Was directional antenna technology used in radio communication during World War II? Why is it so easy to intercept?

The highly directional Yagi antenna in World War II was mainly invented by Nitaroouta, but it was his teacher who first applied for a patent and named it Yagi-Tian Yu antenna. However, the Japanese did not pay attention to it. It was not until the invasion of Singapore that the good performance of Yagi antenna was discovered by the British army. Yagi antenna is used for short wave communication.

In fact, whether it is short wave or long wave, VHF, VLF, ELF, etc. As long as it doesn't frequency hop, it is easy to be intercepted by broadband receivers. Moreover, short wave is the main force of communication, so it is more valued by both warring parties.

Theoretically, the efficiency is the highest when the antenna length is a quarter wavelength, and the long wavelength is generally 1km ~ 100km. If the antenna is made large, the power will increase accordingly, the probability of being accepted by the enemy will increase, and the benefits of good directivity will not be highlighted.

See Weapons Knowledge (20 1 1) for details. Broadcasting knowledge is serialized throughout the year.