Last page update: 8 April 2017
The term "communication performance" is often quite subjective, as the list of conditions that could, or would have to be, specified is sheer endless. We are talking here about the Hell Feldfernschreiber - a particular military field teleprinter of the era 1935-1945. For Hellschreiber, I will use my simple personal definition of the term "performance". That is: "legibility of copy, for prevailing conditions of operational use and range". The competition was voice/phone communication, "Morse" telegraphy, non-Hellschreiber teleprinters, and possibly couriers and courier pigeons. The basic communication channels are:
- wired, i.e., via phone lines ("Drahtbetrieb")
- wireless, i.e., via radio ("Funkbetrieb")
Wired and wireless links are complementary, and each has its advantages and disadvantages. They are listed below, based on ref. 1A. Also see ref. 1B. In ground-war situations, telephone lines were used extensively, as long as the front line was stationary. Obviously, "wired" is not an option for ship-to-ship, ship-to-shore, air-ground, and air-to-air communication.
In general, wired links (for telephone, teleprinter, and telegraphy) :
- are suitable for long messages and high-volume communication between fixed and mobile headquarters, staffs, airfields, etc.
- and where fast and secure communication is required.
- are limited by available material (cables, etc.) and installation manpower.
- require more planning ahead than radio links (radio links can be set up ad-hoc).
- can be interrupted by accidents and bombing and artillery attacks, not only of the end-stations and switchboards, but also of any wiring in between.
- can be built and set up faster than wired links; hence, they are more suited for fast moving front lines. Quicker to establish than line com, when front moves (which could be over 100 km a day), e.g., tank attacks and airfield changes.
- are also more suited to cross large bodies of water and areas that are "still held by the enemy", and terrain where it is hard to install wired lines.
- very easy to broadcast general instructions to own forces, and for broadcasting propaganda material.
- radio equipment is generally smaller, lighter, and more mobile than wired equipment, but require more electrical power.
- limited by the available number of channels, can be jammed and monitored by the enemy.
- medium- and short-wave links are subject to ionospheric conditions. VHF suffers much less from this, but its range is limited by mountains and other obstacles.
- for enemy bombing and artillery attacks to interrupt a radio link, they must hit the transmitter or receiver station.
FELD-HELL COMMUNICATION VIA WIRED LINKS (PHONE LINES)
The Hell Feldfernschreiber has a two-wire phone line connector marked "La Lb/E", short for "Leitung-a" and "Leitung-b/Erde". I.e., Line-a and Line-b/earth (ground). This transformer coupled interface has an impedance of 800 ohms. Note that this is the impedance of a standard standard "600 ohm" POTS telephone land-line for the 900 Hz tone frequency of the Hellschreiber pulses. Hellschreiber can be used over any line connection that is suitable for voice communication. Maximum allowed signal attenuation between two Hellschreiber stations is a respectable 5.3 neper (≈46 dB). Per the Wehrmacht and Luftwaffe Feld-Hell manuals, the range of Hellschreiber communication is 25-50% greater than that of voice communication over the same channel (p. 6 in both ref. 1C and 1D), and is comparable to that of Morse-telegraphy (ref. 1E, 1F).
Fig. 1: Feld-Hell communication range for various types of field telephone cable (no line amplifier)
(source: ref. 1A3; range can also be calculated from tables in ref. 1A4 and the max allowed 46 dB attenuation)
According to the Feld Hellschreiber manual (p. 3 in ref. 1G), the Hellschreiber communication range over phone lines (without line amplifiers) is:
- 35 km (≈22 mi) over type DL500 field phone lines (22 km, ≈14 mi, when wet)
- 60 km (≈38 mi) over regular pupin cables
- 160 km (≈100 mi) over type FL250 pupin cables
Pupin cable ("Pupinleitung", pupinized line, loaded line) is named after the Serbian-born scientist Mihajlo Idvorsky Pupin (1845-1935). He proved that placing a loading coil at regular intervals (½ - 1½ mile) in series with phone lines, greatly extends the communication range. Adding the coils is referred to as "pupinization". Over the years, standard toroidal coils of 22, 44, 88, 118 and 135 mH were introduced successively. Siemens-Halske made pupin cables in Germany. In phone systems, these (series) loading coils do not permit modern data communication (e.g., POTS-modem, DSL, ADSL, etc.) with a data rate that is beyond voice bandwidth (300-3400 Hz).
