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Easy Chair path loss survey system

TEC Mk II was a path loss survey system for Easy Chair Mark V bugs (EC Mk V), developed in 1963 by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term research contract under the name Easy Chair (EC). The TEC system was intended for predicting the propagation of radio signals between a target area and an EC listening post (LP).
An EC device is a covert listening device (bug) of a passive nature, which means that it is powered by a strong RF signal that is transmitted from the listening post. In CIA/NRP terminology, such a device was known as a passive element or PE.

Both the activation signal and the PE operated in the 370-390 MHz band, which means that in a two-antenna system at the listening post, the antennas had to be separated as far as possible, and sufficient spillover attenuation is necessary. In a single antenna system this attenuation is generally provided by the antenna's duplex filter.
TEC receiver, antenna and RF source

As the PE is powered by a remote RF signal, it operates at a very low power and transmits a very weak signal in the order of 100 µW. To guarantee a proper reception of this weak signal at the LP, it was important to carry out a site survey with the TEC, prior to installation of the PE, preferably at the actual target site. Note that the path attenuation or path loss is applicable twice: once to the high-power activation signal from the LP to the PE, and once to the PE signal back to the LP.

TEC was intended for studies on the propagation of radio signals over an obstructed path, but was also considered a valuable tool for training CIA technicians when preparing for an operation. Based on the way the board of the unit's internal receiver is built, all TEC units were hand-made. As the device shown here is marked as Mark II, we assume it was the successor to an earlier one. In 1977, the TEC Mk II was succeeded by the more advanced URS-1 path loss survey system.
TEC receiver, antenna and RF source Easy Chair Mark II test device Control panel Frequency table Frequency dial Model number Carrying strap RF source


The receiver is the largest part of the survey system. It is fully self-contained and is housed in a blue metal enclosure, with all controls and connections at the largest surface (front). The receiver is powered by a Mallory TR-135R 6.75V battery 1 that should be installed in a slot at the right. An adjustable neck strap is provided to allow the receiver to be operated with both hands free.
The interior of the receiver can be access by removing the non-recessed screws from the edges of the front panel and removing the lid of the battery compartment at the right. The front panel can now be removed from the case shell.

All internal parts and sub-assemblies are mounted to the inside of the front panel, as shown in the image on the right. The rectangular blue box at the top left holds the IF and AF stages of the receiver. The RF stage, the local oscillator (LO) and the mixer are mounted in the rectangluar aluminum unit at the front left.
Receiver interior

The remaining space is taken by the controls, the battery holder and the dB-meter. In the above image, the lid of the IF/AF section has been removed in order to show the circuit board. Judging from the manufacturing method of this circuit board, each TEC unit was completely hand-made.
  1. This was an industrial stacked mercury battery, that was also used in military equipment, where it was known NSN 6135-00-801-0587. Mercury batteries are illegal for domestic use today.

Receiver interior Receiver interior Directional coupler and local oscillator Battery compartment Inside the metal box Receiver deetail IF/AF circuit board Directional coupler and local oscillator

A small handheld transmitter is provided as an RF energy source for performing calibrated path loss measurements. It produces just 100 µW of RF energy and is powered by a Mallory TR153R 4.05V battery that is installed at the bottom.

The entire solution is built inside one half of a ½λ dipole antenna and weights just 100 grams. The transmitter operates in the 370 to 390 MHz range and can be modulated with a 1500 Hz tone. The frequency can be adjusted by means of a slider that is located close to the feed point. The design is similar to an EC target element.
RF source

RF source Battery compartment Spring-loaded battery holder Tuning adjustment Two separate sections Antenna feedpoint Two antenna halves Holding the RF source in the hand

The installation was supplied with a reference antenna that should be connected to the TEC receiver. The antenna consisted of a 300 Ω ½λ folded dipole, with a 90° corner reflector.

The antenna could be fitted on a tripod or held in the hand using the supplied grip, which is actually a modified screwdriver. The hinged relector could be folded in such a way that the entire antenna could be stored on top of the receiver when the installation was in transit.
Unfolded antenna with grip

Antenna stored on top of the receiver Folded antenna Unfolding the antenna Unfolded antenna Turing the fixation arm Fixating the reflector Grip Holding the antenna in vertical position

AF   Audio Frequency

CIA   Central Intelligence Agency
United Status intelligence agency.  More

EC   Easy Chair
CIA codename for the super-secret project to develop covert listening devices based on the principle of the Russian resonant cavity microphone, also known as The Thing. The name EASY CHAIR is also written as EASYCHAIR.

IF   Intermediate Frequency

LP   Listening Post
Common expression for the monitoring station. In the case of a PE, this is also the position of the activation transmitter.

NRP   Nederlands Radar Proefstation
Dutch Radar Laboratory, at the time located in Noordwijk (Netherlands).  More

PE   Passive Element
NRP name for derivatives of the resonant cavity microphone. In this sense, passive means that it does not have its own local power source.

RF   Radio frequency

TEC   Test Easy Chair
Test equipment for Easy Chair installation.

  1. Manual for TEC - Mk II
    14 June 1963.

  2. Operating Manual - TEC Mk I
    Date unknown.

  3. Technical Manual - TEC Mk I
    Date unknown.

  1. Anonymous, TEC Mk II survey receiver
    July 2016.

  2. NRP/CIA, Collection of documents related to TEC
    Crypto Museum Archive, CM302457 (see above).

  3. Wikipedia, Corner reflector antenna
    Retrieved January 2017.

Further information

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Crypto Museum. Created: Wednesday 04 January 2017. Last changed: Sunday, 05 February 2017 - 14:19 CET.
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