> surprisingly not used that often. VLF (and the lower frequency parts of the radio spectrum) aren't used much anymore because power-efficient antennas are too

> surprisingly not used that often. VLF (and the lower frequency parts of the radio spectrum) aren't used much anymore because power-efficient antennas are too large to be practical. If RF energy enters an antenna that doesn't resonate at the correct frequency, much of it will be reflected back and forth within the antenna until it all dissipates as heat due to internal resistance. A simple kind of antenna is a vertical antenna. These need to be 1/4 the wavelength of the RF signal. You can get by with more or less than 1/4 wavelength, but you'll waste increasingly more energy. Wavelength is inversely proportional to frwquency. If you've got a microwave signal with a wavelength of 30cm, that's practical. VLF frequencies go up to 30 kilohertz at the high end, which is about 1 kilometer (!). So if you want to transmit at decent power, you need a big antenna and a big power source, because you're still not going to have a perfect match and some will be wasted. This is what a VLF antenna array looks like: https://en.wikipedia.org/wiki/VLF_Transmitter_Cutler https://en.wikipedia.org/wiki/VLF_Transmitter_Cutler Notice it's absolutely enormous. Back when we didn't have high frequency electronics, commerical communication was done in the hundred kilohertz range. You can see here what those coast station antennas looked like: https://www.radiomarine.org/historic-coast-stations https://www.radiomarine.org/historic-coast-stations