According to ref. 1H, the estimated range of voice communication over German phone lines (again, without line amplifiers) was:
- 50 km (≈31 mi) over type FFK phone lines ("Feldfernkabel", field telecom cable)
- 120 km (≈75 mi) over 1.5 mm bronze FD lines ("Feld Dauerleitung", permanently installed field cable)
- 200 km (≈125 mi) over 2 mm bronze FD lines
- 425 km (≈265 mi) over 3 mm bronze FD lines
As stated above, the Feld-Hell range would be 25-50% larger.
With such cables, end-to-end networks of over 4000 km (≈2500 mi) were created (p. 9, ref. 1H). Ref. 1J1 has additional estimates for voice-communication ranges over wired lines. At division level, typically single-core heavy field cable was used, usually with earth/ground connections used for the return current. Along roads and where suspension points were available, the cables were suspended about 3 m (10 ft) above ground ("Hochbau"). Away from roads, near villages and in areas under enemy observation, the wires would typically be laid on the ground, close to the ground, or suspended in cable trenches ("Tiefbau"). To minimize the risk of interception, the used of double cables were prescribed within 3 km of the front line, preferably suspended in cable trenches or on other low suspension points. It was forbidden to lay double field cables directly on the ground or suspend it in tree tops.
Fig. 2: various types of Feldkabel
(b & e: pre-WW2, c & f: mid-war, d & g: late war; source: ref. 1K, used with permission)
A "Kabelaffe" (lit. cable monkey, a disrespectful nickname for a cable-laying soldier) could install a kilometer of field cable ("Feldkabel", Fkb) in about 5-60 minutes. This depended on local terrain conditions and whether the "monkey" was on foot, on horseback, or using a special Kettenkraftrad für Feldfernkabel such as the Sd.Kfz. 2/1 cable-laying tracked motorcycle or 2/2 tracked vehicle, both from NSU Motorenwerke AG. Heavier long-distance field cable ("Feldfernkabel", Ffkb, shielded 4-wire cable) took longer. Cable removal took 5-30 minutes per kilometer. See the tables in Fig. 3 below. That same text suggests a voice-communication range of 10-20 km (≈6-12 mi) over light field cable, and 50-60 km (≈30-37 mi) over heavy field cable (range is reduced when cables are lying on the ground, rather than suspended).
Fig. 3: times required for installing 1 km (≈0.6 mile) of field lines (left) and voice-ranges (right)
(source: probably "Handbuch der Fernmelder")
Instead of 2-wire cables, it is possible to establish a telephone or telegraphy link, with a single wire in combination with ground-return. The return is literally via the ground (earth): both stations have a ground electrode stuck into the soil. I have no information about achievable communication ranges via ground. Obviously this fully depends on the conductive properties of the soil. "Single-wire + ground return" was widely used during the trench warfare of WW1. Note that it is not secure: ground currents of the signals can be intercepted by the enemy, by simply placing two sufficiently spaced sensing-electrodes into the ground, and amplifying the differential signal. Ref. 1H, 1L, 1M. German regulations banned the use of single-wire operation within 3 km (≈2 mi) from the front.
Note that "civil" Hellschreibers, and Feld-Hell machines used in civil applications, have also been used over 60 kV and 100 kV high-voltage power lines (ref.
1N, 1P). This required special modems that operated with carrier frequencies in the 50-300 kHz range.
FELD-HELL COMMUNICATION VIA WIRELESS LINKS (RADIO)
The "Hellschreiber" was invented by Rudolf Hell, explicitly for news agency teleprinting via radio ( = " wireless"). Numerous Wehrmacht transmitters and receivers were explicitly intended for use with the Feld-Hell machine. See this page. It is important to note that the Feld-Hell was not conceived for use with any particular receiver, transmitter, or transceiver. In principle, any radio suitable for fast CW (on-off keyed "Morse" telegraphy), AM, or FM communication could be used. FM modulation was used on UHF directional radio links, such as the "Michael" system. Feld-Hell does not appear to have been used with FM portable radios. During WW2, the Feld-Hell system was used with transmitters with an output power of several watts to 1 Megawatt!
