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EASYCHAIR Mark II - Carrier Pigeon

Easy Chair Mark II, also known as EC Mk II or EC II, was a passive covert listening device (bug), developed in 1956 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). It uses the same Passive Element (PE) as the EC I, but adds bi-directional communication to the concept [B].
In addition, EC II also introduces a duplexer that allows transmitter and receiver to be connected to the same antenna. At the same time a 2-bay 4-element Yagi antenna is introduced, which offers a gain of 15 dB. This partly compensates for the coupling loss caused by the duplexer.

The reason for adding two-way communication is currently unknown, but it was probably done to allow the operative who was covertly installing the Passive Element (PE), to receive instructions from the Listening Post (LP), allowing him to test the PE and find the best possible position for it.
EC II detector, amplifier, T-adapter and earpiece

Adding two-way communication required quite a bit of additional circuitry, both in the activation transmitter (actuator) and in the receiver at the LP. At the target area (TA), the operative only had to connect a pair of headphones in parallel to the terminals of the crystal detector. This way, the operative received instructions from the LP, but he could also hear himself, which indicated that the PE was powered properly. Once the PE was installed, the headphones were removed again.

Two-way communication was developed for the CIA under a separate research contract with the name CARRIER PIGEON (CP). It is likely that it was intended for one, or perhaps a few, specific CIA operations, as it's features are not found on any of the later systems. In 1958, the EC Mk II was succeeded by the much improved Easy Chair Mark III (EC III), which had a fully redesigned PE.
Easy Chair II passive element (PE) ready for transport Easy Chair II passive element (PE) T-junction for connection of headphones High-impedance crystal earpiece EC II detector with TELEX hearing aid plug TELEX plug connected to the EC II detector EC II detector, amplifier, T-adapter and earpiece Detector, amplifier and earpiece

The EC II features the following improvements over the EC I:
  • Duplex unit (single antenna)
  • Redesigned activation transmitter
  • Two-way communication
  • Audio muting (pilot tone)
The diagram below shows how an EC II installation works. At the left is the listening post which consists of an activation transmitter (the so-called actuator), a receiver, a duplexing unit and a suitable antenna array. As the EC II system is suitable for two-way communication, a microphone with additional circuitry is added to the transmitter, and a muting switch is added to the receiver.

At the Target Area (TA), a Passive Element (PE) is installed. The PE is identical to the one used with the EC I system, with the only addition that a pair of headphones could be connected in parallel to the line between the detector (antenna/crystal) and the amplifier. It allowed the operative to receive instructions from the LP during installation of the PE. Once installation had been completed, the headphones were removed and the PE could be used as in the EC I system.
Passive Element (PE)
The Passive Element (PE) used at the target area of an EC II installation, is identical to the PE of the earlier EC I system. The circuit diagram is unchanged, but only the Model B production variant was available. This was the one that is housed inside a perspex enclosure, as shown below.
It came with a three-stage transistor amplifier that was housed in a separate perspex case, optionally with a Fortiphone FM5 microphone fitted inside. Unlike the EC I however, the EC II detector did not have regular screw terminals.

Instead the terminals were a receptacle for a standard TELEX miniature plug of the era, such as the ones that were used on hearing aids. The image on the right shows an EC II detector with a TELEX plug fitted. Note the presence of a red dot on the plug, which has to be aligned with the red dot on the detector case for proper operation.
TELEX plug connected to the EC II detector

The reason for changing the terminals, is the fact that in the EC II system it was possible to connect a pair of high-impedance headphones in parallel to the line that connects the detector to the amplifier. This was done by inserting a T-adapter between the detector and the amplifier.
The T-adapter is also known as a Y-cable or a splitter cable. Initially the adapter consisted of three twisted wire pairs, connected in parallel, with TELEX plugs at the end. One of these plugs was later swapped for a 2 mm jack socket, in order to allow the headphones to be removed more easily once the PE setup was completed.

In the latter case, the T-adapter was commonly left in place after the headphones were removed. The image on the right shows the later variant of the T-adapter. It consists of a jack socket and 2 cables for connection of detector and amplifier.
T-junction for connection of headphones

Note that a 1µF capacitor is mounted to the rear of the jack socket, in order to block DC currents to the headphones. The twisted cables with TELEX plugs were usually ordered as spares from companies like Telex, Fortiphone, Maico or Audium. The sockets for these plugs were machined at the NRP, and were constructed in such a way that they could also be used as solder terminals. For further details about the PE, refer to the description of the Easy Chair Mark I Passive Element.

