The Messerschmitt Me-109 Radio

by William P Lear,
President, Lear Avia, Inc

Here is an interesting description of the radio equipment now being used in one of the most popular Nazi fighters.

On a murky afternoon last December, a low-flying Messerschmitt 109 was shot down in action over the Thames estuary. Its pilot, mortally wounded by a machine-gun bullet which pierced the left side of the cockpit, managed to bring his ship down intact to a wheels-up landing on the mud flats. Then he died.

This was one of the few Messerschmitt 109s to land on English soil virtually undamaged. Its yellow nose signified that it belonged to one of the better Luftwaffe squadrons. The markings on its tail testified to its bag: one English, two Dutch and two French airplanes shot down.

Salvage crews removed the airplane from the mud flats and turned it over to the British Air Ministry, who removed from it the pilot's papers and most of the instruments. The rest of the airplane — airframe, engine and radio apparatus — was shipped to the United States by Lord Beaverbrook at the request of Mrs Winston Churchill, who is Honorary Sponsor of Bundles for Britain, for use in their drive for funds in this country. The airplane was crated and shipped in convoy to Canada, and arrived in New York late in June. Through the courtesy of Mr Rand Sutherland, of Bundles for Britain, the radio apparatus was removed from the Messerschmitt by our mechanics from Lear Avia's New York office, and the entire radio apparatus was air expressed to Lear Avia's engineering department in Dayton, Ohio, for tests, investigation and critique.

This marked the first instance that an American radio manufacturing concern was afforded an opportunity to examine at first hand German military aircraft radio apparatus. Since a record of the findings should be of some interest to our radio engineers and technicians, the following brief summary and description of this radio apparatus is submitted.

General Observations

The Messerschmitt 109 Fighter type was developed originally in 1933, and its manufacture continues to this day with only slight variations and refinements of the original design. Powered with a 1150 hp Daimler-Benz engine, the Messerschmitt 109 has a top speed of 354 mph at 12,500 feet, cruising speed of 298 mph at sea level, ceiling of 36,000 feet, and a range of 620 miles. Its wing span is 32 ft 6 in, its length is 32 ft. Weighing empty 4,180 lbs, it carries 1,340 lbs of bombs, and is armed with a 23-mm cannon, two .30-calibre machine guns in the fuselage and two .30-calibre machine guns in wings.

The radio apparatus installed in this airplane consisted of a low-power fixed- frequency transmitter, a companion receiver, a dynamotor power unit operating from a 24 VDC power source, together with the necessary cables, junction boxes and controls.

Although the exact date of its manufacture could not be ascertained the radio apparatus, as a type, dates back to around 1933, and although it is definitely known that this type of equipment is still being manufactured, it by no means represents the latest and the best in German military aircraft radio apparatus. Parenthetically, it may be added that information on more advanced types of German radio apparatus is on record in this country but exact data and performance figures are not available for public presentation.

The Messerschmitt 109 radio, despite its comparative obsolescence, is significantly symptomatic of German design and manufacturing practices and of the tactical uses of aircraft radio under actual warfare conditions.

From the design standpoint, it is important to note the high degree of standardization of tubes and component parts of German aircraft radio transmitters and receivers. Thus, the Messerschmitt 109 transmitter design, as well as most subsequent and more modern designs, are made around only two basic tubes: Telefunken REM-904 and RES-1664D. Similarly, the Messerschmitt 109 receiver and most subsequent and more modern designs have been built around only one basic tube, Telefunken RENS-1264.

It is evident that the Germans have "frozen" on this design a good many years ago and have concentrated on making a large number of these units available for installation into the Messerschmitt 109s.

The equipment is built very ruggedly. The chassis of both the transmitter and the receiver are light-weight castings; the material used is Elektron, a special aluminum alloy having a high percentage of magnesium, which does not burn even when subjected to intense heat. The casting and the rest of the equipment is painted a dark gray which is the standard color of the German military apparatus.

The various units are interconnected with cables complete with their coupling plugs. These plugs are waterproof and include a ceramic terminal board for the various connections. The practice in this country is to provide cable connections at the receiver or transmitter proper, thus eliminating the disadvantage of having a length of cable with its coupling plug permanently affixed to the apparatus. Noteworthy is the fact that while shielded rubber-covered cables are used in this country for maximum dependability under humidity and abrasion, Germans employ the inferior fibre-covered cables, probably indicating a shortage in these essential materials.

