Spy radio
Burst encoders
Dead Drop
Lock picking
• • • Donate • • •
   Logo (click for homepage)
EASYCHAIR Mark V - multi-channel

Easy Chair Mark V, also known as EC Mk V or EC V, was a passive covert listening system (bug), released in 1962 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). EC V is similar to its predecessor EC IV, but introduces the simultaneous use of up to three Passive Elements (PE) [1].
The EC V was the result of a number of customer requirements and several research experiments in which each part of the existing concept was revisited. It was developed between 1960 and 1962 and was in fact a complete overhaul of the earlier systems. It offered superior performance and stability, and was in use until at least 1967.

The image on the right shows the redesigned EC V Passive Element (PE), which looks similar to the PE of the earlier EC III system, but contains a new internal design. The three wires extending from the bottom are for connection of the microphone.
Three different types of PE

The Listening Post (LP) consists of a powerful 500W transmitter operating at 378 MHz, with an external antenna matching unit, a very sensitive receiver with spill-over cancelling, and separate antennas for receive and transmit. In order to speed up the development, the transmitter, and probably also the antennas, were built to NRP specifications by an unknown third party in the US.

A complete EC V system was extremely big and heavy, and was transported in several suitcases of different design and make, in order to attract the least attention. The receiver rack came in three suitcases, whilst the transmitter, the accessories and the two antennas required several more.

The EC V was the last system that used passive elements at the target site. Although modifications were made in later years, including a completely redesigned PE, the CIA eventually made the move to active target elements, such as the SRT-52, which featured advanced audio masking facilities.
Three EC V passive elements with microphone EC V Passive Element (PE) Body of the Thin PE compared to the standard PE Rectangular version of the thin PE Three different types of PE One of the EC V antennas Test setup for the EC V. The suitcase on the right contains the (externally built) 500W transmitter EC V receiver. At the front left is the remote control box of the transmitter EC V receiver rack

The diagram below shows the new version of the Passive Element (PE), known as the EC Mark V PE. The slimline design is nearly identical to that of the EC III PE, and was ideally suited for embedding inside a piece of furniture, such as a table leg, and could easily be adapted to the environment.

The PE is in fact an open dipole, consisting of a tick and a thin element. The thicker part contains the electronic circuit, whilst the length of the thin part (the other half of the dipole) can be aligned in order to match the environment (air, wood, concrete, etc.). The interior is not easily accessible as the entire electronics circuit is potted in black epoxy. At the bottom end are three wires for connection of a low or high impedance microphone. It was commonly used with a Shure MC30.
Compared to its predecessors, the EC V offers the following improvements:
The diagram below shows how the EC V system works. A the left is the Listening Post (LP) which consists of a powerful transmitter (TX), a rather complex receiver and two directional antennas. The transmit antenna is aimed at the Target Area (TA) on the right, where it provides energy to one or more Passive Elements (PEs). As each PE uses a different subcarrier frequency (SC), they can be received at the LP simultaneously. Up to three PEs in the same target area can be used this way.

As the receiver in this system is much more sensitive than in the earlier concepts, plus the fact that the tranmitter is 10 times stronger than before, a separate cancelling unit is added in order to suppress any signal spill-over from the transmitter, before it reaches the receiver's pre-amplifier. The receiver has three individual IF/AF stages, one for each PE channel (i.e. subcarrier frequency).
Development of the EC V started in 1959 with several studies into the use of subcarrier signals, transmitters, seperate receivers, etc. This led to the conclusion that it was necessary to review the entire Easy Chair concept and reconsider every design decision that had been made in the past.
As in each Easy Chair system, the Passive Element (PE) that was covertly installed in the Target Area (TA), was the most important part of the concept. Over the course of 1959 and 1960, aided by the arrival of better components and subminiature transformers, the design of the PE was improved.

The result was a drastic increase in sensitivity, an improved temperature range, an improved power range (200-2500mV), and a much more stable sine-wave subcarrier frequency (±2kHz) over the entire voltage and temperature range. The sine-wave subcarrier also resulted in better secrecy.
Hoekstra testing the EC V. Note a PE in his hand, but also inside the left closet (taped to the right side).

When the first prototypes of the redesigned PE were available in 1962, they had to be tested under simulated real-life conditions, in order to avoid implementation difficulties, as had been the case with the EC III when bugging the Russian Embassy in The Hague (Netherlands) back in 1958.
For this reason it was decided to book a room in the nearby Hotel Belvedere, approx. 200 metres north east of the rear side of Villa Wave Guide. 1

Thijs Hoekstra, who in 1958 had been involved in the bugging operation against Russian Embassy, took position in the room and installed several PEs. The image above shows Hoekstra holding a PE in his right hand. Note that another one has already been installed inside the leftmost closet behind him, taped to the right separation wall. In the image on the right, Hoekstra watches a PE that has just been taped to the door at the right.
Hoekstra testing the EC V PE. Note the PE taped to the door at the right.

