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--
HEADQUARTERS XIV CORPS
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.
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.