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"German Two-Way Radio Telephones" from Tactical and Technical Trends

The following U.S. military report on the German Feldfunk-Sprecher two-way radios, similar to the U.S. walkie-talkie, was originally published in Tactical and Technical Trends, No. 43, January 27, 1944.

[DISCLAIMER: The following text is taken from the U.S. War Department publication Tactical and Technical Trends. As with all wartime intelligence information, data may be incomplete or inaccurate. No attempt has been made to update or correct the text. Any views or opinions expressed do not necessarily represent those of the website.]


The German two-way radio telephone, type Feldfunk-Sprecher b, is used for tactical communication between small combat groups. It is the Germans' so-called "walkie-talkie" and is used in almost the same manner as our SCR 536. It may be operated on the move as well as from a stationary position. Its frequency range is considerably higher than the range of any of our tactical sets. However, if captured in quantity, the German sets could be used under some of the conditions which are favorable to the use of the SCR-194, SCR-536, SCR-300 and SCR-195. The distance over which satisfactory operation may be expected from the German sets is theoretically about one-fourth the distance over which the American sets can operate. One disadvantage is that the use of a storage battery as a primary power supply necessitates the use of charging equipment. The Germans have a companion set which is exactly the same except for frequency band which is approximately 120 mc to 138 mc in 30 steps.

a. General Characteristics

(1) The Feldfunk-Sprecher b is a compact portable radio telephone, complete in a single carrying case. It consists of a three-stage transmitter and receiver. It is designed for voice operation only. The set is a transceiver, that is, the same tubes are used for transmitting and for receiving. Push-to-talk operation is used, and once communication has been established, all that is necessary to carry on two-way traffic is to press the microphone switch to transmit and release it to receive. The practical range is about one mile, but the performance is dependent largely upon terrain features. Communication is best over line-of-sight paths. A fresh battery will operate the set up to 13 hours at normal temperature.

[German Two-Way Radio Telephone, Feldfunk-Sprecher]

(2) Technical Data

       Transmitter      Receiver
Frequency range 100-119 mcs 110-119 mcs
Preset frequencies 30 30
Power input (total) 2.4 volts at 2.75 amp 2.4 volts at 1.5 amp
Power input (oscillator) 102 volts at 10 ma* (1 watt)  
Vibrator-rectifier output 115 volts at 23 ma 135 volts at 13 ma
Filament Ratings    
   RV2.4P700** 2.4 volts at 0.06 amp
   RL2.4P2*** 2.4 volts at 0.15 amp
   RL2.4T1 2.4 volts at 0.15 amp
Relay Ratings
   #1 2.2 volts at 50 ma
   #2 2.2 volts at 250 ma
Tube Functions
   RV2.4P700 R.F.**** amplifier Monitor
   RL2.4P2 A.F.***** amplifier Modulator
   RL2.4T1 Superregenerative detector R.F. oscillator
Battery Rating 28 amp hrs at 2.4 volts
Antenna 80 cm (31 1/2 inches)

(3) Military Data -- Range in miles on open terrain, from hill to hill, from hill to valley is up to 1 and 1/2 miles; on hilly terrain, amidst foliage, near telephone lines, it is 1/3 to 1 mile; mobile operation -- up to 3/4 mile.

(4) Dimensions

         Length        Width        Height
Set Case 13 7/8 in 4 3/4 in 13 in
Battery  4 1/2 in 2 1/8 in  6 15/16 in

(5) Weight

         Set case (complete) 24 lbs
         Battery             5 lbs

b. Physical Characteristics

(1) The complete set is housed and transported in a single case moulded of some sort of resinous plastic. The moulding is ribbed to give it strength and it should withstand considerable abuse. A removable harness is attached to the case at top and bottom to permit the set to be carried on the back of the operator. The receiver volume and receiver fine tuning may be controlled from a remote control box strapped to the belt of the operator. Frequency channel selection, however, can be accomplished only by adjusting the frequency selector knob on the front panel.