From his room at Hotel Belvedere, Hoekstra is able to see the rear side of Villa Wave Guide, where the antennas of the powerful activation transmitter and the sensitive receiver are located. From behind the windows of Villa Wave Guide, the antennas are aimed at Hoekstra's room at the hotel.
During the experiments, Hoekstra is continuously in contact with Gerhard Prins, the main Easy Chair developer at the NRP, via his 'portable' Motorola FHTRU (H13) 2 VHF radio telephone [5]. Inside the suitcase on the table in front of him is a simple field-strength indicator used in earlier Easy Chair experiments, which allows him to find the best possible position for installation of the PEs. 3

The image on the right is the last one that was taken at the test site. It shows Hoekstra on the phone with Prins, with the PE taped to the wall behind him, just to the right of the closet door.
Hoekstra testing the EC V PE. Note the PE taped to the wall behind him.

Judging from other photographs in this series, the pictures were taken in March or April of 1962, whilst the development of the PE had just been completed. The powerful 500 Watts activation transmitter was built in the US, whilst the sensitive and complex receiver was made by the NRP.
  1. Villa Wave Guide was the NRP's head quarters, located at the North Sea coast in Noordwijk (Netherlands).
  2. Judging from the length of the antenna it is the H13 model operating in the VHF-H band (approx. 153 MHz).
  3. This field-strength indicator is made from an EC I passive element with a meter connected to it.

Hoekstra testing the EC V. Note a PE in his hand, but also inside the left closet (taped to the right side). Hoekstra testing the EC V PE. Note the PE taped to the door at the right. Hoekstra testing the EC V PE. Note the PE taped to the wall behind him. Villa Wave Guide (NRP) seen from Hotel Belvedere Hotel Belvedere seen from Villa Wave Guide (NRP) Villa Wave Guide (NRP) seen from Hotel Belvedere Hotel Belvedere seen from the NRP


EC V Passive Element (PE)

Passive Element   PE
The Passive Element (PE) of the EC V does not have a local energy source, but is powered by a strong RF signal beamed at it from the nearby Listening Post (LP). It is based on the design of the EC III, but has been improved in a number of ways. First of all one transistor has been removed (now 3 transistors instead of 4) and transformers have been introduced between the stages [D].
Furthermore, the subcarrier frequency on which the audio is Frequency Modulated (FM) is now determined by a tuned LC circuit, which makes it more accurate and produces a sinewave shape. 1

In the EC V system, up to three PEs can be used in the same Target Area (TA) simultaneously. This was done by assigning a specific subcarrier (SC) frequency to each PE during the manufacturing process: 90 kHz, 120 kHz or 150 kHz, usually identified as L, M and H (low, mid and high). The SC frequencies are spaced by 30 kHz to allow for frequency variations due to temperature changes.
The two halves of the PE dipole

The image above shows a typical EC-V PE that consists of two halfs that can be screwed together at the center. Together, the two halves form a ½λ dipole antenna, whilst the thick half contains the electronic circuit. The detector diode (crystal) is located at the threaded end of the thick half.
The two halfs can be separated to save space when transporting the device and to protect the sensitive detector diode from excessively strong RF signals prior to installation. Three PEs were usually supplied with each EC-V system and their SC frequency (L, M or H) was printed on the serial number label at the center of the thick half.

The design of the new PE was finished in 1960 [C], but it was not used with the intermediate EC-IV system. Instead it was improved several times before it was taken into production at the end of 1962 [D]. The first units were delivered in 1963.
Close-up of the detector

The fact that multiple PEs could be used simultaneously in the same target area, was an enormous improvement. They could be powered by the same activation beam and allowed multiple rooms to be bugged and monitored in a single action. When two PEs were installed in the same room, it greatly improved the legibility of the signal (much like a stereo signal) which was a bonus when transcribing a conversation. In 1964, the design of the PE was upgraded once more, with the introduction of the transformerless PE [2].
  1. In the EC III, the subcarrier was more or less squarewave, making it produce a number of unwanted sidebands that could potentially disclose its presence. In the EC V this has been corrected.

Fortiphone coils and transformers Three EC V passive elements with microphone Two EC V passive element with microphone EC V Passive Element (PE) The two halves of the PE dipole EC V Passive Element (PE) Close-up of the detector

Circuit diagram
Although the new PE was based on the PE of the earlier EC III, the circuit was completely revisited, redesigned and modernised. The final result is shown in the circuit diagram below. Unlike the EC III, which consisted of four transistors, the new PE had just three, all of which are Philips OC44.