Of course, had transmitters with Single Side Band (SSB) modulation been commonly available for military communication, they could also have been used. Ref. 2A-2C. The SSB concept dates back to 1914 and the 1915 US patent US 1449382 "Method and Means for Signaling with High Frequency Waves" by John Carson/AT&T. Commercial transatlantic SSB radio telephone service on long-wave dates back to 1927. The actual 55 kHz SSB transmitter for the London - New York radio link was in the RCA transmitter plant at Rocky Point/NY, on Long Island, some 150 miles east of New York city.
In order to reduce interference susceptibility on overseas HF radio-telephone circuits, the Deutsche Reichspost inaugurated SSB traffic between Berlin and New York in 1931, as proposed by Dr. W. Runge of the Telefunken company.
In WW2, SSB techniques were also used for speech scrambling/encryption. Radio telephone conversations between the US and England, including some early conversations between Roosevelt and Churchill, were intercepted and 'decrypted' by the Germans.
The radio range with Hellschreiber is similar to that of CW "Morse" telegraphy (ref. 1D). The table below list the CW range of a number of standard Wehrmacht transmitters. As the war evolved, the German military recognized that the use of shortwave radio became necessary, and 200 - 800 watt transmitters were used on 3-20 MHz, instead of the 1.5 kW longwave transmitter. Though not until 1941. The 15 watt transmitter for 3-7.5 MHz did not prove itself for medium ranges. Ref. 3A.
Fig. 4: communication range of several Wehrmacht transmitters and transceivers
(source: Fig. 54 in Chapter 8 of ref. 1H)
Terrestrial radio communication implies the exchange of modulated electro-magnetic energy between a transmitter and a receiver, via the atmosphere. The propagation path that this radiation follows, and hence the communication range, depends on many factors, such as operating frequency, location and characteristics of the antennas, properties of the terrain and of the atmosphere. Obviously, the range also depends on the transmitter power and receiver sensitivity, modulation techniques (incl. data redundancy), as well as local background noise levels. Depending on the operating frequency, the radio waves tend to follow the curvature of the earth as a "ground wave", are limited to slightly more than the straight line-of-sight, pass through the atmosphere into space, or are refracted (and in extreme cases reflected) by layers or areas of ionized gasses in the atmosphere. The ionosphere is the part of the atmosphere where the density of ionized gasses is high enough to affect radio wave propagation. Radio waves that are reflected back to earth may actually bounce back and forth between the ionosphere and the earth several times, and "hop" over great distances. Note that there may be a silent zone (reduced or even zero strength of the received signal) between the range of the ground wave and the area where the first "hop" touches down, as well as between subsequent "hops". This is referred to as the skip zone. Ref. 3B.
Fig. 5: simplified presentation of ground-wave and sky-waves
(source: Fig. 2-19 in ref. 3C)
Gasses in the ionosphere are photo-ionized ( = a photon strips an electron from a neutral atom) by space radiation, primarily from the sun. Hence, this radiation is at its greatest during the daylight hours. There are many layers in the ionosphere because the gases absorb the sun’s radiation at different wavelengths. The free electrons can reflect (actually refract) radio waves, hence, affect radio propagation. Note that ionization can also be caused by meteorites (solid space debris, the size of dust particles or much larger) that enter the atmosphere at high speed.
