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Universal low-band survey transmitter

URT-4L is a portable transmitter, developed around 1978 by the Dutch radar Laboratory (NRP) for the US Central Intelligence Agency (CIA), as part of the URS-4 path loss survey system. It was used for analysing the link budget 1 of a VHF-H band 2 communications system, and works on a crystal operated frequency around 170 MHz. It was supplied complementary to the URT-4H.

The image on the right shows the URT-4L with its batteries and case shell removed. At the top it has a socket for connection of a VHF 170 MHz Sleevex antenna, a socket for an external marker button, an ON/OFF switch and a marker button.

The marker push-button on the front panel is connected in parallel to the SMB socket for an external one. This was done to allow body-worn operation during a survey. From 1982 onwards, the front panel also held a red power LED. The unit is typically powered by a 6V battery pack that is connected to the terminals at the rear.
URT-4L transmitter

The unit transmits a narrow-band 30 kHz wide signal, on a predetermined crystal-driven spot frequency in the VHF band (either 168 or 169 or 170 MHz), with an average output power of 160 mW (+22 dBm). It produces an on-off keyed audio tone of 1250 Hz. When the MARK push-button is pressed, this tone changes to 1350 Hz, to allow easy identification at the receiving end.

 Check out the complete URS-4 system

  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
  2. In the context of the URS-4 system, the VHF-high band (170 MHz) is known as the low band, hence the letter 'L' in the model name. The 316 MHz band was known as the high band (H).

URT-4L transmitter Controls and connections Battery terminals Variant with poer LED on the front panel URT-4L with antenna and remote MARK button connected Remote switch Remote switch
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URT-4L transmitter
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Controls and connections
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Battery terminals
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Variant with poer LED on the front panel
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URT-4L with antenna and remote MARK button connected
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Remote switch
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Remote switch

The diagram below shows the URT-4L transmitter without batteries, but with the Sleevex antenna and the external MARK button connected at the front panel. The unit is powered by four AA-size 1.5V batteries that should be connected to the terminals at the rear panel. The transmitter is turned ON with the miniature switch at the front panel, which also enables the 1250 Hz tone.

Remote switch
The 130 Hz marker tone can be activated with the push-button on the transmitter's front panel. However, when the unit is body-worn during a survey, it might be difficult to reach this button.

For this reason, a wired push-button was supplied with the URS-4 survey set. It can be connected to the female SMB socket of both transmitters and is connected in parallel to the push-button on the front panel.
Remote switch

Sleevex antennas
The URS-4 set was supplied with two complete sets of Sleevex antennas (three models each). One set was tuned for the VHF frequencies, whilst the other set (the shorter ones) was tuned for the UHF frequencies.

Each Sleevex antenna has a colour coded ring at its base, that indicates the environment (medium or diëlectricum) for which it has been designed.

 More information

Yellow Sleevex antenna

The URT-4L was designed for operation in combination with the URR-4 receiver, which was also supplied as part of the URS-4 survey set.

When working with the URT-4L, the low-band (VHF) plug-in had to be installed in the URR-4 receiver and the required frequency has to be selected with the rotary knob on the plug-in.
No image of the URR-4 is currently available, so we are showing an image of the very similar URR-3 instead

URT-4L with antenna and remote MARK button connected Remote switch Remote switch Operating the remote MARK button URT-4L with remote MARK button and Sleevex antenna With BNC antenna fitted
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URT-4L with antenna and remote MARK button connected
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Remote switch
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Remote switch
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Operating the remote MARK button
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URT-4L with remote MARK button and Sleevex antenna
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With BNC antenna fitted

The URT-4L transmitter is housed in a strong brass enclosure, that was probably painted in the same bright green colours as the receiver. It consists of a double-sided printed circuit board (PCB) that is mounted inside a brass frame, which in turn is enclosed by a brass case shell.

The case shell is longer than the transmitter itself, in order to accomodate the batteries. The case shell is held in place by four screws located close to the front panel (two at either side). After removing these four screws, the transmitter can be extracted from the case shell. The image on the right shows the extracted 'bare' transmitter.

