十九試空一号無線電信機十二型 (19-shiki Ku-1 Gō musen denshinki 12-gata, Prototype 19 Ku 1 Wireless telegraph Model 12). The FK-3 was an airborne patrol and search radar developed by Kawanishi. It was developed from the H-6 radar and was considerably smaller and lighter with 40-60 kg (depending on source). Research on the radar started October 1944, and production halfway 1945. A few of the 200 units produced were installed on aircraft, however none were used in combat.

The radar had two indicators in parallel, the normal one for the observer and a sub indicator for the pilot.

Technical Specifications

十九試空一号無線電信機十二型
Protoype 19 Ku 1 wireless telegraph Model 12
Designation FK-3
Object Patrol and Search
Research started October 1944
Finished June 1945
Operational status not yet used
Installation Small Aircraft Observer’s seat
Frequency 2 meter / 150 kHz
Power Output (Peak) 2 kw
Pulse Length 10 µs
Repetition Frequency 1000 c/s
Weight 40 or 60 kg (conflicting sources)
Units build ~ 200
Transmitter Oscillation Circuit   Blocking Oscillator  
  Oscillator Valve U-233 x2
Receiver Intermed. Freq. 10 mc
Detector 1st UN-954 2nd FU-2A05A
Local oscillator UN-955
Scope
Representation
Diameter 75 mm
Scanning Axis Linear, Logarithmic
Scale Mechanical
Antenna Type Head: Yagi Sides: Folded Doublet
Gain 16db 6.5db
Beam Angle Horiz. θƒ = 30° θƒ = 28°
Vertical θƒ = 35° θƒ = 30°
Max. Range (Max.Effective Scale) 150 km theoretical
60 km against large ships such as aircraft carriers
Minimum Distance 3 km at 1000 m
Accuracy of Range ≃± 5%
Distance Discrimination 4~8 km
Accuracy of Bearing ± 3°
Angle Discrimination ≃ 60°
No. Of Operators Observer
Device Dimensions Weight
Transmitter 37.5 x 28.6 x 41 cm  
Indicator Receiver 28.9 x 15.6 x 37.9 cm  
Sub Indicator 28.9 x 15.6 x 39.7 cm  

Reports of the U.S. Naval Technical Mission to Japan

Overview of the Kawanishi J6K mock-up with the alleged FK-3 top left on the instrument panel

Kawanishi J6K

The cockpit of the wooden mock-up of the Kawanishi J6K interceptor shows an interesting device. On the internet this is often speculated as an FK-3 radar indicator. And while both the FK-3 and J6K were developed by Kawanishi, this is quite unlikely. Both the timeline nor intended purpose match.

The J6K mock-up was presented in February and June of 1944, with the plane cancelled the 8th of July 1944. Development of FK-3 however didn’t start until October.

The intended purpose of the FK-3 as well was a long range search and patrol radar. It was intended to warn about incoming aircraft, but not precisely locate them as would be useful on an interceptor such as the J6K. It was also not an easy device to operate, requiring a 2nd person to do so, even if the pilot would have their own indicator.

The prototype doesn’t show any antenna either that would be required for the FK-3, nor does it show any other controls inside the cockpit, making it unclear if the device even is a radar in the first place.

While the device may indeed have been an theoretical radar or another, to date unknown device under development by Kawanishi, it is most likely not an FK-3.

Sources

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