The are regular and irregular ionospheric variations. The regular variations are primarily driven by:
- the time of day ("diurnal" effects, such as appearance of the D and E layers after sunrise)
- the time of year ("seasonal" effects, such as the variation of the height of the F2 layer between winter and summer)
- geographic location (ionization is strongest in the equatorial regions, electron densities are lower in the polar regions)
- the sun's cyclical nature:
- short-term: 27-day rotation period of the sun
- long-term: the sunspot cycle (ca. 11 years)
Fig. 6 below shows the cyclic sunspot variation during the period 1935-1945. During the peak of the cycle, propagation conditions for long-range communication on frequencies at the high end of HF (30 MHz) are generally very good, whereas during the minimum of the cycle, ionospheric propagation at those frequencies is typically very poor. Note that the sunspot number of and by itself is not a dependable indicator of propagation conditions for a specific situation.
Fig. 6: the sun spot number peaked in April 1937 and May 1947, with a deep cycle-minimum in February of 1944
Irregular solar events cause irregular ionospheric and, hence, propagation variations. Examples of such solar events are solar flares, coronal mass ejections (CMEs), proton events, and coronal holes. These effects may be so intense, that all skywave propagation is blocked.
Radio wave propagation is a very complex phenomenon that is well beyond the scope of these pages. General discussions of this topic are provided by ref. 3B-3L.
Clearly, it was of strategic military importance to be able to predict, with some degree of accuracy, what the optimum and maximum usable operating frequency would be. Several Luftwaffe radio information centers (Funkberatungsstellen, such as at the major research and test center at Rechlin, about 100 km northwest of Berlin) provided twice-monthly propagation forecasts for ground-ground and ground-air communications over several ranges and for several time periods. Ref. 3M, 3Q. The Allied forces had similar services, e.g. ref. 3N.
Fig. 7: examples of Luftwaffe propagation predictions (operating frequency vs. distance) for September 1941
(left: usable frequencies, right: max usable frequencies; source: ref. 3M)
Clearly, the best real-time propagation indicators are are provided by vertical sounders (ionosondes, mid 1920s technology) and beacon networks. The earliest record of radio propagation beacons goes back to World War II, when the German military operated such beacons on wavelengths of approximate 80 m and 10 m.
The next two diagrams show maps centered on Berlin, allowing to determine ranges from there to the major war theatres in Europe and around the world.
Fig. 8: azimuthal-equidistant map, centered on Berlin
(background map generated with the OK2PBQ map generator)
Fig. 9: Azimuthal-equidistant map, centered around Berlin
Variations in propagation "quality" may cause short-term and frequency-selective fading (reduction in the strength of the received signal). The Hellschreiber system is not immune to this, though probably less sensitive than competing start-stop teleprinter systems of the era (possibly with the exception of FSK systems, ref. 4B). In the presence of fading, the Luftwafe radio info center recommended minimizing the effects by operating near the maximum usable frequency (MUF):
Fig. 10: Luftwaffe recommendation for using the maximum usable frequency (MUF) to minimize fading (1942)
(source: ref. 3M)
Variations in propagation "quality" may also cause the duration of transmitted pulses to be modified, be it reduced or stretched (smeared). The test print-outs below show that that Hellschreiber system is very insensitive to such effects:
Fig. 11: effect on print quality when transmission signal propagation causes pixel duration change
(a: reduction by 60%, b: increase by 60%; source: Fig. 67 in ref. 3P)
Fig. 12: print-outs of April 1938 long-distance Hellschreiber tests
(50 kW transmitter station DLo at Rehmate (near Berlin) and receiver at Santiago de Chile; source: ref. 4A)
As late as 1956, the ITU/CCIR determined required SNR and fading allowances for Hellschreiber (FSK):
Fig. 13: CCIR/ITU performance parameters for Hellschreiber (FSK)
(source: ref. 5)
For a more modern view on Feld-Hell performance, see ref. 6.
- Ref. 1: wired communication
- Ref. 1A: "Drahtnachrichtenverbindungen Richtfunkverbindungen 1939 - 1945", Part 2 of "Der Weltkrieg", which is Volume 2 of "Ln- Die Geschichte der Luftnachrichtentruppe", Karl Otto Hoffman, Kurt Vowinckel Verlag, 1973, 762 pp., ASIN: B002N0OROA.