At the left is the front panel which holds the controls and connections to the outside world. At the right is a pertinax panel with four battery terminals. It accepts two battery holders with two 1.5V AA-size penlight cells each (6V total).
URT-4L transmitter

At the bottom right is the crystal that determines the output frequency. Below the crystal is a CD4011 that delivers the 1250 Hz square-wave audio tone, that is directly fed to the on-off modulator built around the BD228 at the centre. Although the circuit is largely built with silicon transistors, there are still two germanium types as the bottom left. They are part of the power supply (protection against reverse voltage) and are used because of their low voltage-drop.

The unit shown here was rediscovered in 2017 [1]. It was found without the case shell and with some of the wiring missing (cut away). After restoration at Crypto Museum — using the original circuit diagrams as a guide [2] — it is now fully operational again.

URT-4L transmitter URT-4L transmitter Top view Battery terminals Upper part Crystal oscillator and tone generator Two Germanium transistors
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URT-4L transmitter
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URT-4L transmitter
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Top view
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Battery terminals
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Upper part
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Crystal oscillator and tone generator
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Two Germanium transistors

Technical specifications
  • Frequency
    168 or 169 or 170 MHz ± 2 kHz
  • Output power
    +22 dBm (160 mW) ± 1 dB
  • Impedance
    50 Ω
  • Harmonic suppression
    > 30 dB
  • Tone frequency
    1250 Hz ± 10 Hz
  • Mark frequency
    1350 Hz ± 10 Hz
  • Modulation
    on-off keying
  • RF bandsiwdth
    ≤ 30 kHz
  • Supply voltage
    4.5 to 7 V (minimum 4.5 V)
  • Current
    < 320 mA
  • Battery
    4 x AA-size penlight (1.5V each)
  • Dimensions
    61 x 21.5 x 171 mm
  • Weight
    ~ 320 grams (without batteries)
  • Temperature
    -18°C to +50°C
  • Antenna type
  • Antenna socket
  • Mark socket
    Sub-Minax/F (SMB)
  1. Tentative Proposal for Prototype XURS-4 Equipment
    NRP, 8 March 1978. CM302554/x.

  2. Technical Attachment to Proposal for Prototype XURS-4 Equipment
    NRP, 8 June 1978. CM302554/x.

  3. Proposal for Prototype XURS-4 Equipment
    NRP, June 1978. CM302554/x.

  4. Operating Manual for URS-4 Path Loss Measuring System
    NRP, June 1980. CM302554/x.

  5. Environmental Test Report on XURR-4 Receiver
    NRP, August 1980. CM302554/x.

  6. Environmental Test Report on XURT-4L and XURT-4H Transmitters
    NRP, August 1980. CM302554/x.

  7. Engeneering Considerations Related to XURS-4 Protype Equipment
    NRP, September 1980. CM302554/x.

  8. URT-4L Component Layout (technical drawing)
    NRP, November 1982. CM302554/x.

  9. Operating Manual for URS-4 Path Loss Measuring System
    NRP, January 1983. CM302554/x.

  10. Collection of correspondence between NRP and CIA about URR-4
    NRP/CIA, 21 January 1983. CM302554/x.

  11. Environmental Test Report on URS-4 Path Loss Measuring System
    March 1983. CM302554/x.

  12. Environmental Test Report on URS-4 Path Loss Measuring System
    May 1991. CM302554/x.

  13. XY-YT Miniaturschreiber Minigor, Type RE 501, Bedienungsanleitung
    Goetz, Metrawatt AG, Nürnberg (Germany). Date unknown. CM302559/A.
  1. Anonymous, Partly complete URT-4L transmitter - THANKS !
    Crypto Museum, July 2017.

  2. NRP/CIA, Collection of documents related to URS-3
    Crypto Museum Archive, CM302554 (see above).
Further information
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Crypto Museum. Created: Thursday 10 August 2017. Last changed: Sunday, 13 August 2017 - 12:39 CET.
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