The Defeat of the 193rd Tank Btn at Kakazu Ridge... - Axis History Forum

The Defeat of the 193rd Tank Btn at Kakazu Ridge...

Discussions on WW2 in the Pacific and the Sino-Japanese War.
Mil-tech Bard
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Joined: 06 Jan 2010 15:50

Postby Mil-tech Bard » 02 Feb 2013 22:19

...was caused by poor FM radio signal security?

American tanks were equipped with FM radios and their operators were told the Japanese didn't have any FM gear, so they took no elementary security precautions. The poor performance of the 193rd Tank Battalion on Okinawa at Kakazu Ridge could be easily explained if that were indeed the case.

Previous topics here (See link below) never touched that aspect of the battle for the 193rd.


This is clipped from the Signal section of the Luzon After Action report of the XIV(14th) Corps showing that the idea of FM communications by the Japanese was not thought possible and the security issues with Army troops thinking that FM voice radio was inherently secure--

APO 453
Page 50

f. In the BATANGAS area there were reports of Japanese voice radio transmissions from operators of an artillery F.M. radio set of the SCR600 series. A sensitive monitoring receiver (BC-787) was set up at an, elevated spot in CANLUBANG end the intercepted traffic was piped into a loudspeaker in the Lang-age Section of AC of S, G-2, at CorDs Headquarters. Intermingled with our own artillery radio traffic, numerous Jap voice signals were heard and translated by Nisei at the headquarters. It was noted that such transmissions were only intercepted between, 1100/I and 1800/I, with the signals best about 1330/I on frequencies ranging between 26 megacycles and 38 megacycles, indicating that the stations were at a great distance (over 1200 miles). Translation of the Japanese heard revealed that the nets were of the Air-warning and Air reconnaissance type, some of them being on a training status, while others seemed to be reporting the approach of hostile (American) aircraft with reference to some of the larger cities in Japan itself, verifying the suspicion that the Jap nets heard were most likely not in the Philippine Islands.


Page 56

Security in the M-1 Operation was only fair because too many voice operators (especially on Air-ground radio sets) are not security conscious. There is some idea that FM has a security feature. This is entirely false. Officers seem to be the worst offenders on voice radio. During the practice landings, by intercepting voice radio nets of ground units, it was an easy matter to reconstruct the complete order of battle. This led to the schooling of our people in increased use at brevity codes and careful message writing. On CW circuits, where most traffic is usually enciphered, security was very good; what few breaches of security rules there were came from clear text and/or improper procedure between radio operators.

Carl Schwamberger
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Postby Carl Schwamberger » 02 Feb 2013 23:26

Sounds like business as usual... A couple of the observations in that report would have applied to practices I observed in the US military of the 1980s & 1990s. I'd strongly suspect poor or incomplete training with the 193d Tk Bn. I am not sure how the "skip" radio transmissions from Japan relate to possible compromise of the 193ds radio comm. but it is interesting.

Mil-tech Bard
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Postby Mil-tech Bard » 03 Feb 2013 19:25


If by "ill-trained" you mean poor radio security procedures in the 193rd Tank Battalion, I agree with you.

The text quoted below was from the Vol III No 12 August 1945 issue of Intelligence Bulletin from the Military Intelligence Division of the War Department.


Tests conducted with four Japanese Type 94 (1934) Mark
6 radios on Luzon reveal that it is possible for this amplitude modulated
set to communicate with both the U. S. SCR-610
and SCR-608 FM sets. A small two-unit set of the Walkie-
Talkie design, the Type 94 Mark 6 is standard equipment
within the Japanese infantry battalion.

Results, of the texts show that the enemy sets operate satisfactorily
with the SCR-610 within a range of 1/4 to 1 & 1/2 miles,
and with the SCR-608 within a range of 1/2 to 4 & 1/2 miles. Operation
outside these ranges is not recommended. The tests were
conducted by the Signal Technical Intelligence Team, I Corps,
in conjunction with signal personnel from the 25th Division.
The SCR-610 was set up in a truck, with a battery pack and

page 25

an AN-29 antenna. The signal transmitted by each set was
tuned in on the receiver of the other with the two radios side
by side. The signals in both sets were very weak, but readable.
Following this, the SCR-610 was moved to successive locations
ranging from 200 yards to 2 & 1/2miles away. Within the
range of 1/4 to 1 & 1/2 miles the two sets operated together satisfactorily
for two-way communication without retuning the Type
94 Mark 6. After the initial move to a location 1/4 mile from
the Type 94 Mark 6, however, it was necessary to retune the
Jap set. Communication was sporadic at distances from 1 & 1/2
to 2 & 1/4 miles.

Installed in a command car, the SCR-608 was operated from
a stationary location, while the Type 94 Mark 6 was carried
in a truck. The signal transmitted by each set was tuned in on
the receiver of the other with the two radios side by side. The
signals in both sets were loud and clear.

Tests were conducted by moving the Jap radio to successive
locations 200 yards to 5 miles away. As was the case with the
SCR-610, retuning was necessary after the initial move. Within
the range of 1/2 t o 4 & 1/2 miles, the two sets operated together
satisfactorily, with tuning very critical. Beyond 4 & 1/2 miles
communication was sporadic and subject to interference from
adjacent channels. The signal from the Type 94 Mark 6 was
not strong enough to operate the "squelch" x of the U. S. set
beyond 1/2 mile.

Two flat No. 4 batteries for filament and-six Type B-18 batteries
for plates—the batteries delivering 3 and 135 volts, respectively—
are used by the Japs for the Type 94 Mark 6.

These batteries fit in the battery box of the radio set.

