In the first part of this article, I presented the history of the B5N2 Kate aircraft “KEB-306” along with some details on the onboard radar antennas of the H-6 Type 3 Model 4 radar. Now, it’s time to describe the core of this device’s system. Photos of our Kate come from the 1944 TAIC archives. Archival photographs and English-language system diagrams are drawn from a Naval Radio Laboratory research document from 1945. The drawings and descriptions in Japanese are from the original H-6 Radar Operations and Maintenance Manual. The contemporary color photos of electronic components depict the current remnants, likely now part of the NASM collections.
Beginnings
The formal project to create a sea surface search radar was approved by the Navy just before the outbreak of the Pacific War (in November 1941). The Navy’s Electrical Engineering Department at Yokosuka Arsenal took charge of its development. The original plan involved using a 300 MHz frequency and a 1-meter wavelength. However, Japanese industry could not produce vacuum tubes with the necessary specifications. At that time, only the British (and subsequently the Americans) and Germans produced such components. Obtaining suitable parts from Allied countries was, of course, impossible, and the Third Reich was reluctant to share the results of its secret research with its ally. Thus, the H-6 radar was developed using a frequency of 150 MHz and a wavelength of 2 meters. Laboratory tests of prototypes were conducted in the spring of 1942, culminating in flight tests with a modified Aichi H9A seaplane. The results were promising, although the system’s size and weight were problematic. As a result, the Arsenal recommended its use on a limited number of large Navy aircraft, including the Mitsubishi G3M3 Nell and G4M1 Betty bombers, as well as the Kawanishi H6K Mavis and H8K Emily flying boats. Work on “miniaturizing” radar components for smaller IJNAF aircraft continued, and by mid-1943, it resulted in the H-6 Type 3 Model 4—the very radar installed on our subject aircraft.
System Components
The complete H-6 radar system included five components: transmitter, receiver, operator panel, power module, and transmitting/receiving antenna system. In the archival photo of our Kate’s interior, we can see the large transmitter box (bottom right), the receiver (top left), and the operator panel (below the receiver). The same components (removed in the USA in 1945 after sea trials) appear in the next photo, with the transmitter on the left and the receiver and display module on the right. The power module is also shown on an electrical schematic developed by the Naval Radio Laboratory (lower right corner) but does not appear in any photographs. Regarding their weights, the transmitter, receiver, and operator panel weighed 49 / 8.5 / 22 kg, respectively. Combined with the power module, the system’s weight reached approximately 110 kg, increasing to about 150 kg with wiring and antenna structures. Interestingly, test pilots at the Yokosuka Arsenal reported that the radar-equipped Kate was overloaded, slow, sluggish, and generally unresponsive. This is surprising, considering that the B5N2 could carry up to 900 kg, often flying combat missions with an 800 kg aerial torpedo or an equivalent load of anti-ship bombs. Why did 150 kg of equipment affect flight performance so significantly? The radar antennas likely contributed, but to this extent? Unfortunately, American research reports on the KEB-306 (TAIC-6) don’t address this either. Regardless, the Navy Arsenal recommended that radar-equipped Kates should not carry offensive payloads. It made sense that a plane carrying valuable detection equipment should not engage directly with the enemy, a view held as of late 1943/early 1944. Later, however, specialized Kyūshū Q3W1 Lorna aircraft, also utilizing H-6 radar, were tasked not only with detection but also with the destruction of enemy submarines.
Performance Specifications
The theoretical range of the H-6 radar was approximately 150 km—a reasonable distance at the time. Realistically, the equipment on Kate had a range of 90-110 km (depending on conditions), which was still respectable. The radar cone allowed observation within 30 degrees to the left and right of the aircraft’s axis. In the vertical plane, the cone was tilted slightly toward the sea surface. A large object, such as an aircraft carrier or battleship, could be detected beyond 110 km, while formations of smaller surface ships produced a similar, though different, radar echo. A surfaced submarine could be detected from about 25-30 km. Interestingly, a lone bomber like a B-24 would register at a distance of 70-80 km. Although Kate was designed for sea surface scanning, H-6 radar later proved useful in day and night fighter operations on Gekko aircraft within Japan’s Air Defense (such as the 302 Kokutai).
An operational altitude of 1,000 meters was deemed optimal for H-6 radar missions, with typical patrols conducted at this altitude. If a different vantage point was required, the plane’s altitude could be increased, but a maximum of 2,500 meters was set to avoid “corona discharge,” a phenomenon that risked damaging the equipment and potentially endangered the crew. The radar’s minimum range was 3 km; within this range, targets would fall into the “radar shadow” and disappear from the screen. In Kate’s case, which conducted daytime operations, this was no issue, as the crew would visually observe the enemy long before. The H-6 radar had no link to any weapons control system, so this limitation was even less of a concern. Machines like the Q3W1 Lorna and Aichi E13A1 Jake (meant for both detection and anti-submarine warfare) were also equipped with magnetometers, allowing target tracking even during emergency dives.
Readings and Analysis
The radar operator’s panel was a crucial part of the system. Its core was a large oscilloscope tube (labeled “Brown’s Tube” on American schematics—a term derived from its inventor). The display, 120 mm in diameter, had two working modes. The first mode displayed a “comb” graph indicating the target’s distance based on reflected signal strength (peak of the graph). Since signal strength was affected by distance, the operator had a reference table to help estimate target size (e.g., probable ship tonnage). A “clean peak” indicated a single large object, while a “blurred peak” suggested a group of smaller targets. The operator’s experience in interpreting the display was critical. Standard procedure also involved approaching the target from different angles to refine measurements on whether it was a single or grouped target. The distance measurement error of the H-6 Type 3 Model 4 radar was about 5%, a reasonable figure.
The second mode displayed a bar graph, showing the target’s position relative to the aircraft’s course. This allowed for a relatively accurate determination of the enemy’s geographical coordinates. Included Japanese instruction illustrations depict both graph types and detail the proper interpretation and conversion methods to refine data. For particularly interested readers, original H-6 diagrams and descriptions of each module (in Japanese) are also attached. The contemporary images show FZ-064A vacuum tubes, the transmitter base with sockets for these tubes, and other smaller details of the equipment.
Summary
The H-6 radar family was the most widespread onboard radar type of the Imperial Navy Air Force. Approximately 2,000 units were produced across several variants. While the H-6 lagged behind comparable British, American, and German products in some respects, it ranked among the global elite. However, its full potential was arguably underutilized. Between 1942 and 1945, H-6 radar evolved mainly in reducing component size and weight, rather than improving operational parameters. Technological constraints on more advanced electronics production hampered progress, and while newer radar designs incorporated improved components, these appeared too late in the war to impact significantly.
Operationally, the Japanese failed to integrate radar-equipped aircraft effectively. Small scout planes played a crucial role early in the war (December 1941–November 1942), but this was largely forgotten once the “Japanese blitzkrieg” stalled. The Babs reconnaissance aircraft, later replaced by Gekko aircraft, carried out crucial intelligence tasks, but without proper assignments, most were eventually swapped out for A6M Zero fighters. Similarly, radar-equipped Kates were often reluctantly accepted, with unit commanders struggling to find uses for them. Only a few exceptional commanders, such as Commander Kozono and the legendary Commander Genda, recognized and leveraged their potential, using radar-equipped aircraft as mobile command centers.
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