The workmanship is very good. However, the equipment as a whole will not meet our requirements which we impose on our commercial equipment with respect to performance under various temperature and humidity conditions. The German equipment will not pass our tests for operational characteristics over a temperature range from -40° to +150° F. Furthermore, since the various metallic surfaces of the German equipment were not protected against corrosion, it could not pass our standard humidity test. In this connection, it should be noted that none of the coils are impregnated with a protective coating of wax; our standard humidity tests would certainly render the equipment inoperable, if not wreck it.

The high degree of standardization of German radio apparatus is further evidenced by the use of identical connection plugs and subassemblies, and even minor components, in a great number of various types of German military aircraft radio apparatus. Thus, beginning with the types developed around 1933 when Germany first started actually preparing for war, and down to the latest types developed only last year these same components and subassemblies are constantly recurring.

This is symptomatic of large-scale quantity production of radio equipment, and indicative of interchangeability not only of the various components within the radio apparatus, but also of the radio apparatus itself in the various types of aircraft. It should be noted that this procedure, although effective in overcoming production problems, imposes certain grave penalties on the nation adopting it.

In the case of German equipment in general, and that of the Messerschmitt 109 in particular, the heaviest penalty is imposed by the necessity of adhering to obsolete and obsolescent design practices. In some respects German equipment is at least ten years behind commercial practices in this country, although it is known that better designs are available. These better designs, however, only began to become evident in the latest German aircraft, indicating that a basic new re-standardization is apparently in progress, and that more advanced designs and production methods are about to replace the type of equipment exemplified by the Messerschmitt 109 radio apparatus.

Description of Apparatus

Examination of the radio equipment from the Messerschmitt indicates very definitely that it is intended primarily for communication over very limited ranges. The equipment is comprised of a low-power transmitter sufficient for inter-plane communication, a receiver, and a power supply unit. The receiver is relatively insensitive, and the transmitter output is so limited that it cannot be picked up by the enemy at a distant location. The frequency range of the equipment is from 2500 to 3700 kHz which in itself represents a very narrow band for communication purposes. Both the receiver and the transmitter are tunable in this range of frequencies.

The equipment weighs in the neighborhood of sixty pounds and the approximate dimensions for the various units are as follows:

Receiver13¼" x 97-7/8" x 6-7/8" high    13 lb
Transmitter8¼" x 13½" x 7½" high  20 lb
Dynamotor & Filter9½" x 4¾" x 9" high  13 lb
Junction Box & Cables  4-5/8" x 9¼" x 3-7/8" high  13 lb
Resistance Units4½" x 5¼" x 2" high    2 lb

As far as the size and weight of the Messerschmitt radio equipment is concerned, it should be mentioned that it is possible to obtain commercial equipment in this country which will give much better performance and yet be considerably lighter and take up much less space. Specifically, Lear Avia manufactures a transmitter and a receiver, complete with a shielded loop for aural-null direction finding, which weighs less than half that of the Messerschmitt equipment and the entire three-way Learadio system takes up less space than either the German receiver or transmitter. The Learadio equipment has much better sensitivity, and the output of the transmitter is about four times as great as that of the German equipment. From this comparison it would appear that we, in this country, have progressed further with the development of lightweight aircraft equipment in commercial fields than the Germans have with their military equipment.

The receiver has a single band, tunable over the frequency range of 2500- 3700 kilocycles. It employs a superheterodyne circuit and contains five shielded Telefunken RENS 1264 tubes used as follows: first RF, detector oscillator, first IF, second detector, and audio output. The filament voltage on these tubes is 4 volts, and the plate lead is brought out at the top of the tube. These tubes are the equivalent of our Nos 24s or 35s; their design is at least ten years old. They have been obsolete for more than five years and are no longer used in this country. The RF detector uses an old type autodyne circuit which is not at all suitable for high frequency operation and has been radically revised through the use of newer types of tubes.

This article was originally published in the August, 1941, issue of Aviation magazine, vol 40, no 8, pp 100, 102, 224.
The original article includes 7 photos and the data table above.
Photos are not credited, but are presumably from Lear Avia.