SE-450 / II (SF station / LF station)

Radio station SE-450 / II or High Power radioteletype station (with 1000 W power amplifier) ​​resp. Low Power radioteletype station (with 400 watt power amplifier); manufactured by SRT.

With the introduction of the ARQ operation, a radioteletype mode with automatic error correction, which is also referred to as SITOR, the embassy radio stations and the mobile stations for training were equipped with new station material.

Most of the radio station equipment now came from SRT, the ARQ modem from Philips. Earlier, a configuration with the modified Siemens 1 kW transmitter as power amplifier stage was used, even longer in some embassies abroad.

The 1000 W solid state power amplifier from SRT was only used at the Swiss embassy radio communication central, all older morse telegraphy stations still in use were replaced by the variant with the solid state 400 W amplifier.

Embassy radio SF station

Technical data


Power Supply

  • Mains: three-phase mains 3 x 380/220 V
  • Generator: ONAN JB 6 kVA, 3 x 380/220 V petrol generator on single axle trailer

The entire power supply of the stations was controlled from a central control panel ST 84. With the MAIN Switch, the station can be operated from the local AC network or from a generator in case of an emergency or power failure, this can be started and stopped by remote control. Timed operation for automated reception and transmission of messages during the night hours with best propagation conditions can also be activated on the ST 84 control panel.
With the METER SWITCH, the three phases of the mains network or of the generator can be checked. A Schaffner FR 211 line filter is used to filter out interference pulses from poor quality mains network which could impair the functioning of the station.
Voltage regulators were sometimes used to compensate for unstable mains voltages. In countries with a 60 Hz mains frequency, a 220V voltage with a frequency of 50 Hz must also be prepared for the teletype machine PACT-220.


The training mobile radio station was installed on a Berna 4×4 truck, the generator in a single-axle trailer.


  • Antennas
    • AGD antenna: 2 - 13 MHz, 1 kW max. load; dipole with a distance of 30 m between the masts
    • AGV antenna: 9 - 30 MHz, 700 W max. load; four segment mast antenna
    • Roof antenna on the truck; only for training purposes, low transmission power
  • Antenna selector ASU 84

SF station New Delhi (relay)

Station equipment

The centerpart of the radio station is the TD90 exciter transmitter driving a solid state SSA 400 power amplifier or a 1000 W power amplifier SSA 1000 in the embassy radio central; in the embassies, usually the final stage of the 1 kW Siemens transmitter continuied to be used as power amplifier.

At the control desk, the ARQ modem STB 750, the tranmitter control unit TCU 90, the receiver CR 91 and the transmitter exciter TD 90 are located.

The ARQ modem STB 750 made by Philips generates the SITOR signal (ARQ). The SITOR teletype mode is an FSK radioteletype mode with error correction for omitted characters.
For „point-to-point“ connections between two stations, the radio station works in ARQ mode („automatic repeat request“, SITOR-A). If one character fails, the opposite station requests the transmission of the character again. For broadcast messages, the station is set to FEC mode („forward error correction“, SITOR-B), in which characters are transmitted twice with a time offset.
Only for emergency situations, F1 (FSK) standard teletype mode 50 Bd without error correction is provided. For service communication, an SSB voice radio channel is programmed; a Mikrotel is connected to the control unit for this purpose. Transmission in Morse telegraphy A1 is only possible in exceptional cases.

All functions of the station can be controlled from the control unit TCU 90. The frequency memory and station identifier are called up in various modules.

Receiver CR 91 (SRT)

The receiver used is the double conversion CR 91 from SRT, which provides sufficient frequency stability for ARQ operation.

The exciter TD 90 provides an RF output power of 100 mW in the complete fequency range 1 - 30 MHz and drives the power output stage in use.

Exciter TD 90 (SRT)
The transmitter power amplifier consisting of the final amplifier of the original 1 kW FSK transmitter from Siemens S-502 can be remotely operated. The power apmlifier can be tuned to the 1.5 - 30 MHz range in three ranges: 1.5 - 4/4 - 11/11 - 30 MHz.
During the eighties, the tube power amplifier was replaced by the solid state power amplifier SSA 400 with a power of 400 W, only at the Swiss embassy radio headquarters, the 1000 W power amplifier SSA 1000 was used.

As teleprinter system to print the messages, instead of the noisy rattling teletype machine Siemens Stg-100, an electronic teleprinter PACT-220 from Philips was used.

From 1995 on, the MVS messaging system was introduced, so transferring messages to punched tape was no longer required. A switch automatically selected between the teleprinter or the MVS as the output channel.

The antenna selector ASU-84 (Antenna Switch Unit) feeds the RF to the antenna system, the station has an automatic antenna tuner KGA.

Technical principle


SE-450: SSA 400 The original centerpart of the station, the 1 kW F6 transmitter, was developed by Siemens in 1969, as were the teleprinters. Instead of the E-629 (Collins 51J-4) receivers in the original configuration from 1979, the CR 91 was used after changing over to the ARQ error correction mode.

Also in the mid-1980s, the Siemens power amplifier was replaced by a solid state power amplifier from SRT, usually the SSA 400 and in special cases the more powerful 1000 W unit SSA 1000 was used.

Field use

The radio station SE-450 was used for embassy radio communication, the mobile stations with the same setup were procured for training the staff in Switzerland and foreign embassies; in around 1984/85 the station equipment was modernized and converted to radioteletype with automatic error correction.

In regular operation, the foreign embassy stations operated on timed control. A timer on the main control panel activated the station at a pre-programmed times, the frequencies were selected according to the propagation conditions at the planned transmission time. Only as exceptions, the station could be powered up manually.

For use in crisis regions, there were plans to construct some stations as container stations.

Technische Unterlagen

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en/se-450_ii.txt · Zuletzt geändert: 2020/01/01 20:06 von mb