(2) A removable cover protects the front panel when the set is not in use or when it is operated from the back of the operator after frequency selection has been performed. The back of the case has a hinged cover which permits removal of the accessories or change of battery. The accessories include two headsets, one throat microphone, one remote control cable and one back protector. The remote control box is mounted on the front panel where it may be used to control the volume and fine tuning of the receiver. It is believed that the back protector has been eliminated from the accessories in later sets. Apparently the harness for carrying is to be left strapped to the outside of the case as the list of accessories to be carried in the back of the case does not include this item.

(3) There are six controls to the set. Three of these are permanently mounted on the front panel: namely, the On-Off switch at the bottom of the panel, the frequency-channel selector at the top of the panel, and the receiver aligning adjustment which is accessible through an aperture at the upper right of the panel. Two controls are operated either from the front panel or remotely; these are, the receiver volume control (to the left on the remote control box) and the receiver fine tuning (to the right on the remote control box). The sixth control is the microphone switch on the microphone cable.

c. Electrical Characteristics

(1) The set can be conveniently divided into four electrical components: namely, the transmitter, the receiver, the power supply and the control circuits.

(2) The transmitter uses a modulated oscillator. A type RL2.4T1 tube is used as a series-fed Hartley oscillator. The oscillator coil consists of a silver-plated groove, which winds about a ceramic coil form four complete turns. There is an adjustable powdered iron core which can be screwed into the coil form about half the length of the coil. The winding itself has five taps. The main tuning condenser is connected across the whole coil, and two trimmers are connected across different taps. Plate current is delivered through an off-center tap. This current first flows through one half of the modulation choke, the other half of which carries the current to the modulator tube. Plate modulation is effected by transformer coupling between the two windings.

[German Portable Radio Telephone]

(3) The modulator tube is a type RL2.4P2. The input of the tube is across the high resistance winding of the transceiver transformer. The microphone in series with the 2.4 volt battery is connected across the primary winding of this transformer. Output of the R.F. oscillator is transferred to the antenna by an antenna winding coupled to the oscillator coil. This antenna winding consists of one third of a turn of silver plating on the same coil form. It has its lower end grounded and its upper end, near the oscillator winding, connected to the antenna.

(4) Some of the energy delivered to the antenna is fed through a condenser (C1) and a coil (L1) to the grid of the monitoring tube RV2.4P700. When a strong signal, such as that fed through the condenser (C1), is applied to the grid of this tube it acts as a detector. A headset is connected across the output of this tube to reproduce the modulation.

(5) The receiver is a three-stage set comprising an untuned R.F. amplifier stage, a superregenerative stage, and an A.F. amplifier stage. The antenna is connected to the grid of the RV2.4P700 R.F. amplifier tube through coil (L1). The plate circuit of this tube is capacitively coupled to the grid of the RV2.4T1 detector tube. The audio signal is taken from the detector stage through the primary audio winding of the transceiver transformer. R.F. is filtered out of this circuit by choke coil (L6) and condenser (C16). The secondary winding of the transformer is in the grid circuit of the RL2.4P2 A.F. amplifier tube. The plate load of this tube is an audio choke. Across this are a condenser (C17) and a resistor (R9) in series with the earphones and a 1,500 mfd.****** condenser (C28). The higher audio frequencies are by-passed around the earphones by a 0.03 mfd. condenser (C18).

(6) The primary power supply is a 2.4 volt nickel-iron-alkaline storage battery type 2.4NC28. Current for the microphone, the two relays, and the filament of all the tubes in parallel is drawn from the battery through the power switch. Plate voltage is developed by a combination vibrator-rectifier voltage-doubler.

(7) Battery voltage is applied through one set of vibrator contacts to the primary winding of a power transformer; the alternating current appearing across the secondary winding passes through a second set of vibrator contacts where half-wave rectification takes place. This output voltage is then doubled. During one half cycle the high voltage of the secondary winding is applied through one set of vibrator contacts to condenser (C30) (top positive), and during the next half cycle it is applied to the condenser (C31) (top positive) without passing through the vibrator. The resultant voltage across the two condensers is approximately twice the voltage across each. Additional filtering of the ripple voltage is effected through condenser (C29) and choke coil (L9).

