Text and Drawings by Greg Springer

 The need for better tactical control of fighter planes was recognized in the early years of combat between groups of aircraft.  Due to the limits of technology, only visual communication methods were available to commanders of airborne formations.  Hand signals or ‘wing wagging’ and flare pistols were useful but limited to signaling the intent of the leader prior to entering combat and rounding up the unit after the fight.  The air forces of the major powers recognized this shortcoming and all strove to incorporate voice communication when short wave radiotelephony sets were developed between the wars.

  It is not my intent to present a technical treatise since my knowledge of old radio systems is quite limited.  My intent is to provide accurate drawings to enable modelers to better detail the cockpits of their model 11, 21, 22 and 32 Zeroes.  I will provide a little background of the experience of the Imperial Japanese Navy with their aircraft radio systems. 

A posting from John Acosta on the news groups at sparked my interest in this subject.  He provided the URL for a web site called Army Radio Sales at, owned by a Mr. Ben Azari.  This is a site that combines commercial sales of military radios from many nations as well as pages of historical information on such radios.  These technical pages are written by Lt. Col. William Howard, U. S. Army (Ret.) One of Col. Howard’s interests is collecting Japanese radios and he devotes a lot of bandwidth to documenting them.  The main document he outlines is called “Operational History of Japanese Naval Communications: December 1942 - August 1945” published by the Military History Section of Headquarters, Army Forces Far East.  This is a translation of a document written by Japanese officers at the behest of the occupying authorities.  With regard to aerial radios, problems were encountered both in technology and in the attitudes of personnel that caused difficulties throughout the war.  These resulted in poor performance of aircraft radio systems in general.

The following conclusions were reached in the report.  The radio sets were well designed and at the beginning of the war, well built.  As the war went on, shortages of raw materials began to affect the quality of the radio components.  One of the earliest problems encountered was a lack of coordination between the manufacturers of the radios and the aircraft builders.  Frequently, no provision was made for mounting points for the radios in the initial cockpit designs of aircraft.  As a result they were fitted as an afterthought in whatever space was available.  This led to problems of accessibility in some aircraft types.  Crewmembers had difficulty adjusting frequencies, volume and other parameters of the radios.  The greatest problem encountered was that of correctly installing the radios with proper wiring, shielding and grounding of the equipment.  It seems that little attention was devoted to this problem until late in the war.  Insufficient shielding of the ignition system of the aircraft caused interference with reception of signals to a great degree, as did static charges generated by the passage of the airframe through the atmosphere.  It seems that there were very few officers at fighter group level who were familiar with radio systems or who cared to conduct effective programs to maintain them.  The resulting poor performance quickly led fighter pilots to cease using the radios and resort to the old visual methods.  In the case of some land-based groups, they removed all radio equipment to enhance the performance of the planes.  Ship-based planes needed to retain their radios for navigation and homing purposes.

 Very early in the war the lack of radios severely limited tactical control options that could be exercised by flight commanders.  Sakai, Saburo wrote of the death of fellow pilot Miyazaki, Yoshio and the near ambush of Lt. Sasai, Junichi under circumstances where formations had drawn apart and he was unable to tell the straying aircraft that they were about to be bounced.  These incidents occurred over New Guinea in May 1942.  Later, during the air battles around Guadalcanal, Japanese fighter formations had great difficulty coordinating escort actions due to heavy cloud conditions degrading visibility at multiple altitudes.  John Lundstrom cites this on page 192 of “The First Team and the Guadalcanal Campaign”.

The radio systems installed in the early A6M2 and A6M3 variants of the Zero were the Type 96 ku (aviation) Model 1 voice/telegraph system and the Type 1 ku Model 3 Radio Compass or Radio Direction Finder.  The Type 1-3 was the standard RDF unit for most carrier-borne IJN aircraft.  The Type 96-1 system was previously used in the A5M4 fighter series.  The Type 96-1 system consisted of three components, all of which were installed in the cockpit.  The transmitter and receiver were separate units that were placed low on the right side of the cockpit.  They were hung in the standard shock mount that consisted of frames above and below the radio which had bungee cords secured to them.  The bungees were looped around spools mounted on the radio casing.  The suspended radio was protected from shocks by the flexibility of the cords.  (see illustration 3)

