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To transmit Hellschreiber signals via a radio transmitter, there are basically three options for interfacing the Hellschreiber to a transmitter:

	Directly key a CW transmitter with the contacts of the character-drum (or the Morse key of the Hellschreiber keyboard): This interface is only possible if the Feldfernschreiber has the optional 12-pin round connector on the front of the Amplifier & Interconnect Unit. The keying contacts are available at pin 3 and 4 of this connector.  The connector and plug carry a Luftwaffe part-number (Fl. 32620-1 for the "Steckerdose" socket and -2 for the "12-pol. Stecker" plug). Do not apply more that 200 volt to these contacts, and draw no more than a couple of milli-amps through them (2 watts max combined); ref. 4 (1942) actually even suggests limits of 0.1 mA and 100 volt (1 watt max combined). Note that nearly all German telegraphy transmitters used "grid-block keying" (as opposed to cathode keying): simultaneously blocking the control grids in all stages (allows for near full break-in). Ref. 8.
	Connect the 900 Hz tone that is keyed by the character-drum, to the microphone input of an AM or SSB transmitter: if the transmitter has a VOX function, use that to key the transmitter. Make sure that the off-delay is sufficient. The first and last column of each character is blank (2 x 1/7 x 1/2½ = 114 msec), but few characters (if any) have a pixel at the bottom of the second and at the top of the sixth column. So the off-delay should be at least double that to be safe. A delay of about 1 sec will work fine. The on-delay is not an issue if a pre-amble is used for each transmission (e.g., standard "RRR" or "..."). if the transmitter does not have a VOX function, then use a manual "push to talk" (PTT), or use an external VOX-detector. The output of this detector is connected to the "push to talk" (PTT) input of the transmitter. Examples of such detectors are here (Stephen Smith, WA8LMF) and here (Ramsey Electronics). There are commercial radio-PC interface boxes that generate a PTT control signal with such a detector. E.g., Signalink. Note that I am not endorsing these products.
	Connect the keyed 900 Hz tone to the key jack of a CW transmitter via a keying-circuit. The keying-circuit recovers the original binary (DC) pulses by rectifying and filtering the tone pulses. The recovered pulses are passed to a keying tube (or transistor), the output of which is connected to the keying input of a CW transmitter. The Hell Co. designed tube-based keying devices to do exactly that. See further below. For use with a solid-state CW transmitter, a keying circuit can be made by expanding a  demodulator (ref. 5, 6a, 6b) with a keying transistor. A keying circuit  can be as simple as a diode rectifier, an RC-filter, and a switching transistor. Two diodes and two capacitors can be configured as a  full-wave voltage doubler, that consists of two half-wave rectifiers operating on alternating polarities (ref. 7).

Note that if the12-pin round plug is used,  a contact behind the connector is opened. This interrupts the path of the keyed 900 Hz tone such that it is no longer coupled to the "Leitung" ("line") I/O transformer. Also, own-text is no longer printed during transmission, unless the side-tone of the CW transmission is fed back to the "Empfänger" ("receiver") or "Leitung" input of the Hellschreiber.

Most modern transceivers only require (and can handle) a small signal at the microphone input - in the 10 millivolt range. Typically an appropriate voltage divider (e.g., a potentiometer) must be installed between the Hellschreiber output and the microphone input.

To print signals from a radio receiver, simply connect the audio output (loudspeaker, headset, line) of the receiver to the Empfänger (receiver) input of the Hellschreiber. Note that the Empfänger input has an impedance of 4000 Ω at 900 Hz; if the audio output of the receiver has a low impedance (e.g., 8 Ω), it may me a good idea to use an audio transformer (even 8 Ω : 1 kΩ should be fine).

Note that all AF interfaces (line I/O, phone jack, monitoring) of the Hellschreiber are transformer-coupled, and the keying contacts (character drum, telegraphy key) are completely isolated from the Hellschreiber's electronic circuitry. So there is no need to add additional galvanic isolation (e.g., transformer- or opto-isolators) between the Hellschreiber and a transceiver (unlike when hooking up a PC to your transceiver!).

The diagrams below illustrate the interface options outlined above.

Interface for keying a CW transmitter
 

Keyed-tone from line output to microphone input of transmitter with VOX function
 

Keyed-tone from line output to microphone input of transmitter without VOX function,
plus external VOX-detector with output to PTT input of the transmitter

 

Audio output from the receiver to the Empfänger (receiver) input of the Hellschreiber
(note that the Empfänger input has an impedance of 4000 Ω at 900 Hz)
 