The first two transistors (T1 and T2) are the audio amplifiers. T3 is the subcarrier oscillator that now delivers a pure sinewave signal, modulated with the audio for the amplifier stages. Depending on the selected subcarrier frequency, some components are hand-picked during production. The components in the red section at the right, are selected manually during production in order to obtain the best matching of the antenna. The 1N416 crystal is more efficient that the old CS2A.

Thin PE   transformerless PE
Despite the fact that the PE was much smaller than the initial one that was used with the EC I and EC II, there was an ongoing desire to make it even smaller. Although its length is dictated by the frequency (after all it's an antenna), the standard version of the PE has a diameter of 13 mm, which is largely caused by the Fortiphone transformers that are the widest components in the unit.
This this reason a research project was started in 1962 for the development of a transformerless PE, which was intended to make the PE much thinner. The research took most of 1962 and 1963, but the goal was ultimately achieved. [2]

In January 1964, the new 'thin' PE shown in the image on the right was introduced. Is has the same length as the previous version, but is only 7 mm thick, which is nearly half the previous one. Unfortunately, the other half of the thin PE and the detector diode are currently missing from our device, which is why only one half is shown here.
Thin PE with detector and protective polythene centre part

The thin PE is fore-shorted for installation in wood, but can be extended for use in free space by adding extra elements to one end. According to the original development report [2], the other half of the thin PE was equally thick, and the two halves were separated by a polythene section that contained the crystal. Furthermore, four wires were running from one half of the antenna to the other, and there were no provisions for replacing the detector diode in case it got damaged.
The manual also states that the device was delicate, especially at the center, and that it had to be shipped in a special transport container.

A mechanically stronger solution was provided in the shape of a rectangular black botted variant of the transformerless PE, as shown in the image on the right. It measures 94 x 25 x 7 mm and weights less than 30 grams. It has two terminals for connection of a microphone and two flat contacts for connection of the antenna elements. The detector diode (crystal) is held in place by a screw, allowing it to be replace when damaged.
Rectangular version of the thin PE

Although the new design does not contain any transformers, it does have a couple of coils, one of which even has a tap. For the thin PE they had to be made equally thick to an OC44 transistor, or else they wouldn't fit the tight space. The new coils were purpose built by the NRP themselves.
Miniature coils and transformers One half of the thin PE Body of the Thin PE compared to the standard PE Thin PE with detector and protective polythene centre part Detector diode inside polythene tube Wires at the bottom Old detector versus new one PE marked 'L' (90 kHz subcarrier)
Rectangular version of the thin PE Flat variant of the thin PE Microphone terminals Antenna terminals Close-up of an antenna terminal Antenna terminal and crystal replacement

Circuit diagram
Below is the circuit diagram of the transformerless PE. The circuit contains one extra transistor, but the use of the fairly large Fortiphone transformers is avoided. T1 and T2 form the first amplifier stage, whilst the second stage has only one transistor (T3). Note that the design now includes one NPN transistor (T2). The 4th transistor (T4) is used as a temperature-dependent diode, which takes care of the base bias compensation of T1 and base bias stabilisation of T2. Furthermore, the base bias voltage of T2 is temperature-compensated by using an NTC resistor.

The last transistor (T5) acts as subcarrier oscillator and modulator. Its circuit is based on that of the standard PE, but has been modified to make it more temperature independent. This is done by moving the tuning capacitor from the collector of T5 to the base end, and by replacing part of it by a so-called hi-K capacitor that has a negative temperature coefficient (NTC). In this design, the 1N416 crystal has been replaced by an 1N53, which has a better efficiency and is much smaller.
The image below shows a collection of dynamic microphones that were used with Easy Chair equipment over the years. The second one at the top is the Shure MC-14 1 . The Shure MC-30, that was recommended for use with the EC Mk V, is the leftmost one on the second row. They were later superceeded by the Knowles BA-1501 and BA-1502, shown here at the front right.

 More about Knowles microphones
A collection of microphones that were used with Easy Chair equipment.

 Shure MC-30 datasheet
  1. No details about the Shure MC-14 microphone element are currently available. The device is not found in any vintage Shure catalogue, but is believed to be a variant of the Shure MC-11.

