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Plexiglass directional antenna for 1500 MHz

SRN-59 is a vertically polarized end-fed directional 1500 MHz dipole antenna, developed around 1969 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). It was intended for the transition of CIA covert listening devices (bugs) from 300 MHz to the newly allocated 1500 MHz band. [1].

The SRN-59 was made for use in combination with covert listening devices (bugs), such as the SRT-107, and is in fact the directional variant of the nearly identical omni-directional SRN-58.

Like the SRN-58, the SRN-59 is a vertical dipole, constructed from two short brass pipes that are mounted inside a perspex (plexiglass) stick. It is half a wavelength long (½λ), but is fore-shorted in order to compensate for the dielectric effects of the environment (the perspex stick). Unlike the SRN-58 however, the antenna is not placed at the centre of the stick, but towards one side.
SRN-59 antenna

At the circumference of the perspex stick, opposite the dipole antenna, is a narrow brass strip that acts as a non-resonant reflector. It is this strip that gives the antenna its directive property, which results in an isotropic gain of +2 to +6 dB. It should be clear that this antenna can only be used in situations where it can be positioned accurately with respect to the position of the nearby listening post (LP). If this is not the case, the omni-directional SRN-58 should be used instead.

SRN-59 antenna shown in horizontal position (in practice it was used vertically)

The diagram above shows the construction of the SRN-59, seen from its side, whilst the antenna is in horizontal position. In practice the antenna was used straight up, with the red arrow showing the direction of optimum radiaton. For further information, please refer to the SRN-58 page.

 About the SRN-58

SRN-59 antenna SRN-59 antenna in vertical position SRN-59 antenna compared to the size of a hand SRN-59 seen from the top Dipole antenna inside the perspex stick
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SRN-59 antenna
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SRN-59 antenna in vertical position
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SRN-59 antenna compared to the size of a hand
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SRN-59 seen from the top
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Dipole antenna inside the perspex stick

Transmitter   SRT-107
The SRN-59 (or the omni-directional SRN-58) was commonly supplied as an integral part of a transmitter (bug) such as the SRT-107 shown in the image on the right. In that case the 25 cm long coaxial cable was fixed to the body of the transmitter.

 More information
SRT-107 transmitter

The SRN-58 is the omni-directional variant of the SRN-59. Although it does not provide the +2 to +6 dB isotropic gain of the SRN-59, it was preferred in most situations, as it makes the positioning of the target area (TA) concealment and the listening post (LP) much less critical.

 More information
SRN-58 sleeve antenna for 1500 MHz

The SRN-58 and SRN-59 were developed at the NRP between December 1968 and November 1969, as part of a CIA research contract to investigate the feasibilty of moving the operational frequency of covert listening devices (bugs) from 290 MHz and 350 MHz to the newly allocated 1500 MHz band [1]. For this project, the following new components were developed:

In September 1969, the first equipment was sent to the CIA for evaluation. This resulted in an upgraded version of the SRT-56 bug, of which the SRK-35 RF-module was replaced by the new SRK-145, making it suitable for 1500 MHz operation. It was fitted with an SRN-58/59 antenna.

The Rejected Pulse (RP) audio masking scheme of the SRT-56 was kept, as it was housed in a separate cylindrical enclosure. The image on the right shows a complete set as it was discovered by the Russians in one of their buildings in the USA. It consists of an SRK-145 RF-module, an SWE-56 video coder and a large Mercury battery array. At the right is the SRN-58 antenna stick.

The combination shown in the image on the right was in production from 1971 to 1974, after which it was succeeded by the integrated SRT-107, which was smaller and easier to conceal.

The SRT-107 was in fact a combination of the SRK-145 RF-unit and an SWE-56 video coder (i.e. the audio-masking unit), housed in a single cylindrical enclosure that was approx. 15 cm long. With the SRT-107, the SRN-58 (or SRN-59) antenna was permanently fitted to the transmitter.

  1. NRP/CIA, Manual for 1.5 GHz system
    September 1969. CM302484.
Further information
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Crypto Museum. Created: Thursday 23 March 2017. Last changed: Tuesday, 13 June 2017 - 06:06 CET.
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