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Japanese WWII miniature VHF transceiver

The 94-6 (94 Mark 6) was a miniature single-valve portable transceiver that was used by the Japanese Army during WWII, as part of a series of Army radios. The name is derived from the Japanese year of development 2594 (1934), with the Mk.6 version being the smallest model. Althoug the radio was intended for the infantry, it was also used for various 'special' purposes.
The radio is extremely small for the era, and is suitable for voice and morse transmissions in the VHF band (25-45.5MHz), divided over three frequency ranges. It was intended for tactical (i.e. short-range) communication, up to 2km.

Development of the radio began in 1934 and the first units were delivered in 1935. Initially the radio was only suitable for a single band, but a few years later (around 1937) the range was extended and a three-position band selector was added. According to the name plate, the radio shown here was manufactured in February 1943.
 Japanese 94-6 radio in letter case with open top lid

Japanese radios like the 94-6 are very rare. In WWII, during the heavy fights at Guadalcanal, American Marines captured a number of them and immediately put them to use for their own communication [1]. Apparently the radios worked well, but were only suitable for short-range communication. Immediately after WWII, the US ordered the destruction of all German and Japanese war-time equipment, which is why so few of these beautiful radios have survived.
The closed leather case Japanese 94-6 radio in letter case with open top lid  Japanese 94-6 radio in letter case with open top lid Removing the radio from the leather case Morse key The radio after taking it out of the leather bag The closed radio Top view of the controls

Frequency ranges
  1. 24.7 - 34.1MHz
  2. 29.5 - 41.2MHz
  3. 35.0 - 50.5MHz
The 94-6 is one of the most compact radios of its era. The entire unit measures no more than 18 x 13 x 8 cm, and is normally packed inside a high-quality leather bag, from which it can be operated directly. Even the morse-key at the side can be accessed directly through the leather.

Most of the controls are located on the top surface, just below the top lid of the case. There are four adjustements and a meter. Once tuned to the desired frequency, the top lid can be closed during the conversion. The operational controls are all located on the side panel. The leather case has been constructed in such a way that the operational switches are always accessible.

On the side panel is a classic 3-position lever-operated telephone keyswitch, that is used as the power switch. In the center position, the radio is turned off. In the upper position the transmitter is activated and the lower position is for reception (RX). Above the power switch is a 2-position switch that allows the microphone circuit to be interrupted when the radio is used in CW mode.

The actual morse key is also on the side panel. It has a large flat circular knob and can be accessed through a thin flexible spot of the leather case. The 94-6 was usually carried on the chest with the leather strap hanging from the neck and the controls at the right. The operator could then control the radio, and send messages in morse, with his right hand.
The controls of the Japanese 94-6 A thin insert in the leather case allows the morse key to be operated Operating the morse key Top view of the controls Controls on the side panel The 94-6 radio and the antenna plug

The radio is very compact and is extremely well built. In some respects it is even better built than German radios of the same era. The entire radio is housed inside a rectangular metal box that is carried inside a leather bag. The unit can easily be dismantled by opening the top lid and holding the radio upside-down. The inner frame that holds all components will then easily come out. If it got stuck somehow, there is a hole at the bottom that allows you to push the frame out.
The entire radio is built around a single Japanese UM30MC valve (tube). This is a double triode that performs more than one function and forms the heart of a so-called Rushbox. An excellent description of the circuit was published by Dick Rollema, PA0SE, in March 1986 [3]. The article is available for download below. The full circuit diagram of the radio is stored inside the top lid.

As becomes clear from the images below, the unit is extremely compact, making one wonder how the Japanese got all these large components connected together inside such as small box.
Interior of the Japanese 94-6

The radio actually consists of a single aluminium frame that is open at two sides. The top surface acts as the control panel and holds the adjustements, the connections to the headphones and microphone, and the meter. The valve (tube) can be accessed through a hole in the top surface.
Extracting the radio from its case Interior of the Japanese 94-6 The coils and the valve Circuit diagram inside the top lid Close-up of the Japanese valve Close-up of the RX/TX switch Transformer and ajustments Close-up of the coils and the tuning capacitor

Power supply
Although the 94-6 was a very compact radio, power supply certainly was a major issue in those days. For that reason, the 94-6 had two power sources: a battery pack that was used for reception, and a hand-crank-operated generator that was used to power the transmitter.
When receiving, power was supplied by a battery pack that was connected to the left side of the radio. It contained batteries for 3V and 135V.

In transmission mode, the batteries would not be able to produce the required current and a manually operated generator had to be used instead. The generator was connected to the right side of the radio, where a socket is hidden behind a small flap, just below the power switch.

The generator was capable of producing both 3V at 350mA and 135V at 30mA simultaneously.
Hand-crank generator for the Japanese 94-6 radio

When both the batteries and the generator are connected, the lever-operated power switch at the side of the radio (S2) automatically selects between the two power sources when switching from RX to TX. At the bottom of the generator are two flat metal hooks that slide outwards.

