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S-6800 →
  
ASG-6800
Antenna tuner

ASG-6800 is a passive antenna tuner (German: Abstimmgerät) for the short-wave (SW) frequency bands, developed and released around 1970 by AEG Telefunken in Germany, as part of the SP-20 spy radio set. The unit is intended for use with the 30 Watt AEG Telefunken S-6800 transmitter.

It is connected to the transmitter by means of a short coaxial cable with SMB plugs at either end, and converts the 50 ohms unbalanced line of the transmitter into a balanced one, suitable for connection of a long-wire antenna and suitable counterpoise (ground) to the sockets at the left.

Note that the ASG-6800 is a passive device, which means that it doesn not have to be connected to a power source. It matches the output of the transmitter to the connected (wire) antenna and ensures that as much energy as possible is actually passed to the antenna.
  
ASG-6800 Antenna Tuner

Furthermore, it protects the transmitter - and in particular its power amplifier (PA) - against damage caused by impedance mismatches. Note that the antenna tuner is suitable for the entire 2...24 MHz frequency range, but that the best results are obtained when the length of the antenna wires have a relation with the selected wavelength (λ), for example ½λ or ¼λ.

The antenna tuner is usually mounted on top of the S-6800 transmitter, using the metal rails on both devices. When carrying the two units in the pockets of the webbing kit however, they should be fitted the other way around, so that the two banana sockets for connection of the antenna wires, can be reached through two perforations.

The image on the right shows the two devices installed in one of the pockets of the webbing kit, with antenna and counterpoise wires connected to the side of the antenna tuner.
  
Antenna connected through perforations in pocket

ASG-6800 antenna tuner ASG-6800 Antenna Tuner S-6800 transmitter (bottom) and ASG-6800 antenna tuner (top) Close-up of the perforations in the side of the pocket Antenna tuner and transmitter in carrying pocket Antenna connected through perforations in pocket Transmitter and antenna tuner in pocket (connected) Transmitter and antenna tuner inside pocket
Controls
All controls of the ASG-6800 are at the unit's front panel. There are three rotary controls that should be adjusted for a maximum reading of the indicator on the transmitter. A red indicator between the second and the third knob can be used for checking the output of the transmitter.


The output of the transmitter is connected to the SMB socket at the top right of the front panel. A long wire antenna should be connected to the lower banana socket at the left side of the front panel, whilst a suitable counterpoise should be connected to the upper socket. To test the unit, proceed as follows: set the rightmost dial to 'Test' and briefly press the morse key on the transmitter. If full power (30W) is available, the red lamp on the antenna tuner should be lit.

Transmitter   S-6800
The ASG-6800 was designed especially for use in combination with the S-6800 transmitter, which was also developed by AEG Telefunken.

The two units are normally stacked together, as shown in the image on the right. For this reason, the ASG-6800 has a metal rail at the bottom surface of the case, which mates with the metal rail on top of the S-6800. The only connection between the two units is the short coaxial cable at the right.

 More information
  
S-6800 transmitter (bottom) and ASG-6800 antenna tuner (top)

Antenna
The antenna tuner makes it possible to use virtually any type of antenna with the S-8600 transmitter. In practice, a long antenna wire was used on most occasions. The antenna, and a suitable ground, are connected to the left side of the tuner, via 2 banana-type sockets.

The images below show some of the typical wire antennas that were supplied with the SP-20. Please note that transmitter and receiver each needed a separate antenna.
  

Wooden reel with wire antenna Simple set of wire antennas Military wire to be used as antenna Antenna spool in original packaging
Interior
The interior of the ASG-6800 antenna tuner is easily accessible by removing a single screw from the rear end, and sliding off the case shell.

All parts are held in place by a frame that is mounted to the front panel, as shown in the image on the right. Note the dummy load resistor that is mounted to the frame, adjacent to the large tuning capacitor. The full circuit diagram is avaiable for download below [A].
  
Interior - top view

ASG-6800 with case shell removed Interior - top view Interior - top view Top view Bottom view Dummy load resistor Tuning section Detail
Documentation
  1. Antenna Tuner ASG-6800, full circuit diagram
    AEG Telefunken, 1971-1976. 7 pages. 1
  1. Document kindly supplied by Jim Meyer [1].

References
  1. Helmut 'Jim' Meyer, HS0ZHK, My way to Ham - Radio and beyond
    Website QRZ.COM. Personal correspondence.

  2. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN 0952063-36-0, September 2004.
Further information
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Crypto Museum. Created: Monday 29 August 2016. Last changed: Friday, 15 December 2017 - 08:51 CET.
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