The dynamotor module that powered the radios was mounted on a shelf in the left rear corner of the cockpit behind the pilot’s elbow, as seen in the rough sketch.  Dynamotors are generators that are driven by electric motors.  They were a common means of powering aircraft radio equipment for several nations during World War Two as they avoided current fluctuations that occurred in the main electrical system of an aircraft.  The type 96-1 operated in the frequency range of 3.8 to 5.8 megahertz.  This is the lower end of the High Frequency or Short Wave band. Quartz crystal oscillators controlled frequencies.  Power of the transmitter was 8 to 10 watts in voice mode and 30 watts in telegraph mode, called CW for ‘continuous wave’.  In ‘At Dawn We Slept’ Gordon Prange relates that one of the problems encountered in the IJN’s preparation for Pearl Harbor was that they had never operated fighters farther than 90 miles from their carriers.  50 miles was the practical limit of utility of the voice radios under optimum conditions.  Since the fighters would be venturing around 250 miles outbound on the mission it was necessary to initiate a training program in the use of the longer ranged telegraph code communication.  It may be seen in the photo that Hirano’s A6M2 did not have a code key installed and his transmitter is set to voice function.  It seems that not all fighter pilots were trained in code transmission.  Perhaps only unit leaders were supplied with the necessary equipment.  The receiver circuit was a superheterodyne type.  Total weight of the system was 38 pounds.

The control box for the Type 1-3 RDF was mounted above and between the two Type 96 sets.  The control for rotating the RDF antenna was directly to the rear of the control box.  The directional loop antenna was mounted in the fuselage deck beneath the canopy behind the rollover pylon.  It consisted of two round stamped-metal frames with a center post.  The actual antenna was made of insulated wire wrapped around and between the frames.  Cloth was then applied over the wire and doped.  This gave the assembly the appearance of a thick loop with a center post.  The photo-etched ‘twin loop antenna’ found in many aftermarket detail sets today is in fact only the support frames which survived after the cloth and antenna wire disintegrated.  These frames are seen in a number of restored aircraft that no doubt led to the error.  Credit goes to Ryan Toews and James Long for discovering this fact.  The drive motor for rotating the loop antenna is contained in a box inside the fuselage just below the antenna and the large central unit for processing the radio signals was housed in the rear fuselage behind the rear cockpit bulkhead.  The dynamotor power supply for the RDF system was likewise located in the rear fuselage.  A visual indicator for flying along a directional radio range signal was mounted in the lower left corner of the instrument panel.  The face of this instrument has an arc painted on it with characters for ‘right’ and ‘left’.  A needle indicated the relative position of the aircraft to the signal.  One further control is a round switch box mounted next to the pilot’s right shoulder.  This was for switching the antenna’s reception function between the communication radio and the RDF system.  This was to allow the RDF operator to use the antenna to monitor AM radio broadcasts.  Commander Fuchida used this feature on December 7 1941 as he approached Oahu.  The regular music broadcast from Honolulu assured him that the American forces were unaware of his impending attack.  I am informed that many pilots still engage in this practice to fight boredom on long flights.

The interesting thing about the Type 1-3 is that it was a copy of the Fairchild Aero Compass, manufactured in New York by the Fairchild Aerial Camera division of Fairchild Aircraft Company.  The Zero coded AI-154, which was shot down at Fort Kamehmeha during the Pearl Harbor attack had a Fairchild-built direction finder Model RC-4, serial number 484 installed in it, as did the Zero, which was captured in the Aleutian Islands seven months later.  Illustration 4 (see below) shows the faces of the Fairchild control box and one manufactured by the Naval Aerial Navigation Technical Depot.  The dimensions of these units are tentative ones made by comparing them with known dimensions of airframe components in photos.  The control box appears to be 140mm long by 108mm wide by 80mm deep.  According to the TAIC manual these units operated in the 160 to 385 kilohertz range, which is in the Low to Medium Frequency bands.  The selector switches on a captured unit are labeled from 170 to 1200 kHz, however.   In the IJN the Type 1-3 was known as the ‘Kruesi’ after its designer, Geoffrey Kruesi of Dayton, Ohio.

The direction finding function works as follows.  When the axis of the loop antenna is pointed at the transmission source it will receive no signal.  When the plane of the loop is pointed at the source it will receive the strongest signal.  The pilot finds the angle at which he can hear no signal and sees that course on his ‘route meter’ on the face of the control box.  He uses his compass to bring his aircraft to that heading.  However there is a drawback to the system.  When the area of no signal is found, it is on a bearing both towards and away from the source.  If the pilot has good weather conditions he may be able to tell from the position of the sun the general direction in which he needs to fly.  If overcast or darkness is prevailing he may inadvertently fly a reciprocal course away from the source.  This occurred many times with frequently tragic results.

