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EASYCHAIR Mark I - passive element

Easy Chair Mark I, also known as EC Mk I or EC I, was a passive 1 covert listening device (bug), developed in 1955 by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term development contract under the name Easy Chair (EC). The device was powered electrically by a strong RF signal aimed at it from a nearby listening post (LP) [1].
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.
Detector (left) and amplifier (right)

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 research.

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.
  1. 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 source 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.
  2. 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.

Amplifier connected to detector EC I detector with amplifier connected perpendicular PE ready for transport Detector (left) and amplifier (right) Detector with red/blue wires to amplfier EC I amplifier and microphone - top view PE cast in transparent epoxy Part of the EC I antenna array

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.

Complete Easy Chair I passive element (PE)

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.

Three versions of the PE

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 transparent epoxy. The latter was probably a prototype. The potted versions are connected to the microphone and the detector by means of standard hearing aid cables.

PE ready for transport

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.
  1. 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 sensitive receiver, 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 codename EASYCHAIR. CIA engineers built a number of replicas of The Thing, to see if the technology was somehow 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. Philips declined however, 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 post-war radar developments.

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 The Thing along with a full scientific description, plus a suitable countermeasures receiver [4].

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 Thing 1 , 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.
  1. 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.

Passive Element   PE
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.

Bare detector with open dipole antenna

Model A
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 away.

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.
Model B
The more popular second variant is shown below. It consists of a small perspex block 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 runs perpendicular from the centre of the antenna. The wire pair may be up to 10 metres long.
Model C
As an alternative to the two-element dipole shown above, the version below was offered as an alternative. 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 [3].
Bare detector with open dipole antenna Detector with protective (red) wire installed Detector with red/blue wires to amplfier A view from the top side Close-up of the crystal Red and blue wires connected to the detector Detector compared to the size of a hand Empty perspex enclosure for EC I detector

Later models
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, that was probably never used in practice because of its limited 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.
Easy Chair Mark I with Fortiphone FM5 microphone

The PE amplifier was suitable for virtually any type of high-impedance dynamic microphone. 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.

A popular hearing aid was sold by Fortiphone in the UK and its dynamic FM5 microphone element was ideally suited for use in a PE. Measuring just 21 x 21 x 8 mm, it could easily be placed inside the enclosure of the amplifier. The parts were ordered from Fortiphone as hearing aid spares, which were taken apart at the NRP laboratory.
Fortiphone FM5 dynamic microphone

In the amplifier shown here, 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.
Black potted PE with external microphone and detector

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 black dot was used for the microphone. Using pre-assembled hearing aid cables, made installation of the bug much more convenient and faster.
Easy Chair Mark I with Fortiphone FM5 microphone EC I amplifier - bottom view EC I amplifier and microphone - top view Amplifier transistors and capacitors Inductors and transformers at the centre Close-up of Fortiphone FM5 microphone Amplifier connected to detector EC I amplifier compared to the size of a hand
EC I detector with amplifier connected perpendicular Fortiphone coils and transformers EC I amplifier with two different dynamic microphones Fortiphone FM5 microphone showing its sound port Fortiphone FM5 dynamic microphone Fortiphone FM5 compared to a regular dynamic microphone Regular dynamic microphone element Fortiphone FM5 microphone compared to the size of a hand
Black potted PE with external microphone and detector Easy Chair Mark I PE potted in black epoxy Close-up of the connectors (left: microphone, right: detector) PE cast in transparent epoxy Sockets on the PE Top view Bottom view Black and transparent variants side-by-side

Circuit diagram
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 [5].

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 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 or over-loading 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 RF spillover 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.
Part of the EC I antenna array Another view of the antenna array Single dipole array assembly General Radio 'sex-less' connector Colour marking on the antenna to indicate the correct phase Dipole antenna detail Magic-T coaxial transformer Two ports of the Magic-T

  1. Easy Chair Mark I, Technical Manual
    CM302531/A, Prototype December 1955.

  2. EC Mk I, Part A - Operational Manual
    CM302531/B, March 1956 (est.).

  3. EC Mk I, Part B - Technical Manual
    CM302531/C, March 1956 (est.).

  1. NRP/CIA, Collection of documents related to Easy Chair Mark I
    Crypto Museum Archive, CM302531 (see above).

  2. Gerhard Prins, Letter to his heirs
    Date unknown, but probably written shortly before his death in April 1993.
    Vertrouwelijk (confidential). Published by [3].

  3. Maurits Martijn & Cees Wiebes, Operation Easy Chair
    De Correspondent. 24 September 2015.

  4. CIA Contracting Group, Report on Research on EASYCHAIR
    14 July 1955. Classification status unknown. Not marked as secret.

  5. Wikipedia, Effective radiated power
    Retrieved February 2017.

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Crypto Museum. Created: Friday 10 March 2017. Last changed: Thursday, 23 March 2017 - 15:55 CET.
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