- Ref. 1A1: pp. 3, 4: "Die Nachrichtenverbindungen Allgemein"
- Ref. 1A2: pp. 4, 5: "Das Drahtnachrichtenwesen"
- Ref. 1A3: Anlage 14 (Appendix 14): "Reichweiten der Fernschreibübertragung in Km"
- Ref. 1A4: Anlage 13 (Appendix 13): "Dämpfung - Richtwerte dür Kabelleitungen und Freileitungen"
- Also see Vol. 1: "Die Anfänge von 1935 - 1939", 1965, and Volume 2, Part 1 "Der Flugmelde- und Jägerleitdienst 1939 - 1945", 508 pp.
- Note: this three volume series (ca. 1600 pages) "Ln- Die Geschichte der Luftnachrichtentruppe" is the one and only encyclopedia on the subject of the Luftwaffe Signal Corps!
- Ref. 1B: "Telecommunications in War", S.A. Angwin, J. of the IEE, Part IIIA: Radiocommunication, Vol. 94, Issue 11, Nov. 1947, pp. 7-15
- Ref. 1C: "Der Feldfernschreiber", Luftwaffen Dienstvorschriften L.Dv. 702/1 Heft 213, "Luftnachrichtentruppe – Ausbildung am Gerät, Teil 1 - Gerätebeschreibungen", Reichsluftfahrtministerium (RLM), 21 November 1940 [25 MB; this is the Luftwaffe Feld-Hell manual; counterpart to the Army manual D 758/1 below]
- "Der Feldfernschreiber", LDv 702/1, Heft 213; pp. 8-27 in "Fernschreibmaschinen", Heft 2 of "Fernschreibgerätelehre", Fsg-2, Arbeitsunterlagen für nachrichtentechnisches Unterricht, Luftnachrichtenschule, Halle (Saale), 2. Auflage, July 1943, 107 pp. [high-resolution scan - 47 MB]
- Ref. 1D: "Der Feldfernschreiber", document D 758/1 of the Oberkommando des Heeres, Heereswaffenamt, Amtsgruppe für Entwicklung und Prüfung, Berlin, 1 April 1941; this is the official original army manual in German for model T typ 58 (a.k.a. model 24a-32, a1, a2, based on drawing numbers). Here is another scan of this document, with high image quality. [30 MB]
- "The Feldfernschreiber", the above D 758/1 "Der Feldfernschreiber" document, translated into English and annotated by me, Frank Dörenberg; updated 2 May 2009.
- Ref. 1E: last § in Section I of "Der Siemens-Hell-Feldschreiber", by Rudolf Hell's co-workers G. Ege and H. Promnitz, pp. 11-20 in "Hell - Technische Mitteilungen der Firma Dr.-Ing. Rudolf Hell - Gerätentwicklungen aus den Jahren 1929-1939", Nr. 1, May 1940.
- Ref. 1F: p. 525 in "Fernwirktechnik", in "ETZ: Elektrotechnische Zeitschrift. Ausg. A.", Jg. 59, Heft 20, May 1938
- Ref. 1G: "Instruktion för Hellskriv-apparat", Kungl Arméförvaltningens Tygavdelning, SiB/040:3045, 13 July 1945, 40 pp. (courtesy Willi Reppel, SM6OMH)
- Ref. 1H: "Some aspects of military line communication as deployed by the German armed forces prior to 1945", Arthur O. Bauer, (PA0AOB), 26 December 2004, 21 pp. Source: www.cdvandt.org
- Ref. 1J: "Handbook on German Military Forces", War Department Technical Manual TM-E 30-451, 15 March 1945. (revision of TM 30-450 of 17 December 1941)
- Ref. 1J1: Fig. 59
- Ref. 1K: "German Field Line Communication equipment of WW2", R. Campers, Funksammler Publ., 2010, 147 pp. [17 MB] See note 1
- Ref. 1L: "Erdtelegrafiegeräte", Werner Thote, pp. 21-25 in "Radiobote", Jg. 5, Heft 28, Sept-Oct 2010 [pdf]
- Ref. 1M: p. 192 "The role of the First World War in the rise of the electronics industry", F. Nebeker, in "Engineering Science and Education Journal", Vol. 10, Issue 5, October, 2001, pp. 189-196
- Ref. 1N: "Der Betrieb von Siemens-Hell-Schreibern auf Hochspannungsleitungen im Elektrizitätswerk-Nachrichtendienst" [operating Hellschreibers on 60 kV and 100 kV power lines], Fernmeldetechnik, Siemens & Halske, Berlin-Siemensstadt, 1939, SH. 7802, 1,5 9. 39, 4 pp.
