. It was difficult, for example,
to hide the EC I inside a table leg, as in that case the wiring
had to run in parallel with one of the arms of the antenna.
The PE amplifier was suitable for virtually any type of high-impedance
Although suitable microphone elements were available from
a variety of manufacturers, such as
this one (made in the UK), in practice
they were often too large to be fitted inside a concealment.
For this reason it was decided to use the microphone elements and other
parts from electronic hearing aids of the era. Although such hearing aids
are extremely large by today's standards, they were advertised as
miniature at the time.
In the amplifier shown above, the microphone is held in place by
a piece of green felt.
The coils and transformers, also from Fortiphone,
are located at the centre,
between the microphone and the transistors.
The amplifier shown above measures just 55 x 25 x 15 mm, and should be
seen as very small for the era. Nevertheless it appeared to be very
difficult to hide the device inside a concealment, such as a piece of
furniture, as one had to drill a hole of more than 25 mm for it.
Another problem of the version shown above is that its components are
clearly visible and can be accessed by opening the perspex box. As this
would help an adversary in case the bug was discovered, it was decided
to cast the entire circuit in black epoxy, as shown on the right.
In this case, thin hearing aid cables are used to connect the microphone
and the detector. Note that for this variant, the detector needed a 2-pin
socket rather than the screw terminals shown above. Furthermore it was
important to maintain the correct polarity of the interconnection cable.
This was achieved by adding coloured dots to the plugs and the sockets.
The red dot was used for the connection of the detector, whilst a blue
dot was used for the microphone. Using pre-assembled hearing aid cables,
made installation of the bug much more convenient and faster.
The circuit diagram of the PE is remarkably simple. It consists of two
parts: (1) the antenna with detector, and (2) the audio amplifier. The
microphone can be integrated with the latter part, but was sometimes
connected separately. We will first examine the detector, which is nothing
more than a half wave open dipole with a
CS2A detector diode, or
crystal, fitted across its feed point.
When a strong RF signal is beamed at the antenna, a small DC voltage
will become available at its terminals, pretty much like
in a simple field-strength indicator. This energy is just enough to feed
the audio amplifier below. Two RF chokes are fitted in series with the
terminals, in order to block any RF energy. We now have a blue (-) and a
red (+) terminal for connection to the amplifer.
The audio amplifier is built around three Philips OC71 PNP transistors,
and is designed for low-current operation.
The first two transistors actually amplify the microphone signal,
whilst the last one is an emitter-follower which causes a load
on the power lines and, hence, the antenna.
This change in load causes small variations in the amount of incident energy
that is absorbed or reflected by the antenna, the result of which
can be picked up by a sensitive receiver at the LP.
The activation transmitter, also known as the actuator, consists
of two valve-based stages: an oscillator built around a QQE06/40,
and a power amplifier (PA) built around another QQE 06/40. The latter produces
an output power of 30 Watts, which is fed into a 4-element
directional antenna with a gain of 12.5 dB, resulting in an effective
radiated power (ERP) of ~530 Watts .
Mechanical vibrations of the individual elements inside the valves
and also in the tuned circuits,
may cause an undesired kind of amplitude modulation (AM), known as
microphony. As the faint signal reflected by the PE is also
amplitude modulated, transmitter microphony is intolerable.
An extra valve-based circuit is therefore added to eleminate
any AM component in the output signal.
It rectifies (part of) the output signal, amplifies it, and uses it
as a negative feedback into the PA.
The transmitter has a built-in power supply unit (PSU) that is powered
directly from the 110V or 220V AC mains, so that it can be deployed in
nearly any part of the world. The PSU is fully solid-state and delivers
the LT (6.3V) and HT (-500V and +500V) voltages for the transmitter valves.
The design of the receiver is very simple, yet extremely effective.
The signal from the antenna is rectified by a crystal detector and then
amplified several times. Next, suitable high-pass and low-pass filters are
inserted so that only the audio spectrum of 400 Hz to 7000 Hz remains.
A switch is available to select a 4000 Hz cut-off.
The resulting signal is then amplified to speaker level.
Due to the simplicity of the input circuit, plus the fact that there is
no RF pre-amplifier, the receiver does not suffer from blocking
as a result of the strong RF signal from the transmitter. Although
TX and RX antennas have to be isolated properly, the circuit works by the
virtue that some of the RFspillover from the transmitter
reaches the receiver's detector, where it stimulates detection and
improves the sensitivity. This process is known as homodyne or
synchronous detection. Although this works very well,
the signal phase is critical, which means that in practice one had to
adjust the position of the antennas in order to obtain the best result.
The transmitter and receiver each had its own directional antenna which
was aimed at the target area. The antenna consisted of two radiating
elements (A and B) that were mounted in front of a metal sheet reflector.
Each radiating element consisted of two cross-connected folded dipoles.
The diagram above shows the construction of the antenna, which is known
as a combined broad-side end-fed array with four driven elements. The
two sections are interconnected by a so-called Magic-T, or coaxial-T,
which provides a 50Ω impedance at all three ports of the 'T'.
The antenna is optimised (peaked) for 376.5 MHz and provides a gain of
approx. 12.5 dB. This means that in the main direction, it amplifies
the power output of the transmitter by more than 17 times.
As an identical antenna is used for the receiver, the same 12.5 dB gain
is applicable to that path as well.