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
(2) Technical Data
|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)||
||115 volts at 23 ma||
||135 volts at 13 ma||
||2.4 volts at 0.06 amp||
||2.4 volts at 0.15 amp||
||2.4 volts at 0.15 amp||
||2.2 volts at 50 ma||
||2.2 volts at 250 ma||
||28 amp hrs at 2.4 volts||
||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.
||13 7/8 in||
||4 3/4 in||
|| 4 1/2 in||
||2 1/8 in||
|| 6 15/16 in||
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
(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.
(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