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UVK-21
In-line VSWR tester

UVK-21 was an in-line Voltage Standing Wave Ratio (VSWR) meter for RF signals in the 240 - 400 MHz frequency range, developed in the late 1980s by the Dutch Radar Laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of a long-term development contract named Easy Chair.

The device was made especially for measuring the quality of the RF match between an installed surveillance transmitter (bug) and a generally unpredictable antenna system, operating in the CIA's favorite 380 MHz band. It can also measure the actual RF output power of the transmitter.

At the left, just below the meter, is a BNC-twin socket to which the probe is connected. The unit was supplied with a collection of fully passive sampling line modules, that can be inserted into the transmission line, as close to the transmitter as possible, with the probe placed on top of it.
  
UVK-21 in-line VSWR meter (prototype)

Development of the UVK-21 started in the mid-1970s, alongside the UVK-20 — a single-ended VSWR meter with built-in RF source. The first prototype was presented to the CIA in 1978 [A] and was thoroughly evaluated [B] before it was taken into production in 1980 [C]. The CIA used it to validate the performance of bugs and the accompanying antennas under simulated conditions.

UVK-21 in two storage boxes UVK-21 in storage box UVK-21 in-line VSWR meter (prototype) Meter Twin-BNC socket Probe Sampling module with SMA connectors UVK-21 kit
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UVK-21 in two storage boxes
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UVK-21 in storage box
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UVK-21 in-line VSWR meter (prototype)
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Meter
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Twin-BNC socket
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Probe
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Sampling module with SMA connectors
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UVK-21 kit

Features
The photograph below gives an overview of the items that were present in the UVK-21 kit. The largest part is the actual indicator unit. It is powered by a single 9V block battery that is installed behind a lid in the bottom left corner. Depending on the output power of the bug under test, the appropriate probe (1-10 or 5-50 mW) should be connected to the BNC-twin socket at the left.


The unit was supplied with 10 universal sampling line modules, one of which had to be inserted (soldered) between the transmitter and its antenna, as close to the output of the transmitter as possible. This is considered a disposable part, as it was left in place during the operational life of the bug. For measurements in a laboratory, sampling units with SMA sockets were also available.

Sampling module with SMA connectors Probe (5-50 mW) Probe (bottom view) Placing the probe on a sampling module Probe placed on top of a sampling line module Various sampling modules Various sampling modules Example of a sampling module which can be soldered directly to a bug
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Sampling module with SMA connectors
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Probe (5-50 mW)
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Probe (bottom view)
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Placing the probe on a sampling module
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Probe placed on top of a sampling line module
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Various sampling modules
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Various sampling modules
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Example of a sampling module which can be soldered directly to a bug

Operation
The diagram below shows how the UVK-21 is used. At the top left is the transmitter under test (i.e. the bug), powered by its own local battery or
PSU
. A (disposable) sampling line module (SLM) is now soldered directly to the antenna output of the transmitter, preferably with a ground wire at either side. At the other end of the sampling line module – which is actually an open piece of 50Ω transmission line – the antenna is connected. This can be a piece of wire of arbitrary length, or a proper antenna, like a Sleevex, in which case a coaxial line should be soldered to the SLM output.


The probe is now placed in the cradle of the SLM, with the arrow pointing from the transmitter to the antenna. The transmitter is now activated and the meter is adjusted for maximum reading. Next, the probe is reversed, so that the arrow points from the antenna to the transmitter. The meter now shows the amount of energy that is reflected from the antenna, directly in VSWR.

A mid-scale reading (VSWR = 6) means a loss of 50% and is considered a bad match. Likewise, anything below a 10% reading (VSWR ≤ 2) is considered satisfactory. Although the SLM introduces an insertion loss of 0.25 dB, this will outweight the gain of a properly matched configuration.


Similar equipment
Single-ended VSWR meter for testing antenna matching Transmitter tester Path loss survey system for 316 MHz Path loss survey system (wideband) for 1380 MHz Path loss survey system (narrowband) for 170 and 316 MHz
Specifications
  • Frequency
    240 - 400 MHz
  • Power
    6.5 to 10 V (typically a 9V block battery)
  • Current
    ≤ 4 mA
  • Connection
    BNC-twin
  • Impedance
    50Ω
  • Probes
    2 (see below)
  • Sampling
    Module 30 x 19 x 4.5 mm
  • Loss
    ≤ 0.25 dB
  • Dimensions
    127 x 108 x 67 mm
  • Weight
    550 grams
Probes
  • 1-10 mW
    low-power probe
  • 5-50 mW
    medium power probe
Checklist
  • 1 x Indicator (the actual UVK-21 unit)
  • 1 x Probe module 1-10 mW
  • 1 x Probe module 5-50 mW
  • 10 x Sampling module
  • 1 x Final test data sheet
  • 1 x Operator's manual
Documentation
  1. UVK-21, prototype in-line VSWR tester
    CM-302542/B. NRP, May 1978.

  2. XUVK-21 Evaluation
    CIA, date unknown. Preliminary draft.

  3. UVK-21 Inline VSWR Tester, Technical Manual
    CM-302542/K. NRP, March 1980.
Further information
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Crypto Museum. Created: Saturday 12 October 2019. Last changed: Sunday, 13 October 2019 - 10:19 CET.
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