German Aircraft Radio

by William P Lear,
President, Lear Avia, Inc

Standardized Nazi equipment of early part of war was definitely inferior to ours; modern apparatus on later planes has been excellent in design, construction and performance.

In modern, mobile warfare, radio is a combat weapon. In the deserts of Africa, in the jungles of Guadalcanal, in the snows of Russia, radio is the link between swiftly moving troops and headquarters.

A soldier on patrol speaks low into his walkie-talkie. The voice of the tank commander carries many miles over his static-free frequency-modulation radio. High in the sky, a pilot speaks into his microphone — and his voice is heard at headquarters hundreds of miles away. Radio waves span the globe, carrying war messages to Washington, London, Moscow and Chungking — this is a war of communications as well as movement.

The enemy also is fully aware of radio's value as a combat weapon. Through our intelligence services and through expert inspection of combat booty we keep track of what goes on across the battle lines. We have a more accurate knowledge of the kind of radio equipment our enemies are using against us than can be disclosed in full. Here I will try to tell some of what can be told about the radio apparatus of one of those enemies — Nazi Germany.

Since 1939, I have had several occasions to investigate thoroughly the design, construction and performance of German military aircraft radio apparatus.

Several important facts emerge from careful investigation of radio units installed in various types of German combat aircraft. The first — and the most fundamental — is that Germany "froze" her military aircraft radio designs as early as 1933! When the Luftwaffe raided London six years later, the Nazi raiders carried excellent, workable radio equipment but of definitely obsolescent design.

A high degree of standardization was carried much further than the design of transmitters and receivers as such. Transmitters for fighters and light bombers were designed around two basic tubes: Telefunken REM-904 and RES-1664D. Similarly, their receivers were designed around one basic tube: Telefunken RENS-1264. What they lost in efficiency they hoped to gain in simplicity of maintenance.

They have gone even further. In a given squadron only the squadron leader carries a transmitter powerful enough to communicate with the base station. All other aircraft carry low-power equipment with a range of only about five miles, just sufficient to communicate with the squadron leader without betraying their presence to the listening posts of the defenders. Yet both transmitters are identical in every respect; the squadron leader's transmitter merely contains one extra stage of amplification.

The same degree of standardization applies to the component parts, often at a sacrifice of efficiency. Identical subassemblies, connection plugs, cables and even minor components are to be found in every type of German aircraft radio.

We, in this country, know the advantages of mass production far better than the Germans. The production practices of our aircraft radio industry, which has grown manifold since Pearl Harbor, have undergone many changes. We, too, have standardized. But — and here is the crux of the matter — our degree of standardization is far less stringent and inflexible than that imposed by the Nazis.

The Germans developed a number of substitutes for many essential war materials. A substitute can often be better than the real, natural product. But German aircraft radio substitutes have always been found inferior to the real thing. That is one reason why their equipment is so grossly overweight.

With the loss of the Dutch East Indies we were confronted with shortages of many vital war materials. But as yet our aircraft radio production has not been seriously handicapped by lack of essential materials. Should we exhaust our rubber stocks, we are ready with rubber substitutes possessing possibly better dielectric properties than real rubber, as well as a greater degree of wear resistance under all climatic conditions. We will have enough iron, copper and steel, and more than enough aluminum. Not so the Germans.

Where we use sheet and angle aluminum Germans use a special alloy, "Elektron," which must be cast. As a result. the chassis of a German radio instrument may weigh as much as 10 times that of a comparable American instrument. Furthermore, aluminum may be anodized to prevent corrosion; "Elektron" must be painted.

Another factor which contributes to the overweight of German aircraft radio apparatus is the extensive use of ceramics for insulation. In the United States the custom is to use Bakelite and other dielectric plastics.

Other construction features indicative of material shortages in Germany are the use of fiber covered and shellacked wires; varnished cambric tubing for confining the wiring; cement instead of lock washers to hold the screws in place; and, above all, air-tuned coils.