The transmitter and receiver housings are stated to be the same size and configuration but I have seen variations in the reported dimensions.  I am giving the dimensions of the only currently known surviving example of the transmitter.  The owner, Mr. John Groff, supplied these.  Width of the face is 245 millimeters and height is 195 mm.  Depth is 112 mm.  Photographs of this unit can be seen at  There is no known surviving example of the receiver.  I have reconstructed its appearance from photographs and drawings found in two Technical Air Intelligence Center publications and some Crashed Enemy Aircraft Reports that were kindly provided by James Long and Ryan Toews as well as  photos of the cockpit of Zero AI-154 that crashed at Fort Kamehameha during the Pearl Harbor attack. These were obtained from Jim Lansdale and David Aiken.  On page 129 of Robert Mikesh’s ‘Japanese Cockpit Interiors 1940 –1945’ is a photo that shows the mounts for the radio systems quite clearly.

Zero Cockpit - Left View.  Photo Credit: Hickam Base Photo Lab via LRA

Zero Cockpit - Right View. Photo Credit: Hickam Base Photo Lab via LRA


Drawing number 1 shows the face of the transmitter.


 A.  Latches to remove rear housing.

 B. Threaded antenna connector posts

 C. Ammeter

 D. Frequency selector

 E. Frequency selector lock

 F. Possible test equipment jack

 G. Manufacturer’s data plate

 H. Modification plate

  I. Microphone jack

 J. Unknown jack

 K. Three-position master selector switch - Transmit-Off-Receive

 L. Telephone-telegraph function selector.

 M. Unknown jack

 N. Shock mount ‘spools’

 O. Power supply plug

 P. Antenna connection

 Q. Antenna connection

 Drawing number 2 shows the face of the receiver.


 A. Latches to remove rear housing.

 B. Unknown selector knob

 C. Telephone-telegraph function selector

 D. Housing for frequency crystal

 E. Threaded antenna connector posts

 F. Volume control

 G. Possible frequency display

 H. Headphone jack

  I. Possible frequency selector knob

 J. Possible frequency selector lock

 K. Manufacturer’s data plate

 L. Unknown selector knob

 M. Unknown jack

 N. Shock mount ‘spools’

 O. Power input plug

 The three-view of the dynamotor module shows the following:


A. Transmitter dynamotor.

B. Receiver dynamotor.

C. Power switch.

D. Power outlet to transmitter.

E. Power outlet to receiver.

F. Power input from aircraft’s electrical distribution panel.

G. Mounting lugs to secure module.

H. Unknown.

 I. Fuse box.

J. Link latches secure removable covers for dynamotor maintenance.

K. Lubrication access cover plates.


Drawing number 4 shows placement of these items on the face of the RDF control box.  The Japanese-built unit is on the left, the Fairchild unit on the right.  The spin controller and antenna connections shown attached to the Fairchild unit are also found with the Japanese set.


A.     Sensitivity adjustment

B.     Fuse

C.    Volume Control

D.    Three position switch:

      Left: ‘Route Meter’

 Center: ‘Ordinary receiver’ 

       Right: ‘Max. Sensitivity’

E.     Frequency selector: 450 to 1200 kHz (Japanese set only)

F.     Frequency selector: 170 to 450 kHz (Japanese set only)

G.    Power switch on Japanese set.  Labeled ‘Broadcast’ on Fairchild unit.

H.     Route meter

I.         Switch, labeled ‘Off / AVC’ on Japanese set.  Labeled ‘Beacon’ on Fairchild unit.

J.      Crank.  For adjusting route meter settings?

K.     Route meter light adjustment

L.      Headphone jack

M.    Selector switch labeled ‘Telephone / Telegraph’ in Japanese on both sets.

N.     Switch labeled ‘Phone’ (Fairchild unit only)

O.    Crank to rotate loop antenna

P.     Data plates

Fairchild Logo as found on data plate of the direction finder from Zero - AI-154




‘Crashed Enemy Aircraft Report #6 - Type 99 Carrier-borne Dive Bomber, “VAL”, Model 22, Serial No. 3041’, Office of the Assistant Chief of Air Staff, Intelligence, Washington, DC, 21 September, 1943

Article, ‘Digging for WWII Japanese War Relics’, Kimura, Masuo, Pp. 36-39, ‘Scale Aviation’ Vol. 3, Japan, 1998.

‘Illustrated Zero Fighter’, Nohara, Shigeru,  Green Arrow Books, Japan, 1999

‘Japanese Aircraft Interiors 1940-1945’, Robert C. Mikesh, Monogram Aviation Publications, USA, 2001, ISBN 0-914144-61-8

‘Japanese Aircraft - Performance and Characteristics – TAIC Manual No. 1’

Technical Air Intelligence Center, NAS Anacostia, Maryland, December,1944