DEVELOPMENT OF GERMAN WWII RADIO TECHNOLOGY
(borrowed from the LA8AK site)
INTRO to German WW2 Radio equipment
(Radio Bygones No65, June/July 2000)

Germany was restricted by the Treaty of Versailles from having more than 100,000 men in its military, a restricted navy, and no airforce along with other restrictions. Therefore, to hide its rearmament from allied inspectors, Germany had to develop new military technology in secret. This included communication equipment and strategy. In the 1930s German radio technology was developed in a relative vacuum without much cross-pollination with other countries or institutions. This is why Wehrmacht radio equipment design is so different from the designs of other countries' military radio equipment. This is a unique phenomenon in history, because during most other times, the technological institutions of different countries more or less collaborated on design and inventions. The 1930s saw Germany design its military technology in secret and in isolation.

German World War II Radio Equipment
By Dick Rollema PA0SE

This article is based upon two papers: 'Some Aspects of Unconventional Engineering during Interbellum - as particularly in Germany to enhance the specifications of their communication apparatus' and 'Receiver and transmitter development in Germany 1920-1945', both written by Arthur O. Bauer, PA0AOB, president of the Foundation Centre for German Communication and related Technology 1920-1945. The B/W photographs were taken at Arthur's beautiful collection. His help is gratefully acknowledged (The colour pictures are taken by LA8AK).

HOW IT BEGAN
Development of radio equipment in Germany at first went along the same lines as in other countries. Components were mounted on wooden baseboards and behind a front panel of Bakelite or similar material. More expensive sets featured a cabinet, also made of wood. Electrical shielding was hardly used and coils were of the open ('air cored') type. For a reasonable Q coils had to be of large size and were sometimes wound with 'litz wire', made of a bundle of individually insulated strands. Also special winding techniques were employed to reduce self-capacitance of the coils. By the end of the twenties a different approach was initiated in Germany. There were several important developments that made this possible.

NEW CERAMICS
Ceramic insulation had already been used for many years but only for DC and low frequency AC.  For frequencies above 1 MHz dielectric losses in the material were too high. This was changed in the second half of the 1920s when German firm Hermsdorf-Schomburg-Isolatoren-Gesellschaft, also known as Hescho, marketed new types of ceramic material with very low loss at RF, due to the absence of iron. At first Hescho had problems in measuring the loss. But the solution came from young physicists Dr Rohde and  Dr Schwarz who developed suitable test equipment. That was the start of their successful firm Physikalisch-technisches Entwicklungslabor Dr Rohde & Dr Schwarz'.  After the war well known as 'Rohde & Schwarz' (R&S). Their first export order came from Britain in 1934 for a 'tan delta measuring device'. After 1933/1934 low loss titanium dioxide capacitors with controlled temperature constant became available and these were especially suitable for frequency stabilisation of free-running oscillators, as will be seen later. 

 
Temperature stabilization block of capacitors (EZ6) is shown on uppper left side of the picture). Also note the earliest known implementation of the printed circuit board on the left.
 
Lo40K39a and Lo40K39d temperature compensations are somewhat different.
Ceramics also made possible very good coils, manufactured by vaporising and burning a heavy copper and/or silver layer onto a ceramic cylinder. Parts of the layer were then cut away such that a helix remained, forming the coil winding. Stable coils with high Q could thus be mass-produced. The construction also resulted in a very low temperature coefficient, equal to that of the ceramic material and not of the metal of the winding. Siemens reported temperature coefficient of such coils was up to 200 times lower than that of the best conventionally made coils.
Ceramics also made possible very good coils, manufactured by vaporising and burning copper and/or silver layer onto a ceramic cylinder. Parts of the layer were then cut away such that a heliz remained.

IRON DUST CORES
Coils with iron dust cores were already used by the telephone industry in the early 1920s, for instance as Pupin line loading coils, but they were not suitable for frequencies above about 10 kHz. A great improvement was made by versatile inventor Hans Vogt who had the smart idea of using 'carbonyl iron'. The advantage of this material was that the iron dust particles were spherical and only between 2 and 6 micron in diameter, with a very thin oxide film on their surface. This avoided eddy currents and the associated losses. In the thirties the material became known in Britain and Germany by its trade name 'Ferrocart'. It made possible compact coils of high quality that could be easily screened by a small can without affecting their Q.

DIE-CAST FRAMES
After the radio on a wooden baseboard the use of a metal chassis became popular, and it remained like that till the appearance of the printed circuit board. Early in the 1930s C. Lorenz company (since May 1930 owned by ITT, after WWII known as Standard Elektric Lorenz until the end of the 1980s; now owned by French Alcatel) was looking for more rational construction techniques. Until then the chassis moved all the way from the metal workshop up to the testing station at the end of the production line. So only a few people could work on it at the same time. Lorenz wanted a construction consisting of separate modules which in the final stage of the production line could be bolted or clicked together. Such modules could be manufactured at the most suitable production site, even being tested and calibrated there. The use of die-casting (Spritzguss) was soon found to be the answer. For their application, involving small and relatively complicated shapes and very tight tolerances, Lorenz sought support from outside. That was found at the firm of Mahle, manufacturer of aluminium pistons for internal combustion engines. According to an advertisement in a German technical magazine of 1938, Mahle could produce die-cast samples of 0.5-2000 gram and tolerances of a few hundredths of a millimetre! The alloy used became known as 'Elektron' and consisted of about 8.5-9.5% Al; 0.5% Zn; 0.2% Si; 0.2% Mn and the rest Mg. It had a specific gravity of only 1.8. During the war, shortage of aluminium forced the industry to replace the lightweight Elektron by an alloy based on zinc. After mid 1943 the German Army received more and more equipment made of the much heavier zinc alloy. Only the Luftwaffe (Air Force), the best equipped of the three forces, retained the high quality Elektron for their radios.

LIMITED NUMBER OF VALVES
The German air force, army and navy were trying to standardise their components as much as possible. A problem was the large number of different valves used in communications equipment. It was therefore decided to force the industry to integrate and co-ordinate their activities on commercial and military projects. This resulted in a completely new generation of radio receiver and transmitter valves. Also the number of different types was greatly reduced. Several military receivers used only one type of valve in all stages. This was the universal pentode RV12P2000 (Photo 1). R stood for valve (Rohre); V = low power, 12 = heater voltage; P = pentode, 2000 = amplification factor (mu). More than 16 million were produced during WWII. The valve is inserted upside down into its holder, which completely surrounds it. To withdraw the valve a knob, visible on Photo 1, is screwed into the bottom.

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