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Covert plexiglass antenna

SRN-58 is a vertically polarized Super High Frequency (SHF) antenna with a circular radiation pattern, developed by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA), for use as part of a covert listening device (bug), such as the CIA's SRT-107 transmitter. The antenna was developed as part of a long-term research project under the name Easy Chair.
The antenna is basically a vertically polarized ½λ dipole, constructed of two ¼λ brass tubes, that is fed at the center by means of a coaxial cable that is guided through the lower element. This antenna type is also known as a coaxial antenna or more commonly as a sleeve antenna. [1]

The antenna is constructed in such a way that it can be used in or near a variety of environments, such as open air, wood and concrete. To make it less sensitive to the effects of such materials, it is embedded in a plexiglass tube of which the diameter and dielectric constant (εr) are known.
SRN-58 sleeve antenna for 1500 MHz

By creating a known distance between the antenna and the environment, the influence of the environment on the behaviour of the antenna is reduced. In practice, the antenna was often placed in a pre-drilled hole, for example in a concrete wall or in the wooden leg of a table. The antenna has the same diameter as the SRT-107 transmitter and can be fitted in a 1 1/8" hole.

SRN-58 antenna shown in horizontal position (in practice it was used vertically)

The diagram above shows the construction of the SRN-58 antenna. The plexiglass cylinder was probably pre-ordered, after which a 6 mm hole was drilled through the centre. The actual brass antenna was then inserted into the 6 mm hole after which the opening at the bottom was closed with a 2-component adhesive or epoxy. The intention was to create an air-tight construction in order to avoid corrosion. Nevertheless, some corrosion is visable on this 40+ year old antenna.

The exact dimensions of the two brass elements are difficult to predict. They are subject to the wavelength, their diameter, the diameter of the coaxial feed, the required impedance, the velocity factor of the coax and the dielectric effects (εr) of the outer plexiglass cylinder. The antenna is a fixed part of a number of covert listening devices (bugs), such as the SRT-107. For test and measurements, for example as part of a path loss survey system, the antenna was also available separately. In that case a standard BNC connector is fitted to the end of the 25 cm coaxial cable.
  1. The exact material from which the transparent tube is made, is currently unknown. Because of its high degree of transparency, we assume it to be plexiglass, perspex, or a similar material.

SRN-58 Antenna feedpoint Side view Side view (note some oxidation) Strain relief Insulation between the two elements Top SRN-58 sleeve antenna for 1500 MHz

Transmitter   SRT-107
The SRN-58 was commonly supplied as 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.

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SRT-107 transmitter

Sleevex antenna
Another CIA covert antenna that uses the same principle, is the so-called Sleevex antenna. Contrary to the SRN-58, it is not embedded in a plexiglass stick, but was tailored for each specific environment, and was more flexible.

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Yellow Sleevex antenna

Cryptomuseum has the ability to perform S11 measurements by means of an HP8753C network analyzer in combination with an HP85046A test-set. Apart from networks, such an analyzer can also be used to measure the reflections returned by a particular device when it is driven by RF energy. These measurements are known as the scattering parameters or S-parameters [2].

In our case we are particularly interested in the S11 parameter, which tells us how much energy is returned by the antenna. The lower this parameter is, the better. In the ideal situation, all energy is absorbed (and emitted) by the antenna and no energy is reflected. We would expect to see something like shown in the illustration below. The blue line is our antenna. As it is a coaxial antenna (sleeve antenna) it should exhibit a larger bandwidth than a common dipole (red line).

After pre-heating the analyzer and calibrating it's S11 port, the antenna is connected to the analyzer's S11 port. The first test in open air did not produce the expected results however, as reflection was much higher. We therefore had to assume that the antenna was designed for operation inside a particular concealment such as a wooden frame. After placing the antenna inside a wooden dielectricum, performance improved immediately, resulting in this graph:

In this graph we see the antenna's reflection (in dB) as a function of the frequency. Four markers are positioned at strategic frequencies to give a good impression of the antenna's performance. The horizontal scale runs from 1000 MHz to 2000 MHz. The vertical scale is 5dB/division. Good antenna radiation leads to low reflection, so it is clear that the range from 1350 to 1500 MHz gives the best performance here. Within this range, the reflection is down 20dB, which means that only 1% of the RF energy supplied to the antenna is reflected back to the transmitter.

The sleeve antenna principle is perfectly illustrated here as well: This antenna does not peak at one frequency like a common dipole does, but has a more wideband behaviour. This also proves that the NRP had a good idea of which antennas could best be used for their listening devices. We will now look at the behaviour of the antenna's impedance over the given frequency range.

A Smith Chart gives a complex representation of the performance of RF components in general. Although a full explanation of Smith Charts is beyond the scope of this page, we are aming for the centre of the graph, as this is the point where the antenna is perfectly matched to the 50 Ω output of the transmitter [3]. Markers 1, 2 and 3 (1400 - 1500 MHz) are right at the centre. For further information about the theory behind Smith Charts, please check Wikipedia [3].

The final measurement shows the Standing Wave Ratio or SWR of the antenna, which is usually specified as 1:n [4]. Although the mathematical principle behind the SWR is rather complex, we aim for the lowest possible SWR, which is ideally 1:1. In practice, an SWR between 1:1 and 1:1.5 is acceptable. For a low-power device like the SRT-107 transmitter, 1:1.5 is more than adequate. As we can see in the above diagram, the SWR for markers 1, 2 and 3 (1400 - 1500 MHz) is 1:1.2.
From the above measurements we may safely conclude that the design of the SRN-58 was well thought through, and that the antenna is well suited for field use. The use of a plexiglass stick as a dielectric interface between the antenna and the actual medium has been well researched, as it reduces the effects of the environment somewhat. In practice, the antenna gives extremely good results when embedded in wood or concrete. It is probably the best antenna for this application.
Although the antenna is properly matched to the transmission line and to the environment, in practice there will always be reflections of some kind. This the case for example, when it is positioned close to a metal object and a significant amount of the energy is returned to the transmitter where it must be dissipated. In reaction to this, the transmitter will consume more power in order to overcome the returned energy. This may potentially damage the transmitter.

In transmitters like the SRT-107, this is solved by inserting an isolator between the output of the transmitter and the antenna. An isolator is in fact a 3-port circulator of which the return port is connected to ground, as illustrated in the diagram above. In a circulator, the energy is always delivered to the next port. The energy from the transmitter (1) is delivered at the antenna (2), but the energy returned from the antenna (2) is delivered at port (3) which is connected to ground.
  1. Wikipedia, Coaxial antenna
    Retrieved January 2017.

  2. Wikipedia, Scattering parameters
    Retrieved January 2017.

  3. Wikipedia, Smith chart
    Retrieved January 2017.

  4. Wikipedia, Standing wave ratio
    Retrieved January 2017.

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Crypto Museum. Created: Thursday 05 January 2017. Last changed: Wednesday, 11 January 2017 - 10:15 CET.
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