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← Easy Chair
Universal VHF/UHF listening post antenna

SRN-9 was a universal adjustable open dipole antenna with a non-resonant reflector, developed in 1968 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. The antenna was intended for use as part of a Listening Post (LP) for a covert listening device (bug), and offered a gain of ~ 7 dB.
The antenna is basically an open dipole of which the length of the elements can be adjusted freely (within a certain range) to match the desired frequency. The dipole elements are mounted on a horizontal aluminium boom in front of a non-resonant reflective plane that gives the antenna its directivity and its gain. The boom also acts as a ¼λ BALUN, and has a BNC socket at the end.

All parts of the antenna can be dismounted. The reflective plane is hinged at the centre, allowing it to be folded and transported in an unobtrusive standard Samsonite exectutive style briefcase.
Sen from the rear

Initially, there were two versions of the antenna, both of which were in production from 1968 to 1970. The SRN-9L (low) was intended for the 240 to 330 MHz frequency range, whilst the SRN-9H (high) was suitable for 300 - 470 MHz. In 1972, both models were succeeded by the SRN-9, which was suitable for the SRR-52 and SRR-56 and supported the entire 240 - 470 MHz range.
Sen from the rear Antenna boom Two adjustable elements Dipole antenna Coaxial connection Hinged reflector plate Antenna mount Antenna mounted on the mast head
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Sen from the rear
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Antenna boom
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Two adjustable elements
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Dipole antenna
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Coaxial connection
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Hinged reflector plate
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Antenna mount
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Antenna mounted on the mast head

  • SRN-9-L
    Directive LP antenna covering 240 - 330 MHz, with 6 - 7 dB gain. Originally supplied with the SRR-52 surveillance receiver for the reception of SRT-52 bugs.

  • SRN-9-M
    Non-adjustable directive antenna covering 314 - 316 MHz, with 7 dB gain. Originally supplied with the URS-1 path loss survey system.

  • SRN-9-H
    Directive LP antenna covering 300 - 470 MHz, with 6 - 7 dB gain. Originally supplied with the SRR-56 surveillance receiver for the reception of SRT-56 bugs.

  • SRN-9
    Improved design that replaced all of the above antennas. It is an adjustable directive antenna that covers 240 - 470 MHz, and has a gain of 6 - 7 dB.

In most cases, the antenna was stored in the same briefcase as the matching SRR-52 or SRR-56 surveillance receiver. Depending on the orientation of the target antenna (i.e. the antenna of the actual bug), the SRN-9 was placed horizontally or vertically, in order to match the polarization.
In the images above it is shown in the horizontal position, but this could easily be changed to the vertical position by loosening the mounting head of the short mast and rotating the boom. The short aluminium mast was intended for fitting the antenna onto a regular photographic tripod.

Before the antenna can be used, it should be adjusted to the frequency of the transmitter, in order to obtain an appropriate gain. This is done by removing the adjustable antenna elements and placing them on the frequency table that is printed in white at the rear side of the reflector.
Adjusting the length of an element

Both antenna elements are telescopic and consist of two parts with a compression coupling at the centre. After loosening the coupling, the inner part can be shifted in or out, thereby changing the total length of the element. Once the correct length is found, the coupling is tightened again.
Different length telescopic antenna elements were available, both longer and shorter, to allow the frequency range to be extended somewhat. This effectively converts the SRN-9L featured on this page, into the more universal SRN-9.

Installing an antenna in a concealed listening post (LP), always requires some improvisation. For situations were a photographic tripod could not be used, a modified vise was supplied, as shown in the image on the right. It has a fitting for the antenna boom and can be attached to an arbitrary object, such as a table or window pane.
Modified Vise used as antenna mount

Once the installation was completed, the SRN-9 was connected to the surveillance receiver, which was usually an SRR-52, SRR-56, SRR-90 or SRR-91. It could also be used, of course, with any other surveillance receiver that operates in the supported 240 - 470 MHz frequency range.
Dipole antenna with boom and reflector Frequency / element length table Adjusting the length of an element Adjustable element Different length adjustable elements Modified Vise used as antenna mount SRN-9L with mast and tripod
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Dipole antenna with boom and reflector
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Frequency / element length table
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Adjusting the length of an element
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Adjustable element
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Different length adjustable elements
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Modified Vise used as antenna mount
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SRN-9L with mast and tripod

Suitable receivers
The SRN-9 was recommended for use with the following receivers:
SRR-52 listening post receiver SRR-56 listening post receiver Surveilance receiver SRR-90-A Surveilance receiver SRR-90-B Surveilance receiver SRR-91 Path loss survey system
Further information

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Crypto Museum. Created: Sunday 09 April 2017. Last changed: Tuesday, 13 June 2017 - 06:08 CET.
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