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Covert target area antenna

Sleevex is a tailored end-fed coaxial antenna, also known as a sleeve or skirt 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 [1]. Colour-coded variants of the antenna are available for use in various environments, such as free air, concrete and wood [2].

Initially, the NRP would only develop the Sleevex antennas, and pass the baseline documentation of the design on to the CIA, so that it could be produced by an alternative contractor in the US.

After several failed attempts however, the CIA returned to the NRP with an order for many hundreds of them. They were used with covert transmitters (bugs) operating in the 315 MHz band, such as the NRP-supplied SRT-52 and SRT-56, but also with transmitters from other manufacturers. Sleevex antennas for 316 MHz were also used with the URS-1 survey system.
Yellow Sleevex antenna

In most bugging operations, the Sleevex antenna had to be hidden inside a building or inside a piece of furniture. This was commonly done in a pre-drilled 3/8" hole that had been made by an external party. As the time to place the bug and the antenna was limited, they had to ensure that each Sleevex antenna fitted the a 3/8" hole. For this reason, all Sleevex antennas that were made at the NRP, were first tested in a reference hole of the prescribed diameter at the NRP site [3].

Collection of Sleevex antennas
Yellow Sleevex antenna
Red Sleevex (free air)
Green Sleevex (concrete)
Yello Sleevex (wood)
1 / 5
Collection of Sleevex antennas
2 / 5
Yellow Sleevex antenna
3 / 5
Red Sleevex (free air)
4 / 5
Green Sleevex (concrete)
5 / 5
Yello Sleevex (wood)

The diagram below shows a cross section of the antenna in horizontal position. The antenna is basically a ½λ vertical dipole, of which the length of both elements has been modified in order to compensate for the diëlectric effects (εr) of the medium in which the antenna is placed (free air, concrete or wood), the frequency, the impedance, the velocity factor of the coax, the diameter, etc. The basic principles of this antenna are similar to that of the SRN-58 plexiglass antenna.

Sleevex antennas are constructed from a rigid piece of thick coax, of which the central conductor has been replaced by a thin coax, as shown in the diagram above. At the feedpoint, the braided conductor (shield) is cut. Furthermore, a hole is made through which the shield of the thin coax can be reached. It is connected to the braided shield that forms the lower element of the dipole.

The diagram above shows an enlarged cross section of the feedpoint of the antenna. After removing the core conductor of the original (thick) coax, the hole was enlarged with a very long drill, to make room for the new coaxial feed line. The braiding of the feed line ends at the feedpoint, where it is connected to the braiding of the lower element in the cut-away area.

Off-center-fed dipole   OCFD
Note that the feedpoint of the antenna is not precisely at the centre of the dipole. The reason for this is that the arm through with the coax line is fed, acts as a choke or balun for the asymmetric feed line. The length of the balun is affected by the dielectric constant (εr) of the material inside, as a result of which it must be shorted by a factor of ~ 1.5 (i.e. × 0.66). This is compensated by making the other arm of the dipole longer, so that the total length of the dipole remains equal.

Frequency bands
Sleevex antennas were available in the following band variants:

Band Frequency Colour Remark
0 230 - 260 MHz ? e.g. SRT-57
1 (L) 260 - 288 MHz Blue e.g. SRT-52
1 (H) 288 - 320 MHz Red e.g. URS-1
2 (L) 320 - 350 MHz Green e.g. SRT-56
2 (H) 350 - 390 MHz Yellow e.g. SRT-56-F
3 (L) 380 - 425 MHz Grey  
3 (H) 425 - 470 MHz Black  
X 1100 MHz ?  
The coloured ring at the antenna base specifies the medium for which the antenna is designed:

Colour εr Material
Red 1 Free air
Yellow 2 Most woods, light building material
Green 4 Concrete, heavy building materials

Generally speaking, the denser the material, the higher the diëlectric constant (εr) and the shorter the antenna. The longest of the three antennas (red) is for use in free air (εr = 1).

 Dielectric constants of various materials

Related topics
Sleevex antennas were made from a piece of thick rigid coaxial cable of which the outer shield was cut roughly in the middle. The inner conductor was removed at the centre hole was drilled out until the middle, in order to fit the thin teflon coax feed cable. After mounting the cable, as indicated in the diagram above, both ends were potted in a two-component epoxy.

After that, a shrink sleeve was fitted around the antenna. The heating device shown on the right was used to shrink the sleeve.
Shrink sleeve heater

Path loss survey system
In order to determine the link budget 1 of a given transmission system, the NRP developed the special URS-1 survey system, that was used by the CIA to test the performance of the Sleevex antennas under varying circumstances.

The Universal Radio Set URS-1 consisted of an URR-1 receiver, two URT-1 transmitters and a range of accessories, and was supplied in an executive style Samsonite briefcase.

 More information
Transmitter with Sleevex antenna

  1. In a telecommunications system, the link budget is the sum of all gains and losses from transmitter, through the medium to the receiver.  Wikipedia

Plexiglass variant
The principle of the coaxial antenna was used again several years later with the plexiglass SRN-58 antenna, that was used for 1500 MHz bugs like the SRT-107. The SRN-58 was housed inside a perspex stick in order to reduce the dielectric effects of the environment.

The SRN-58 had the same diameter as the transmitter (approx. 25 mm) so that it could be fitted in a 1 1/8" hole.

 More information


  1. Technical Manual for URS-1
    February 1977.
  1. Wikipedia, Coaxial antenna
    Retrieved January 2017.

  2. Dielectric Constants of Various Materials
    Retrieved January 2017.

  3. Anonymous, Personal account
    Crypto Museum interview. September 2016.
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Crypto Museum. Created: Thursday 05 January 2017. Last changed: Tuesday, 22 November 2022 - 11:03 CET.
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