Extremely low frequency refers to radio waves with frequencies from 3Hz to 3kHz and from 10000 km to 100000 km. Most of them are used by the US and Russian military to communicate with submarines.

Because it is difficult to build an antenna that can radiate such a long wave, ELF frequency is only used in a few artificial communication systems.

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In terms of communication efficiency, the higher the frequency, the higher the communication efficiency; The lower the frequency, the lower the communication efficiency. Moreover, to generate low-frequency, very low-frequency and extremely low-frequency electromagnetic waves, it needs a high-power base station and a huge transmitting antenna, which makes it only a strategic communication means, sending short message instructions in one direction, and it is impossible to conduct communication networking under water.

However, the above shortcomings have not obliterated the value of low frequency, very low frequency and extremely low frequency electromagnetic waves in the communication field, especially the advent of nuclear-powered submarines that can be in underwater combat readiness for a long time.

The disadvantages of low frequency, very low frequency and extremely low frequency electromagnetic waves in communication efficiency are greatly weakened, while their advantages in underwater-to-submarine communication are greatly strengthened, so they are called the cornerstone of the navy.

Baidu Encyclopedia-Extremely Low Frequency