If Japanese batteries are not available, the set may be operated
with two Signal Corps BA-23 and six Signal Corps BA-2
batteries, all of which will fit in the battery box except one
1 "Squelch" refers to the stoppage of 'oscillation.

page 26

BA-23. The extra BA-23 may be carried in the headset pouch
attached to the battery box carrier. For reduced weight the
BA-23 batteries may be replaced by four or six BA-30 batteries
connected in series parallel to give 3 volts, although this
will give shorter battery life.

Due to a shortage of the Japanese UZ-30MG tubes used in
the Type 94 Mark 6, it may be necessary to perform a field
conversion so that Type 19 or similar U. S. tubes can be utilized.

Because Japanese microphones and headsets are of
rather poor quality, it is also recommended that U. S. Type
T-17 or similar microphones and U. S. Type HS-30 or similar
headsets be adapted to replace them.

The Japanese had the ability to hear U.S. tank FM-radios down to the Battalion level and US ground units were not aware of this until late in the Luzon and Mindanao campaigns, and after both Iwo Jima and most of the Okinawa fighting.

Given that the Shuri HQ -- with any English speaking intelligence officers -- was a few miles away from Kakuzu Ridge, and the fact that suicide charges were rushed to Kakuzu Ridge the night before the 193rd's attack (This from a PBS special on the end of the Pacific War), we have pretty good circumstantial evidence that poor radio security was a significant factor in the 193rd's defeat.
Last edited by Mil-tech Bard on 04 Feb 2013 17:41, edited 1 time in total.

Carl Schwamberger
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Postby Carl Schwamberger » 04 Feb 2013 03:21

Wish I had time to write all this up into some sort of magazine article. Might be worth aiming for a US military pub. Could be tailored for either the marine Corps Gazette, or naval Insititute Proceedings. any of a half dozen US Army perodicals are cadadates as well.

Mil-tech Bard
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Postby Mil-tech Bard » 04 Feb 2013 17:40


That is where I was going with this. ;-)

Mil-tech Bard
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Postby Mil-tech Bard » 04 Feb 2013 21:32


This is the transcript of the PBS Show "Victory in the Pacific" that includes an interview of an Okinawan Student Conscript carrying suicide bombs to Kakazu Ridge the night before the failed 193rd Tank Battalion/105th Infantry Regiment attack. ... ranscript/

Donald Miller: They went in on April 1 and they thought: What is this? An April Fool's joke? "Where are the Japs?" We got all this big buildup on the boat. And they told us about the ferocious snakes and the Japs are worse than the snakes, and we're going in there and it's going to be maybe the final battle. Then there's nobody on the beach?

Narrator: For five days, American forces were unopposed as they headed south. An admiral radioed Chester Nimitz, Commander of the Pacific Fleet, "I may be crazy, but it looks as if the Japanese have quit the war, at least in this sector."

Donald Miller: And Nimitz wired back, "Delete everything after 'I may be crazy.' "

Narrator: General Mitsuru Ushijima could smile from his mountain headquarters beneath ancient Shuri Castle. He was luring his enemy into a trap.

Donald Miller: Ushijima was very smart. He followed a theory the Americans called it the "cornered rat" theory. Terrain's everything here. And the southern part of the island, the terrain explains it, a series of ridges in the south. So he builds steel, concrete, and coral garrisons inside the mountains. So he has two things that you need to win a battle. He has concealment, and he's got the advantage of height.

Narrator: The first major line of defense was at Kakazu Ridge. Ushijima's command post was under Shuri Castle four miles south in the main line of defense. These ridges ran the width of the island. There were no open flanks. The Army's commander decided to storm the ridges.

Jerome Connolly: You got up every morning, and there'd be another craggy, rock-piled hill and the Army, infantry, would take off over whatever open ground there was. The Japs would then lay into them, and we'd try to keep them down with our machine guns. And that was like a continuum. Every day it was the same story, only different guys got hit and different guys got killed, I used to look at those infantry guys go across -- go across those fields. God, it's tough to believe.

Narrator: The Army sent in tanks.

Katsuo Nagata, Okinawan Student Conscript: We student conscripts were ordered to deliver bombs to Kakazu for destroying U.S. tanks. We carried 10 kg bombs on our shoulders and headed for Kakazu at night. The Americans launched a star shell and then came the gunfire. We had to hide in the shade whenever a star shell was up, but finally we managed to deliver the bombs to Kakazu. I heard that, the next morning when the tanks came, they armed the bombs and made suicide attacks into the tanks.

Mil-tech Bard
Posts: 435
Joined: 06 Jan 2010 15:50

Postby Mil-tech Bard » 17 Jun 2013 22:03

I found the following quotes in the this document:

History of Technical Intelligence
Southwest and Western Pacific Areas 1942 - 1945

Page 101 of 157 (Page 90)

Lt Ford found that the Japanese type 97 portable wireless
telephone set could receive signals clearly from U. S. frequency
modulated SCR-610 end SCR-300 sets at distances of 2300 to
3000 yards
. This was important, as messages in the past had frequently
been sent in the clear on these two sets. The 41st
Division Signal officer, the G-2, and the Division artillery
were given this information.

page 105 of 157 (page 94)

Though no new Ordnance equipment was recovered, fourteen very
high frequency directional radio transmitters having the same frequency
bands as the Unites States VT fuzes were captured near Cebu
, These sets were inspected by T/3 Borchers (Signal) and
arrangements made in conjunction with the Ordnance and Signal
officers of Eigth Army Headquarters to have the radios tested in
the field with actual AA firing to see if VT fuzes could be
activated by waves from these sets, since this would prove an
effective counter-measure against United States weapons.

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