A collection of microphones that were used with Easy Chair equipment. Shure MC-14 Shure MC-14 Shure MC-30 Shure MC-30 (left) and Knowles BA-1501 (right) Knowles BA-1501 (left) and BA-1502 (right)

In order to shorten the development time of the complete Easy Chair V system, it was decided to have the transmitter developed and built by a third party in the US. It was agreed that the CIA would take care of this, whilst the NRP defined the functional specifications for the design [B].
The image on the right shows a complete EC V transmitter setup in one of the secret rooms at the rear side of the NRP building in March 1962, when carrying out the final tests of the EC V system and its new PEs. The heavy transmitter is housed in the large Skyway suitcase at the right.

The black square at the center is the rear side of the directional antenna, which is placed behind an opened window, aimed at the Hotel Belvedere test site. approx. 200 metres away. At the max. output power of 500 W and an antenna gain of 14dB, it delivered an ERP 1 of no less than 10 kW.
Test setup for the EC V. The suitcase on the right contains the (externally built) 500W transmitter

An output power of more than 500 Watts was considered unrealistic, as experiments had shown that it would hardly increase the operational range of the PE. A stronger signal also means that it could be detected more easily and might cause interference in domestic equipment. Furthermore, any spillover from the stronger transmission signal would be harder to suppress in the receiver.
In practice, spillover cancellation was a two-step process. First of all a small portion of energy was extracted from the transmitter's output by means of a directional coupler between transmitter and antenna, that was used as an antenna matcher. It is shown in the block diagram below in yellow.

This unit supplies a cancellation signal (CANC) that is used by the receiver's cancellation unit to suppress excess spillover. The image on the right shows the antenna matching unit, which is visible in the image above in the red circle. It is probably the only surviving part of the EC-V listening post.
Easy Chair Mark V - antenna matching unit

In the block diagram below the construction of the transmitter is illustrated. At the top left is the crystal-driven oscillator/exciter which produces an output power of approx. 2.5 Watts. Included in the exciter are several multiplying stages to reach the desired 370 to 390 MHz frequency range.

The 500 W Power Amplifier (PA) can be bypassed for low-power applications. All stages are connected to a remote control center (RCC), allowing full control over the transmitter from the receiver location, using a small remote control box (RC), connected via a long multi-cable. The entire transmitter was supplied by a third party, with the exception of the antenna matching unit at the right, which was supplied by the NRP as part of the complex receiver setup shown below.
  1. ERP = Effectively Radiated Power.

Easy Chair Mark V - antenna matching unit Three connections at the top Two knobs for optimum tuning Meter showing amount of reflected energy Three terminals for connection of antenna (ANT), transmitter (TRANS) and receiver (CANC) Tuning Tuning Front Rear

The receiver is by far the most complex part of the Easy Chair V system. Development of the receiver and the accompanying PEs took from 1960 to 1962, during which time each design decision of the earlier EC systems was reconsidered, resulting in many significant improvements.
The EC III's subcarrier FM modulation was kept, but was much improved. Rather than using a square wave subcarrier, which caused unwanted sidebands that might reveal the PE, a pure sine­wave subcarrier was used. By assigning a unique subcarrier frequency to each PE, three PEs could be used in the same target area simultaneously.

The nominal subcarrier frequencies were defined at 90, 120 and 150 kHz, and were commonly identified as L, M and H (low, medium and high). Depending on the temperature and the energy level of the activation beam, the frequencies could drift, which is why they are 30 kHz apart.

The image on the right shows the complete receiver which consisted of five same-size modules, mounted in a 19" frame. The modules were usually transported in two large suitcases. At the bottom is the Cancellation Unit which reduces the amount of transmitter spill-over.
EC V receiver. At the front left is the remote control box of the transmitter

Note the small black box at the front left, in front of the Cancellation Unit. This is the remote control unit (RC) of the transmitter which is placed elsewhere in the building. As transmitter and receiver had to be placed as far apart as possible (to reduce spill-over from antenna leakage and reflections on objects) the RC allowed the transmitter to be fully controlled from the receiver site. At the front right is a Telefunken Model 77 stereo tape recorder that can record two channels.

The module above the Cancellation Unit is the receiver's front-end, which is identified as the RF Unit. It consists of two individual paths with a phase difference of 90° in order to make the system insensitive to phase differences between the transmit and receive signals. A small portion of the transmitter's energy is used as a Local Oscillator (LO) signal, and is mixed with the received signal. After amplification, the system is then fed to an AM detector. This principle is known as autodyne operation or synchronous detection and is also used in the earlier Easy Chair systems.

RF Unit block diagram

After detection, the signal is filtered, phase shifted, and then combined into a single signal which is indifferent of any phase shifts between TX and RX signals. After further amplification, the signal is fed to three identical IF/AF stages: one for each of the PE subcarrier frequencies or channels.