From a design standpoint the use of primitive circuits in combat radio equipment is surprising. This, perhaps, is the greatest penalty which Germany has had to pay for her ironclad standardization and freezing of design a few years before the war. Ten years ago, Germany led the world in the development of ceramics for electrical and radio use, negative coefficient condensers, iron-core coils, and other familiar components. Today the United States is indubitably far ahead of the best that Germany can offer in aircraft radio design, construction and performance.

All aircraft radio apparatus made in this country must undergo certain stringent tests to determine its ability to function under critical conditions of vibration, temperature, humidity and atmospheric pressure, as well as extreme variations in electrical power supply. In addition, our military aircraft radio apparatus must undergo certain further tests and satisfy requirements even higher than those applicable in air commerce. German military aircraft radio apparatus cannot be subjected to these tests without rendering it inoperative, or damaging it beyond repair!

But to say that all German aircraft radio apparatus is inferior to ours would be to convey an entirely erroneous picture of the situation. The first of the new equipment to come to the attention of Allied engineers was that which appeared in 1940, in the Junkers Ju-88. Essentially, this consisted of a medium-power transmitter using a motor reel and a fixed antenna with a remote tuning unit controlled by a Selsyn transmitter. A provision was incorporated for resonating the airplane transmitter with the ground station transmitter, so that the ground station becomes the dominating or master frequency for all aircraft working that ground station.

The same principle could be used for synchronization of transmitters and receivers in a given formation flight. The general design of the transmitter and the companion receiver was excellent throughout. The direction finder installation on board the Ju-88 possessed numerous features based upon the modern advance of the art.

Since then, every new German airplane has been found to be equipped with new and better radio than that in evidence during the early days of the war. In 1944, and thereafter we probably will be confronted with far more efficient radio weapons than heretofore.

To understand just what kind of radio weapons we will find used against us, some of which may not yet be described, it is necessary to remember that until Pearl Harbor we operated under the handicap of a peacetime mentality. Now we have caught up with Germany on production and we are ahead on design.

Typical of German designs used in the early stages of the war is the two-way radio communication equipment used in the Messerschmitt Me-109 fighter. This is a radio-telephone instrument with the transmitter and the receiver both normally pre-tuned to a single frequency in the 2500-3750 kHz range; the frequency cannot be changed in flight.

The transmitter is comprised of a master oscillator, grid modulation, and two output tubes (for a squadron leader's set; only one for other ships in the squadron), and has an output of only about 2½ watts. The receiver, operating from the same antenna system, employs an old-style superheterodyne circuit and has an intermediate frequency of 509 kHz. Its maximum output is about 70 milliwatts, with sensitivity of about 10 microvolts per meter at the high-frequency end of the band, rising to 60 microvolts per meter at the low end of the band.

The antenna system is connected to both the transmitter and the receiver through a variometer, and consists of a short fixed antenna and a conventional trailing wire. The circuit is so arranged that the plane serves as a counterpoise when on fixed antenna position; when on trailing antenna position, the fixed antenna and the plane serve as a counterpoise.

The power supply consists of a dynamotor-and-filter combination, and the voltages supplied are 12 VDC for lighting the filaments of the tubes; 400 VDC to supply the plate and screen voltages for the tubes; 275 V, 90 cycles AC, rectified at the transmitter, to supply the bias voltages for the oscillator and speech amplifier tubes.

The operating controls consist solely of the pilot's on-off and transmit-receive switches.

This equipment is typical of the overweight, ersatz-ridden obsolescent radio apparatus that Germany had when the war began. There is nothing to recommend it except its excellent workmanship and extreme ruggedness. Not so with modern German apparatus.

The two-way radio carried by Ju-88s has four pre-tuned frequencies in the 3860-4220 kHz band, any one of which may be preset on the ground. The frequency cannot be changed in flight. The transmitter employs a Colpitts oscillator circuit, with one tube used as a frequency doubler, the other as a power amplifier. A receiver type pentode tube is used for grid modulation.

Noteworthy is the absence of crystal control for frequency stability; instead, considerable ingenuity has been employed to assure stable RF oscillations by using low temperature coefficient inductances which consist of coils electroplated on silver- or copper-impregnated ceramic forms, the entire coil being built up by depositing electrolytically the coil material in the preformed grooves to the exact required thickness.

