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EASYCHAIR Mark IV - high power

Easy Chair Mark IV, also known as EC Mk IV or EC IV, was a passive covert listening system (bug), developed in 1961 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 system is similar to its predecessor, the EC Mk III, but introduces a fully integrated Listening Post (LP) [1].

Unlike the EC III, which used a single antenna at the listening post (LP), the EC IV uses separate antennas for transmitter and receiver. This was done to improve suppression of the reflected transmitter signal in the receiving path and to extend the range of the system. Furthermore, it allowed the output power of the transmitter to be enhanced with an (optional) linear amplifier.

Instead of the EC III's duplex filter, the EC IV had a cancellation unit for keeping the transmission signal away from the receiver's input, based on a sophisticated integrated directional coupler [A].
Circuit diagram of the EC-IV transmitter

Like its predecessors, the EC IV operated on 378 MHz, and featured the same Passive Element (PE) as the EC III for installation at the target area (TA). The complete base station was packed in two large unobtrusive suitcases, one of which contained the transmitter cabinet. The other suitcase contained the power supply unit (PSU) and accessories (cables, headphones, spare fuses, etc.).

The EC-IV was short-lived, as it was replaced a year later, in 1962, by the completely redesigned and much improved Easy Chair Mark V (EC V), of which the development had already been started in 1960. For this reason, the EC-IV should be seen as a transitional or temporary solution, that was used until the new EC-V was ready. The EC V introduces an increased operational range, a much improved performance and the simultaneous use of multiple PEs in the same target area.

Compared to the earlier Easy Chair system, the EC IV introduced the following changes:

  • High power activation transmitter
  • Separate transmit and receive antennas
  • Redesigned receiver
  • Cancellation unit
The diagram below shows the configuration of the EC IV system. At the left is the Listening Post (LP) that consists of a transmitter, a receiver and a cancellation unit, all housed in the transmitter cabinet, plus two identical antennas, one for the transmitter and one for the receiver. Strong RF signals from the transmitter, reflected on buildings and other objects, are compensated for in the cancellation unit. A small pick-up loop in the vicinity of the transmitter antenna, delivers a small portion of the signal to the receiver's detector for use as the local oscillator (LO) signal.

At the right is the Target Area (TA) where the Passive Element (PE) is installed. The PE is powered by the energy picked up by its antenna and rectified by a detector diode, or crystal. It amplifies the sound that is picked up by its miniature microphone and Frequency Modulates (FM) it onto a 100 kHz subcarrier (SC). The resulting signal acts as a load to the antenna, which causes changes in the amount of energy absorbed or reflected by the antenna. At the LP, the small changes in reflected energy are used to recreate the original sound and deliver it to a pair of headphones.

EC III Passive Element (PE)

Passive Element
In the EC IV system, the Passive Element (PE) of the EC III is used at the Target Area (TA). This device does not need a local power source, but is energized by a strong RF signal beamed at it from the nearby Listening Post (LP).

The PE consists of a half wave dipole antenna with a thin and a thick arm, and a diode detector at the centre. The thick part contains a small transistor amplifier, which modulates the signal from an external microphone onto a 100 kHz subcarrier using Frequency Modulation (FM).

 More about the PE

The two halves of the PE dipole

The basic transmitter, or exciter, was identical to that of the EC III, and produced a variable output power between 1 and 50 Watts. This output power could be supplied directly to the transmitting antenna, or to a linear amplifier, also known as a power amplifier (PA), to any desired higher power value, probably in the range of 300 to 500 Watts. Here is the block diagram:

The transmitter is completely built with valves (tubes). It consists of a free-running oscillator that is tuned to the target frequency of approx. 378.5 MHz, just like with the EC I and EC III systems. A small portion of the energy from the antenna is extracted by means of a pickup loop that is placed in the vicinity of the transmitter, and fed directly to the Local Oscillator (LO) input of the receiver, where it is mixed with the incoming signal from the Passive Element (PE).

The receiver is a hybrid of valve and transistor technology. The first two stages - the AM diode detector and the first amplifier - are built with valves, as they are more tolerant to excessive signals. The remainder of the receiver (i.e. the IF and AF stages) are built with silicon transistors.

In many respects, the receiver can be seen as a redesign of the EC III receiver. The first two stages (here shown in blue) are built with valves (tubes), whilst the remainder of the receiver is built with transistors. For some reason, the double reception system with 90° phase shift (for obtaining phase-indifference) was dropped from the design. It would later be re-introduced in the EC V.

Cancellation unit
The diagram below shows the construction of the cancellation unit, which is the most important part of the EC IV. It consists of a double-deck stripline transmission circuit, with the straight-through connection from antenna to the receiver at the top (green). At the bottom left (red) is the cancelling signal, that is fed in from the pickup nearby the transmission antenna. The majority of this power is dissipated in the 50Ω terminator at the bottom right (R4), but a fraction is coupled into the secondary branch, where most of it is dissipated in the 50Ω terminator resistor R3.

By causing a small mismatch with the tuning capacitors, a fraction of the energy in the secondary branch is reflected and delivered at the top right, at the input of the 7dB directional coupler, where most of it is dissipated in R2. A small fraction of this energy however, is coupled into the upper branch, where it is added to the antenna signal and delivered at the receiver's input. A second function of the tuning capacitors in the front deck, is to control the phase of the signal that delivered at the receiver's input, and hence, cancel out the majority of transmitter spillover.

Note that a small portion of spillover has to be allowed, in order for the receiver's detector to function properly. Two meters are provided for measuring the amount of spillover (top left) and the amount of cancelling power (bottom right). They should roughly show the same value.

At this time, no specific information is available about the antennas that were used at the LP. It is likely however, that they were similar to the antennas used with the later Easy Chair Mark V system. Basically, each antenna consists of a 5- or 6-element Yagi with a corner reflector. Two or four such antennas were sometimes combined using a Magic-T, or coaxial-T, in order to obtain a higher gain. For further information, please refer to the EC V's LP antennas.

 More about the antennas

  1. Easy Chair Mark IV - Handbook
    CM302534/A, 30 March 1961.
  1. NRP/CIA, Collection of documents related to Easy Chair Mark IV
    Crypto Museum Archive, CM302534 (see above).
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Crypto Museum. Created: Friday 10 March 2017. Last changed: Thursday, 22 October 2020 - 19:09 CET.
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