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
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.
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.
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,
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.
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
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© Crypto Museum. Created: Saturday 07 January 2017. Last changed: Saturday, 07 January 2017 - 12:15 CET.