(8) There are two C-bias supplies, one taken from the negative (ungrounded side of the choke coil (L9) and the other, a separate bias supply, having a copper-oxide rectifier in series with a resistor-condenser filter across an additional secondary winding of the power transformer. The first-mentioned bias supply is for the RV2.4T1 tube and the second is for the RL2.4P2 tube. The RL2.4T1 tube develops its own bias as it is in oscillation continuously.

(9) The control circuits change the set over from receive to transmit by the multiple contacts of two relays.

(10) The first relay has one circuit to make on transmit; it connects trimmer condenser (C9) across part of the oscillator coil. This connection is broken on receive.

(11) The second relay has six contacts to make on transmit and five to make on receive. On transmit, the antenna condenser (C1) is shorted out; the end of the antenna is connected to the loop (L3) on the oscillator coil form; the plate voltage supply for the oscillator is connected, through the modulation choke and the oscillator coil, to the plate of the oscillator tube; part of the oscillator tube grid-bias is shorted by the ballast tube (to ground); the earphone circuit is connected into the plate circuit of the monitor tube; and the grid bias on the audio amplifier tube is lessened. On receive, the antenna loop on the oscillator coil form is disconnected from the antenna; the antenna condenser (C1) is connected in series with the antenna and the grid coil (L1); the output of the detector is connected to the audio winding of the transceiver transformer; the circuit for the remote control tuning "motor" for the receiver fine tuning is completed through the ballast tube to ground; and the earphone circuit is connected to the plate of the A.F. amplifier tube. Current is supplied to both relays on transmit through the microphone switch.

(12) A double-cathode neon indicator lamp mounted in an oblong black tubular fitting on the front panel of the set indicates whether the battery is well charged and whether the high voltage is normal. A push-button at the bottom of the fitting permits connection of the lamp to the high voltage lead coming from the positive end of condenser (C29). Above this push-button are two small labelled windows. The top one is marked "Spannung normal" (voltage normal) and the lower one is marked "Sanmler bereitst" (prepare battery). If both cathodes glow, when the push-button is depressed, the set is provided with proper high voltage and the storage battery is sufficiently charged. If only the lower cathode glows, the voltage is below normal and the battery can be used for a maximum of an additional half hour. If the lower cathode also fails to glow, the battery should be immediately replaced by a fresh one.

d. Operational Instructions

(1) Open the back of the set case. Remove the antenna and place it in the antenna socket on top of the case. Remove the headset, microphone, and remote control cable. Close the back of the set case; open the front. Plug in the microphone and the headset; remove the remote control box and insert one end of the remote control cable into the end of the remote control box and the other end of the cable into the receptacle for it on the front panel of the set.

(2) Turn on the main switch and advance the volume control until a rushing noise is plainly audible in the headset. Set the frequency selector dial on the front panel to the desired channel. Pressing the microphone switch will place the carrier on the air for transmission. Releasing the microphone switch will permit reception. Fine tuning of the receiver is accomplished by turning the right-hand knob of the remote control box. Should the receiver fine tuning be unable to tune in the station, it is necessary to readjust the trimmer condenser (C8), accessible through the aperture on the front panel above and to the right of the frequency selector dial. The aperture is covered by a metal lid which can be swung out of the way by turning the screw above the aperture in a counter-clockwise direction. The adjustment of the trimmer is only possible with the use of another set. The frequency selector dials of both sets should be set to channel 233 (red notched) and the fine tuning of the set requiring adjustment should be set so that the red dot of the fine tuning knob is visible in the aperture over the dial. With the other station transmitting (actually speaking), adjust the trimmer (C8) until the speech is at an optimum. The fine tuning can now be used to tune in any station (transmitting) close to the frequency for which the frequency selector is set.

(3) Replace the front panel cover, bringing the microphone and headset leads and the remote control cable out to the left through the rubber-protected slots. The set is now ready to be carried on the operator's back for operation.

(4) Field tests showed that communication is often made possible, or improved, by a small change in location. It is wise, therefore, when establishing communication to experiment for a spot where reception is best.

(5) As these sets are ideal for use by the Germans as booby traps, they should be examined with great care and caution when they come into our possession.

**R=Rohre (tube); V=Volt -- 2.4 volts; P=Amplif. factor -- 700
***R=Rohre (tube); L=2.4 Voltage filament; P=2 Watts power
******A term of measurement - microfarad


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