Each of the metal hooks have two rectangular openings that are clearly indended for attaching it to the belt. One side can be used for a left-handed operator and the other one for a right-handed one. We currently don't know whether the generator was operated by the radio operator himself, or by an extra person walking next to him. In either case, operating the generator must have been very inconvenient, so conversations were presumably kept as short as possible.
Hand-crank generator for the Japanese 94-6 radio Another view of the hand-crank generator Battery connection Generator connector Battery connections The connections for the generator, protected by a metal flap

The antenna should be connected to the back of the 94-6, i.e. the side that faces away from the operator when the radio is carried on the chest. There are two sockets, one for the antenna and one for that counterpoise, that accept a special antenna plug. The plug consists of a pertinax bridge with a thick messing pin and a thin one, so that it can not be fitted the wrong way around.
The 94-6 radio was normally used with an L-shaped antenna, consisting of a 1.4m horizontal boom that acted as the counterpoise and a 65cm vertical rod that was the actual antenna.

The antenna sockets are accessible through two holes in the leather case. When the antenna plug is interted, it is held in place by means of a small leather strap with a buckle. The image on the right shows the 94-6 radio in a leather case with the antenna plug in place. More images below. At the moment we don't know how the antenna rods were mounted to the antenna plug.
Antenna connector fitted to the 94-6 radio

Rear view of the radio (i.e. the side that is located away from the chest) Close-up of the antenna plug Another close-up of the antenna plug Fitting the antenna plug Fitting the antenna plug The antenna sockets are accessible through the leather case Antenna connector fitted to the 94-6 radio Close-up of the leather strap and buckle that are used to keep the antenna plug in place

Determining the age
Determining the age of the radio can be a bit tricky, first of all because all text is in Japanese and secondly because the Japanese have a variety of different calendars. In order to determine the age of a 94-6 radio, we have to investigate the serial number tag that is mounted on the top lid.
The top shield with the red lettering is just a warning. It says: SECRET. The bottom shield is the actual name and serial number plate.

It contains information about the manufacturer (Takanashi), the serial number (17899) and the manufacturing date. The prefix tells us that the Showa calendar is used (昭和), which counts the years of the reign of Emperor Hirohito (1926 is year 1). The engraved numbers tell us that the device was manufactured in December (一二月) of Showa-year 17 (一七年), which is 1942.
The name tags on the top lid of the 94-6 radio

Sometimes it is also possible to dermine the age from the markings on the meter scale. At the bottom left of the meter scale is the text 昭和18昭和 2昭和 which translates to February 1943. Several websites may be of help when translating and interpreting Japanese letters and numbers, such as [5] and [6]. We currently don't know the meaning of the other text on the S/N tag.
Top lid of the case The name tags on the top lid of the 94-6 radio Close-up of the name tag Close-up of the meter

The Japanese clearly had servicability in mind when they designed the 94-6, as all components are clearly numbered. The following list can be used as a rough index to the various parts [6].
  1. Antenna connection
  2. Counterpoise connection
  3. Antenna current meter
  4. Mica capacitor 450/1500
  5. Coil
  6. Variable capacitor
  7. Mica capacitor 450/1500
  8. Mica capacitor 450/1500
  9. Coil
  10. Regeneration control 29K
  11. RFC
  12. Mica capacitor 450/1500
  13. Mica capacitor 450/1500
  14. Valve socket
  15. Power switch (TX/OFF/RX)
  16. Filament rheostat
  17. Capacitor in metal can
  18. Wire-wound resistor (250 Ohm)
  19. Transformer 1:20
  20. Transformer
  21. Mica capacitor 450/1500
  22. CW key assembly
  23. Tone/Voice switch
  24. Socket for headsets/handsets
  25. Generator power socket
  26. Battery power socket
  27. Mica capacitor 4500/1500
  28. Carbon resistor
  29. 3-position band switch
  30. Coil
  31. Coil (mounted on #9)
  32. Mica capacitor 450/1500
  1. James H. Smith, How Jap Radio Transmitters Work - In US Hands
    QST, September 1943. pp. 44, 45 and 59.

  2. Helmut Liebich, Ein japanischer UKW-Transceiver - Kein Testbericht
    CQ-DL, August 1983. pp. 372-373.

  3. Dick Rollema (PA0SE), Japan's Hush-Hush Rushbox
    73 for Radio Amateurs, March 1986. pp. 9-12.

  4. Wikipedia, Japanese Calendar
    Retrieved June 2012.

  5. I18n Guy website, Calendars: Japanese Emperor Date
    Retrieved July 2012.

  6. LTC William L. Howard, Japanese Type 94-6 Radios
    Retrieved June 2012.

Further information

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Copyright 2009-2013, Paul Reuvers & Marc Simons. Last changed: Wednesday, 30 April 2014 - 21:32 CET
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