IF Unit block diagram (3 identical units are present)

The three IF Units are housed in separate enclosures which are mounted at the top of the rack. Each unit consists of an adjustable attenuator, a filter, a frequency converter, various amplifying stages, a limiter and finally an FM discriminator which demodulates the actual PE audio. Once demodulated, the signal is amplified to headphones and recording level. When using a stereo tape recorder, two channels can be recorded simultaneously. When recording the audio from two PEs in the same room, this provides a spatial sound, which will be more legible and easier to transcribe.
One of the EC V antennas Test setup for the EC V. The suitcase on the right contains the (externally built) 500W transmitter EC V receiver. At the front left is the remote control box of the transmitter EC V receiver rack

At present, little is known about the antennas that were used for the EC V system. It is possible that, like the transmitter, they were made by an external party, but it is also possible that the antennas of an earlier Easy Chair set were used.

In any case, it is certain that separate antennas were used for transmitter and receiver, and that they had to be separated properly in order to limit the amount of spill-over. It is also known that the antennas had a gain of 14dB (~20 times).

This means that at the maximum transmitter power of 500W, the effectively radiated power (ERP) in the direction of the target, was approx. 10kW. This was sufficient to power a PE inside a building at a distance of more than 200 metres.

The image on the right shows the receive antenna in one of the secret rooms at the rear side of the NRP building in March 1962, when testing the new PEs at the Hotel Belvedere test site. The antenna is placed in a different room than the transmit antenna, in order to avoid spill-over. For the transmitter, an identical antenna was used.
One of the EC V antennas

From the photograph we learn that the antenna was a vertically polarized 5 or 6-element Yagi with a corner reflector. The entire construction is modular, in such a way that tripod, mast, reflector and Yagi antenna can be disassembled or folded easily, in order to fit a common travel suitcase.

Tape recorder
In the above section about the receiver, the photograph of the Easy Chair Mark V listening post shows a Telefunken Model 77 valve-based tape recorder, which is also shown on the right.

Model 77 was in production from 1959 to 1963 and was Telefunken's first stereo tape recorder [7]. In the EC V listening post it was used for recording two channels (of the available three) simultaneously. At the time (1962) it had a price tag of DM 699 (EUR 350).
Telefunken Model 77 stereo tape recorder

Path loss survey system
In a system like the Easy Chair Mark V, it is extremely difficult to predict the maximum allowed distance between the LP and the PE. Although the system is designed for an operational range of 200 - 250 metres, there are many factors that can cause a reduction of this distance, such as walls, furniture, doors, trees, badly positioned antennas, etc. This effect is known as path loss.
In practice, it was highly recommended to simulate the actual conditions of the LP and the target area, so that the maximum allowable distance could be determined, as well as the minimum required RF power to achieve this.

For this purpose, NRP developed the so-called path loss survey system, or TEC, that consists of a small RF source, an antenna and a receiver. It allowed CIA personnel to accurately predict the performance of the EC-V in a certain scenario, and compare the results to the link budget.

 More information
TEC receiver, antenna and RF source

  1. Report on Separate Receiver Systems
    CM302535/A, 27 April 1960.

  2. Specifications for EC Mk V transmitter
    CM302535/B, 1 December 1960.

  3. EC 1959-1960 Final Research Report (PE model 1960)
    CM302535/C, 28 March 1961.

  4. EC-MK V Final Research Report
    CM302535/D, 30 October 1962.

  5. Manual for EC Mk V - Equipment
    CM302535/E, December 1962 (est.).

  1. NRP/CIA, Collection of documents related to Easy Chair Mark V
    Crypto Museum Archive, CM302535 (see above).

  2. NRP/CIA, Final Research Report on Transformerless P.E.'s
    January 1964. Crypto Museum Archive CM302564.

  3. Gerhard Prins, Letter to his heirs
    Date unknown, but probably written shortly before his death in April 1993.
    Vertrouwelijk (confidential). Published by [4].

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

  5. Geoff Fors, Motorola FM mobile 2-way radio equipment
    Portable Sets to 1957. (2000) Retrieved February 2017.

  6. NRP, Collection of photographs of EC-V test session
    March/April 1962. Crypto Museum Archive CM500273/A.

  7. Philip I. Nelson, Telefunken Model 77 magnetophon (1959)
    Website: Phil's Old Radios, 1995-2017. Retrieved March 2017.

Further information

Any links shown in red are currently unavailable. If you like the information on this website, why not make a donation?
Crypto Museum. Created: Tuesday 24 January 2017. Last changed: Tuesday, 21 March 2017 - 20:44 CET.
Click for homepage