The receiver employs a modern superheterodyne circuit, and presents no special features other than already noted. Both the transmitter and the receiver may be accurately resonated to a ground transmitting station or to the flight leader's transmitter.

The power supply is the familiar dynamotor-and-filter combination, but operates from a 24 V storage battery instead of a 12 V battery as found in early German models. It is capable of delivering a maximum of 10.5 A at 24 V and 210 mA at 400 V to the transmitter; the receiver load is 85 mA at 440 V.

For inter-aircraft communication in a given flight formation, a small separate receiver is carried. Direction-finding equipment of advanced design, and in some instances instrument landing apparatus and certain other radio aids, often are found in this type of aircraft.

One of the latest German aircraft radio units to be investigated by Allied engineers is that in their Heinkel He-111H reconnaissance plane. The same radio equipment is being used in new long-range bombers and may well be typical of the current standardization program of the Luftwaffe.

The entire installation consists of long and short wave two-way communications systems with four pre-tuned frequencies, an excellent direction finder, complete instrument landing equipment, inter-aircraft communications equipment, shipboard interphone system and special emergency radio apparatus to be used in the event of a forced landing at sea.

The transmitters are remotely controlled and any one of the four frequencies may be selected in flight. The long-wave frequencies are used for pulse transmission, enabling German ground direction finder stations to plot bearings on the airplanes in flight. A separate transmitter is used for CW operation on the short waves. In design, both transmitters use a master oscillator driving two power amplifiers connected in parallel. The transmitters are built around a single tube with interesting performance characteristics: the heater voltage is 12.6; heater current, .68 Amp.; it dissipates 30 Watts at the anode with a maximum anode voltage of 800. The maximum screen voltage is 200, with a current of 4 mA.

The receivers are of modern superheterodyne type and are arranged for CW operation. It is interesting to note that no automatic volume control is provided. Especially noteworthy is the fact that only radiotelegraphy in code is employed in long-range communications with the home bases, while radiotelephony is limited solely to short-range inter-aircraft communications.

In the direction finder installation on board the ship an unusual indicator is provided in duplicate: one for the navigator, who operates the DF controls, and a repeater instrument for the pilot. The indicator not only shows the relative bearing of the ground transmitting station but also gives a rough estimate of its distance from the airplane. The two separate antenna systems present no points of interest, but the total weight of the two-way and DF radio installation, including the interphone, is much lighter than previous German models (362 pounds) without sacrifice in ruggedness.

Two types of emergency radio sets are provided, one self-floating, the other for use on board life rafts. Both operate in the 320-532 kHz band. with the frequency of 500 kHz (international marine calling and distress frequency) crystal-controlled. Incidentally, this is the only instance to my knowledge in which crystal control has been found in German aircraft radio equipment.

Either hand or automatic radio telegraph transmission is provided. With the latter SOS is transmitted three times, followed by a long dash to enable the listener to obtain a radio bearing.

Two antennas are provided with either set. With the self-floating set is a collapsible aluminum tube umbrella-type antenna, five meters high, and a 165-foot steel wire reel antenna held aloft by a kite. In the life raft set (in addition to the umbrella-type antenna), a 260-foot stainless steel wire reel antenna is provided, held aloft by a kite or by one of two hydrogen balloons supplied with the set. Noteworthy is the automatic hydrogen generator which need only be attached to a balloon and immersed in the sea water. The power output of either set is not quite one watt at the antenna; with the 260-foot antenna its overwater range is about 250 miles.

Power for either set is supplied by self-contained batteries or by a hand-crank generator with built-in automatic transmitter unit.

The Heinkel He-111 radio apparatus is among the latest and the best that Germany has to offer. In evaluating the potential menace we should be guided by its excellence rather than by memories of the obsolete equipment once used by the Luftwaffe in its forays.

This article was originally published in the December, 1943, issue of Flying magazine, vol 33, no 6, pp 67-69, 120, 126.
The original article includes 6 photos.
Photos credited to Lear Avia, RAF Official Photos from Jupe-Electronics.