- Ref. 1P: pp. 94-96, 162, 163, 170-172 in "Hochfrequenz-Nachrichtentrechnik für Elektrizitätswerke", 2nd ed., Gerhard Dreßler, Heinrich-Karl Podszeck, Springer Verlag, 1952, 183 pp.
- Ref. 1Q: "World War II Battlefield Communications", Gordon Rottman, Peter Dennis, Osprey Publ., 2010, 64 pp., ISBN-13: 978-1846038471
- Ref. 1R: p. 242 in "Recent Developments in Telegraph Transmission, and their Application to the British Telegraph Services", L.H. Harris, E.H. Folley, F.O. Morrel, pp. 237-268, Journal of the Institution of Electrical Engineers, Vol. 80, Issue 483)
- Ref. 2A: "Early History of Single-Sideband Transmission", A.A. Oswald, Proc. of the IRE, Vol. 44, Issue 12, Dec. 1956, pp. 1676-1679
- Ref. 2B: "The Application of SSB to High-Frequency Military Tactical Vehicular Radio Sets", R.A. Kulinyi, R.H. Levine, H.F. Meyer, Proc. of the IRE, Vol. 44, Issue 12, Dec. 1956, pp. 1810-1823
- Ref. 2C: "Long-distance point-to-point communication", A.H. Mumford, Journal of the Institution of Electrical Engineers - Part IIIA: Radiocommunication, Vol. 94 , Issue 11, 1947, pp. 23-43
- p. 26: advantages and (minor) disadvantages of SSB; note: p. 37 missing
- Ref. 3: radio wave propagation
- Ref. 3A: p. 51 in "Aus der Rüstung des Dritten Reiches (Das Heereswaffenamt 1938-1945); ein authentischer Bericht des letzten Chefs des Heereswaffenamtes", General Emil Leeb (last chief of the Heereswaffenamt), in "Wehrtechnische Monatshefte (Zeitschrift für Wehrtechnik, Wehrindustrie und Wehrwirtschaft, Deutschen Gesellschaft für Wehrtechnik)", Beiheft 4, May 1958,70 pp. [62 MB]
- Ref. 3B: "Skip the "skip zone"; we created it and we can eliminate it", David M. Fiedler, Army Communications, Spring 1986, pp. 41-45
- Ref. 3C: "Introduction to wave propagation, transmission lines, and antennas", US Navy Electricity and Electronics Training Series, NAVEDTRA 14182, Module 10, September 1998, 276 pp.
- Ref. 3D: "Radio-Wave Propagation during World War II", K. A. Norton in "Proceedings of the IRE", Vol. 50, Issue 5, May 1962, pp. 698-704
- Ref. 3E: "Radio Propagation Following World War II", L.A. Manning, Proc. of the IRE, Vol. 50, Issue 5, May 1962, pp. 709-711
- Ref. 3F: "Radiowave propagation - the basis of radiocommunication", L.W. Barclay, Proc. Int'l Conf. on 100 Years of Radio, 5-7 Sept. 1995, IEE Conf. Publ. No. 411, pp. 89-94
- Ref. 3G: "An introduction to HF propagation", S.D. Gilbert (G4UCJ)
- Ref. 3H: "Solar activity & HF propagation", P. Harden (NA5N), Presented at 10th Anniversary of ARCI's FDIM Symposium, Dayton Hamfest 2005, pp. 81-88
- Ref. 3J: "HF Radio Propagation", Marc Tarplee (N4UFP), ARRL South Carolina Section (ARRL-SC)
- Ref. 3J: "A simple HF propagation primer", Geoffrey Noles (AE4RV) - nice presentation in Flash file format (so won't work on Apple® products)
- Ref. 3K: pp. 294, 295 in "The fading machine, and its use for the investigation of the effects of frequency-selective fading", W.J. Bray, H.G. Lillicrap, F.C. Owen, pp. 283-297, Journal of the IEE, Part IIIa: Radiocommunication, Vol. 94, Issue 12, March-April 1947)
- §5.4.1 "Effect of selective-fading on Hell-printer signals transmitted by M.C.W."; Modulated CW (MCW) Hell and FSK Hell; "...the performance is most conveniently judged by inspection of the tape records."
