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Covert transmitter - under construction

URT-1 was a crystal-operated reference transmitter for the 315 MHz band, developed 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. The transmitter was commonly supplied as part of the URS-1 path loss survey system. It was also used for training CIA operatives and technicians.

The URT-1 transmit on a single channel, which is determined by an internal quarz crystal. Three channels were allocated: (A) 314.500 MHz, (B) 315.500 MHz and (C) 316.500 MHz. The version shown here is configured for channel (A).

At the bottom is a bay in which a standard 9V battery can be installed. As soon as a battery is inserted, the device starts transmitting in AM. When voice operation is required, an external dynamic microphone should be connected to the solder terminals at the top. Alternatively, it can be made to transmit a beep by installing a wire.
URT-1 transmitter 300 MHz

A the top of the device is a BNC socket to which the antenna should be connected. Although virtually any 50Ω antenna can be used (as long as it is suitable for the frequency), it is commonly used in combination with a so-called Sleevex antenna. Sleevex is a coaxial antenna that does not need any radials or counterpoise and is one of the best antennas for concealed installation.

The URT-1 has a fixed output power of 2.5 mW, which can be calibrated, so that the transmitter can be used for reference measurements. It was used, for example, to test the performance of Sleevex antennas under varying circumstances, such as wooden and concrete concealments.

URT-1 transmitter 300 MHz
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URT-1 transmitter 300 MHz

The URT-1 transmitter is battery operated and does not have any controls. As soon as the battery is connected it start transmitting at a fixed frequency, which is determined by an internal crystal.

The transmitter circuit has been cleverly designed for low power consumption and a precision RF output power level. An oscillator runs a crystal in the 3rd overtone. Behind the oscillator is a tripler stage that multiplies the oscillator signal (x3) to obtain the desired frequency of, say, 314,500 MHz. This signal now goes into a 3rd amplifier stage, beeing the output amplifier. The output from this amplifier is coupled out in a very low impedant way. This RF output signal is fed with a 47 ohm resistor towards the antenna output BNC connector. In this way the amplifier is not sensitive for RF mismatch / antenna reflections. 2,5mW)

Interestingly, the final amplifier stage is controlled by a bias regulator circuit. The RF output of the final amplifier stage is detected and measured. This level is compared with a current controlled voltage reference, which controls the final amplifier bias. In this way the output level of the transmitter is controlled, guaranteed to be stable within 0,5dB.

In addition, a modulator feature is build inside this transmitter. This circuit is build up around 2 transistors that form an 800 Hz oscillator. This to be able to modulate the transmitted signal slightly with 17% amplitude modulation. The oscillator can also be used as an AF-amplifier, just by wire-strapping, decoupling the 800 Hz feedback. By using a microphone element, such as the Knowles 1501 etc. the transmitter can then be used with AM-modulated audio. Extremely versatile.

The battery power supply is stabilized as a whole -at first- to guarantee battery usage until main battery voltage has been dropped under 7V. This guarantees 24 hours of usage with a fresh good quality 9V battery @ 15mA current consumption.

  1. Technical Manual for URS-1
    February 1977.
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Crypto Museum. Created: Saturday 07 January 2017. Last changed: Tuesday, 13 June 2017 - 06:27 CET.
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