If you look closely, Nakajima B5N2 Kate aircraft marked with tactical code KEB-306 was not a standard airframe. This machine was discovered by American forces at Aslito airfield on Saipan, an island captured in June 1944, part of the Marianas archipelago. The distinguishing feature of this specimen was its onboard sea-surface search radar, from the H-6 family. I mention “family” deliberately, as this was a modified variant of the device, adapted for smaller aircraft use and designated as H-6 Type 3 Ku 6 Model 4. The base version of the H-6 radar was developed for Navy twin-engine bombers and large twin- and four-engine flying boats. I will try to describe and illustrate the specifications of the H-6 device in Part 2 of this article. In today’s note, I’d like to focus on the story of our main character and provide (mainly for modelers) an introduction to the external elements of its onboard electronics.
In IJNAF Service
The aircraft with serial number 2194 rolled off the assembly line around mid-December 1943. Unlike most Kates in this production series, it didn’t go to a combat or training unit but was directed to the Research and Development Center known as the 1st Naval Arsenal in Yokosuka. The laboratories and workshops there were responsible for the development and implementation of the H-6 radar system. Debate still exists about how many B5N2 aircraft were fitted with the Type 3 Model 4 device. It’s known that the first batch (a concept test) was around 12 units. In practice, several installations were made (probably 6-8), of which only 2-3 configured this way were put into operational use. In other words, our Kate was an exceptionally rare combat machine.
After undergoing technical tests (conducted at Saeki base in Japan in February 1944), our subject headed to the front, joining the ranks of 931 Kokutai. This unit was part of the Rengo Goei Butai, translated in English literature as the Great Surface Escort Fleet. To accurately convey the character of this formation in Polish, I would say it is the Main (Combined) Escort Force. RGB included escort surface ships (of various classes), naval aviation, and a number of submarines. The term “Combined” is also seen in American terminology, aligning with my interpretation, though this is semantics. RGB’s primary role was to protect key supply sea routes, especially between the Dutch East Indies, the Philippines, Japan, and between the Marianas, the Philippines, and Japan. Thus, the mission of radar-equipped Kates from 931 Kokutai was to patrol designated shipping routes and detect enemy surface vessels (and, under favorable conditions, also surfaced submarines) that might threaten supply convoys. 931 Ku reported directly to the regional Naval headquarters on Saipan. Unfortunately, as far as I know, no documents survive that might shed more light on our Kate’s specific missions or the composition of its crew(s).
In American Hands
As I mentioned, aircraft s/n 2194 was found at Aslito in June 1944, and its radar installation aroused great interest among the Technical Air Intelligence Center (TAIC) specialists. The machine had sustained minor damage during an American fighter attack on the Japanese base, but this was quickly remedied (nearby repair hangars and warehouses were stocked with the necessary materials and spare parts). By July 12, KEB-306 had been transferred aboard the escort carrier USS Copahee (CVE-2) and set sail for the United States. On this journey, Kate was not alone, accompanied by 13 A6M fighters (known as the Saipan Zeros) and 37 captured aircraft engines of various types. Sixteen days later, our subject “landed” at NAS North Island, San Diego, California, where it came into the capable hands of engineers from the Naval Radio Laboratory. The plane, brought to full airworthiness, stripped of camouflage, and marked TAIC-6, participated in a series of intensive tests aimed at revealing the capabilities of its radar equipment. The technical and tactical characteristics of the B5N2 were not studied. After years of Pacific war, both Intelligence and the combat pilots were well aware of Kate’s strengths and weaknesses. Tests conducted in November and December 1944 off the Delaware coast to detect various-sized sea targets provided a comprehensive understanding of the H-6 Type 3 Model 4 radar’s capabilities. The results matched 100% with Japanese materials found in Yokosuka after the war. Detailed data on this will be presented in Part 2 of the article.
After the tests, the onboard electronics were dismantled and stored at NAS Patuxent River, which ensured their preservation. The aircraft was then sent to NAS Anacostia, joining “The Navy Flight Might,” a traveling static exhibit showcasing captured enemy aircraft. After the tour ended in July 1946, our Kate was decommissioned and sent for scrapping.
Antenna System
To conclude today’s description, I would like to focus on the external elements of the radar system, namely the antenna system. I can do this thanks to a series of photos that illustrate each detail well. This visualization excites me both as a military technology enthusiast and a modeler.
Let’s begin by noting that our Kate had four antenna modules, identical in design and size. Two of them (forming the transmission antenna system) were mounted on the wing leading edges. Identical modules, mounted on the sides of the fuselage’s rear section (near the Hinomaru), formed the receiving antenna system. In plastic aircraft models, we typically get separate details to attach, usually chunky and resembling a type of “ladder.” An alternative is to use photo-etched parts, but these are unfortunately flat and usually inaccurate. Worse still, linear parameters of the details present issues in both cases. As we know, the shape and length of each antenna section are closely tied to the operating wavelength. The H-6 radar worked at a 2-meter wavelength.
Let’s move on to the technicalities. I will use one of the photos with a few markings. A single antenna assembly consisted of two dipoles made of steel rods with a circular cross-section, shaped like an “L.” The longer sections of the dipoles (the “active part” of the antenna) were 1 meter long. Thus, each dipole was half the wavelength, with the active part of the antenna assembly corresponding to the full wavelength. The shorter dipole sections (extending outwards from the wing/fuselage) measured 50 cm, making up one-quarter of the wavelength, which was intentional. This allowed the aircraft’s external surfaces (wing/fuselage) to act as wave reflectors. Documents found in Yokosuka after the war indicate that studies with dipoles positioned 100 cm and 25 cm away from the surface were also conducted, but 50 cm proved optimal. Due to the wing’s geometry, the distance from the leading edge was not uniform but functioned as expected.
Each dipole was attached to the aircraft via two “bayonets.” These were thick-walled aluminum tubes topped with insulating material (through which the dipole rods passed). Specifically, they were tips made from high-density ebonite (1.5 g/cm³), produced using high-grade rubber sourced from the Dutch East Indies (NEI). The same material was used for the tubes and dipole guides at the wing/fuselage base and the connector for the shorter dipole segments. This last component stabilized the entire structure and prevented the dipoles from touching if deformed (e.g., during sharp maneuvers). I marked all these parts with green ovals in the photo. The main purpose of the ebonite elements was to isolate the dipoles from the aircraft’s structure and prevent any interference with radar operation.
Additionally, I noted an unrelated element in a yellow circle—a mechanical indicator for main landing gear extension. It’s clear that, unlike many similar designs, it resembles a retractable “shark fin.”
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