Direct keying of a CW transmitter is no problem with solid-state CW transmitters or tube transmitters with a solid-state keying relay. However, older tube transmitters may have significant negative or positive voltage at the key input connector. This depends on whether grid (grid-block) keying, cathode keying, or anode keying is used. Key-down current draw may also be significant. This may exceed the voltage, current and power limits of the Hellschreiber's character drum and slip contacts. Rudolf Hell and his co-workers (ref. 1, ref. 2) recommend such direct keying only for low power tube transmitters. For medium power (up to 100 W) they describe a keying device (a "Tastgerät") between the drum contacts and the transmitter's key input. The circuit takes the continuous 900 Hz tone of the Hellschreiber (available at the 12-pin round connector of the Feldfernschreiber), transforms its 2 Vpp up to 400 Vpp, and uses a rectifier tube to rectify this continuous tone. Ripple smoothing is done with a simple RC-filter. The resulting voltage is passed through a large current-limiting resistor (1 MΩ) to the key input of a grid-keyed transmitter. The Hellschreiber's drum contacts (also available at the 12-pin connector) are connected across that key input. They short the key input when pixels are transmitted. Anode and filament heater power for the rectifier tube is also available at the 12-pin connector. I have not (yet) been able to determine if Hell or Siemens-Halske ever manufactured such keying devices.
 

Keying circuit for medium power CW transmitter
(source: figure 13 in ref. 1, figure 7 in ref. 2)

For high power transmitters, they suggest a keying circuit that uses the keyed 900 Hz tone from the Hellschreiber, taken directly from the La-Lb/E phone line connector of the Hellschreiber, or even from that connector to a remote transmitter via phone lines. The keying circuit is similar to the one that is used in the Hellschreiber itself for "keying" the electro-magnet solenoid of the printer: a transformer coupled pre-amplifier, followed by a transformer-coupled rectifier with RC-filter, and a keying tube.
 

Keying circuit for high power CW transmitter
(source: figure 14 in ref. 1)

As with CW, an electro-mechanical relay (EMR) could be used for keying the transmitter. Hell recommended against this, based on reliability during long term operation (1940). Note that 1 hour of full-time transmission = 60 x 60 x 2.5 = 9,000 characters. Assuming an average 10 distinct pixels per character, this corresponds to 10 x 9000 = 90,000 switching cycles per hour. Modern EMR typically have a life expectancy of 1,000,000 cycles. I.e., the equivalent of 11 hours of continuous Feld-Hell transmission. Reed relays may have up to 100x longer life expectancy. Mercury-wetted reed-relays are an option, but not exactly eco-friendly, and not usable below -39 deg (F and C) as the mercury freezes, hi!
 

Peter (DC9XD) described a Hellschreiber-transceiver interface in the early days of amateur-Hell:

"Bediengerät zum RTTY-Hell System", Peter Stolzenberg, DC9XD, QRV - Amateur Radio, Heft 2, January 1977, pp.  71-75 [in German; courtesy Gerard Wolthuis, PA3BCB].

Modern keying circuit designs use high-voltage/high-current MOSFETs. Designs are available on the web and in kit form, see e.g. towards the end of ref. 3, and references therein. The 2008/2009 ARRL Handbook for Radio Communications also contains a Universal Keying Adapter design from NØXAS (schematic: here).
 

REFERENCES:

	Ref. 1: section 9a and 9b in "Die Entwicklung des Hell-Schreibers" by the inventor himself: Rudolf Hell; pp. 2-11 in "Gerätentwicklungen aus den Jahren 1929-1939", Hell - Technische Mitteilungen der Firma Dr.-Ing. Rudolf Hell, Nr. 1, May 1940   [in German]
	Ref. 2: section 4a in "Der Siemens-Hell-Feldschreiber", by Rudolf Hell's co-workers G. Ege and H. Promnitz, pp. 11-20 in "Gerätentwicklungen aus den Jahren 1929-1939", Hell - Technische Mitteilungen der Firma Dr.-Ing. Rudolf Hell, Nr. 1, May1940   [in German]
	Ref. 3: "Understanding Soundcard Interfacing" by Ernie Mills, WM2U
	Ref. 4: "Siemens-Hell-Schreiber", pp.  149-166 in “Fernmeldetechnik“, Band 9 of “Lehrbücher der Feinwerktechnik“, Fritz Schiweck, 1st ed., 1942, 526 pp.,  C. F. Winter'sche Verlagsbuchandlung
	Ref. 5: "CW Demodulator" [a synchronous CW/Hell demodulator], Jan-Martin Nøding  (LA8AK), Amatørradio 1987, February, Nr. 41, pp. 41-43, March, Nr. 42, pp. 66-69
	Ref. 6a: "Een CW-Hell interface", Klaas Robers, PAØKLS, Electron, 12/1980
	Ref. 6b: "Ein CW-Hell Interface", Helmut Liebich (DL1OY), RTTY - Informationsorgan d. DAFG e. V. u. d. Swiss-ARTG, Jg. 12, nr. 1, January 1981, pp. 5-8 [translation of ref. 6a,with PCB layout added]
	Ref. 7: "CW Rig Keying Circuit for Hell", Mike Blake, K9JRI
	Ref 8: p. 78 in "Receiver and Transmitter Development in Germany 1920-1945", Arthur Bauer, PAØAOB, IEE Int'l Conf. on 100 Years of Radio, London, September 1994

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