SRN-55
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← Easy Chair CIA NRP Antennas Directional 1500 MHz listening post antenna SRN-55 was a compact wide-band directional Super High Frequency (SHF) antenna, developed around 1970 by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term research project under the name Easy Chair. The antenna covers a frequency range of 1300 to 1950 MHz, and was commonly used as part of a covert Listening Post (LP).
The diagram below roughly shows the layout of the SRN-55 antenna. As it is a collection of in-phase stacked dipoles, the direction of radiation is at an angle of 90° from the surface, as shown by the arrow marked Normal. The more dipoles are used, the narrower the beam width will be.
The actual beam width is illustrated here by two purple planes and varies between 10 and 17 degrees over the entire frequency range. The angle of the entire beam (as seen from the Normal) also deviates with the frequency between -25 and +15°. This angle is known as the Squint [4]. The antenna has been optimised for the 1300 - 1950 MHz range, but its characteristics are by no means constant over this range. The following four diagrams are printed on a label at the rear side of the antenna, so that its behaviour at a particular frequency can easily be predicted.
The gain is between 14 and 17.5dB over the entire range, with the best performance between 1500 and 1800 MHz and a peak around 1700 MHz. The relatively high gain is caused by the fact that the beam width is rather small as a result of the stacked dipoles. Note that the antenna is used on its side, in order to obtain vertical polarization, which is done to match the transmitter.
The Voltage Standing Wave Radio (VSWR) is a measure for the impedance mismatch between the antenna and the transmission line [2]. A value of 1.0 indicates an optimum impedance match in which no power is lost. A VSWR below 1.5 would be acceptible for the given frequency range. The diagram above shows the measured VSWR of the SRN-55 over its entire frequency range, which is never worse than 1:1.6. The best performance is obtained between 1400 and 1850 MHz.
The width of the beam decreases nearly linearly when the frequency increases. It practice it will be between 10° and 17°, with the narrowest beam (10°) occuring at 1900 MHz. The narrow beam is partially responsible for the relatively high gain of this antenna over its entire frequency range.
The antenna is constructed from a number of stacked dipoles that are interconnected by a fixed-length transmission line. This fixed-length line can be seen as a non-variable phase shifter (φ). Ideally, we want all dipoles to be in-phase, but this will only occur on one particular frequency (1500 MHz in this case). At other frequencies, the (unwanted) phase shift) produces a virtual tilt of the beam, which is known as the Squint [4]. As the SRN-55 is commonly placed horizontally (as illustrated in the diagram at the top of the page), the Squint adds a virtual rotation to the beam.
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