- Ref. 3L: "Beyond-line-of-sight propagation modes and antennas", David Fiedler, George Hagn, in "Army Communicator", United States Army Signal Center, Vol. 8, No. 4, Fall 1983, pp. 14-22
- Ref. 3M: "Wellenberatung der Truppe durch die Funkberatungsstelle", 2nd ed. of "Frequenzberatung der Truppe durch die Funkberatungsstelle Rechlin", Rawer, Dieminger, Plendl, Erprobunsgstelle der Luftwaffe Rechlin, Abt. F, 12 May 1942, 56 pp. Source: www.cdvandt.org
- Ref. 3N: "Signal Training Pamphlet No. 2 - Part IX -- Aerials and Frequency Selection for Corps and Divisional Signals", 26 G.S. Publications/1525, UK War Office, November 1945, 55 pp. See note 1
- Ref. 3P: "Verringerung der Fehleranfälligkeit drahtloser Telegrafiewege durch Maßnahmen im Niederfrequenzteil der Überttragungssysteme", [Hellschreiber on pp. 56-58] K. Reche, A. Arzmaier, R. Zimmermann, in "Telefunken-Hausmitteilungen", Jg. 20, Heft 80, March 1939, pp. 53-62
- Ref. 3Q: "Atlas günstiger Kurzwellen für 1943" and "Atlas günstiger Kurzwellen für 1944" , Druckvorschrift D 925/1 (1 March 1943) and D 825/2 (29 May 1944), respectively. Available at Bundesarchiv in Freiburg/Germany, Archivsignatur RHD 8!510 and 8/1511
- Ref. 4: performance and distortion
- Ref. 4A: "Eine Untersuchung zur Bekämpfung von Doppelzeichen im Kurzwellen-Hell-Telegrafie-betrieb", H. Haberland, pp. 32-40
- Ref. 4B: "Military Teletypewriter Systems of World War II", F.J. Singer, Transactions of the AIEE, Vol. 67, Issue 2, 1948, pp. 1398-1408
- p. 1403: pre-war: transmission of TTY pulses over long-haul HF radio was not done merely by connecting TTY machine to radio facilities, since no features to overcome fading, QRM, QRN disturbances. Hence, FSK introduced.
- Ref. 4C: "Multitone signalling system employing quenched resonators for use on noisy radio-teleprinter circuits" [Piccolo vs Hell], H.K. Robin, D. Bayley, T.L. Murray, J.D. Ralphs, Proceedings of the Institute of Electrical Engineers (I.E.E.), Vol. 110, Nr. 9, September 1963, pp. 1554, 1556
- Ref. 4D: "Picolo players" in "Communications", Pat Hawker (G3VA), in "Wireless world", Vol. 88, November 1982, p. 42
- Ref. 5: p. 160, 165 in "Documents of the VIIIth Plenary Assembly, Warsaw 1956, Volume I - Recommendations, Reports, Resolutions Questions and Study Porgrammes", C.C.I.R. International Radio Consultative Committee, International telecommunication Union (ITU), 684 pp.
- Ref. 6: "Hellschreiber, Part I - History and description; Part II - getting on the air; Part III - Signal analysis and digital philosophy; Part IV - Frequency domain", Murray Greenman (ZL1BPU), in "Break-In Amateur Radio Magazine" (NZART), Vol. 71, October 1998, pp. 7-8, Vol. 71, November 1998, pp. 4-6, Vol. 71, December 1998, pp. 6-8, Vol. 72, Jan/Feb 1999, p. 6.
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