 More information
Easy Chair II passive element (PE) ready for transport Easy Chair II passive element (PE) T-junction for connection of headphones High-impedance crystal earpiece EC II detector with TELEX hearing aid plug EC II detector, amplifier, T-adapter and earpiece EC I amplifier and microphone - top view TELEX hearing aid cables

The transmitter basically consists of two functional parts: a rock solid valve-based transmitter, and a transistor-based speech modulator with pilot tone generator. The transmitter is crystal-driven and produces an adjustable output power between 0.4 and 40 Watts at approx. 378 MHz. The first stage is built around an E180F that is used as oscillator and tripler. It is followed by two further triplers with QQE 03/12, a QQE 03/20 driver stage and finally a QQE 06/40 exciter (PA).

The upper half of the diagram shows the audio modulator, which amplitude modulates speech, and injects a 20 kHz pilot tone. The resulting signal is directly injected into the exciter. In the receiver, the 20 kHz pilot tone is used for muting the audio circuit in order to avoid howlround.

During speech intervals, the signal-to-noise ratio of the transmitted signal should be as high as possible. For this reason, a voice detection circuit (VOX) is present, as well as a negative feedback from the antenna signal. The VOX circuit interrupts the audio path and controls the pilot tone.
The receiver is a straightforward AM detector, followed by several amplifier stages in order to deliver the signal to a loudspeaker or a pair of headphones. The signal from the first amplifier is also used for detection of the 20 kHz pilot tone that controls an audio muting switch. As an extra feature, there is a negative feedback loop between 3rd and the 2nd amplifier in order to obtain a bass boost. This is done to compensate for deficiencies in the microphone element of the PE [B].

The receiver is housed in a separate ecnlosure and is fully self-contained. It is powered by dry battery cells that last long due to the low power requirements of the receiver. Audio is available through an internal speaker, or via two 6.3 mm jack sockets, one of which mutes the speaker.
The duplexing unit consists of a coaxial directional coupler with two tuners in the arm that is connected to the antenna, as shown in the diagram below. They are spaced by exactly 1/8λ. The advantages of a tuner over an ordinary stub, are the smaller size, the larger bandwidth and the absence of sliding contacts which would cause severe crackling during the tuning procedure.

With this setup it is extremely important that the transmission path (i.e. the antenna and the two tuners) is adjusted for minimum reflection before the receiver is connected, in order to avoid strong and potentially harmful signals at the receiver input. Furthermore, any objects in front of the antenna that could potentially cause strong reflections (such as cars passing by) should be avoided whenever possible. At the operational frequency of 378 MHz, the specifications are:
  • Coupling
    -5.8 dB
  • Insertion loss
    1.7 dB
  • VSWR
  • Directivity
    -31.4 dB
  • Isolation
    37.2 dB
Antenna matcher
The image on the right shows the antenna matching unit that is visible at the top right of the block diagram above. It was unique in that, unlike a common stub tuner, it had no sliding contacts which caused contact noise. It was suitable for the 300-500 MHz frequency range.

This tuner is probably the only surviving part of an Easy Chair Mark II listening post. At the center frequency (400 MHz) it had an insertion loss of just 0.03 dB [C].

 Read the full description

Due to the use of a duplexer, the EC II uses a single antenna for transmitter and receiver. In order to compensate for the losses of the duplexer, the antenna gain has to be as high as possible. For this reason, two 4-element Yagi antennas are used, coupled by a coaxial T-bar with built-in 1/4λ transformers, in order to guarantee a 50 Ω match at all times.
  • Gain
    15 dB
  • F/B ratio 1
  • Frequency
    378 MHz
  • Bandwidth
    30 MHz
  1. F/B = front-to-back ratio.

Top view Input terminal Output termina; Adjusting one of the knobs Double 4-element Yagi antenna with Magic-T coupler

  1. Easy Chair Mark II, Part A - Operational Manual
    CM302532/A, June 1956 (est.). Missing

  2. Easy Chair Mark II, Part B - Technical Manual
    CM302532/B, June 1956 (est.).

  3. A new Stripline Impedance Matcher
    NRP. Date unknown, but probably 1956.

  1. NRP/CIA, Collection of documents related to Easy Chair Mark II
    Crypto Museum Archive, CM302532 (see above).

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Crypto Museum. Created: Friday 10 March 2017. Last changed: Sunday, 19 March 2017 - 08:17 CET.
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