Disadvantages to Using Low Frequency Radio
Very difficult to transmit
Because ELF is indeed extremely low frequency, it requires very large antennae to produce it. If someone wanted to transmit ELF signals using a half-wavelength dipole antennae, the antennae would need to be approximately 1800 kilometers. As this is clearly impractical, methods of using part of the Earth itself as an antenna were developed by both the USA and the USSR.
Communication can only be one way
As a result of the antennae being so big and so power-intensive, the only practical place for a ELF transmitter is on land, not on submarine. Because of this, communication using ELF would have to be one-way, from the transmitting station on land to the submerged submarine.
Security of the Signal
Having the ELF signal being transmitted to literally the entire world is a bad thing for covert operations, particularly for military orders. The solution to this is encryption, which inevitably adds to the length of the message and therefore to the time necessary to transmit it. This is especially problematic because the data rate of ELF and VLF is so slow.
Slow Data Rate
As far as radio communications are concerned, the higher the frequency, the more data can be transmitted in a given amount of time. This means that for very low frequencies like ELF, the data rate is extremely low, on the order of 300 bits per second. While this is enough to transmit orders to submarines, any moderate sized message would take a while to be transmitted and received.
Because ELF is indeed extremely low frequency, it requires very large antennae to produce it. If someone wanted to transmit ELF signals using a half-wavelength dipole antennae, the antennae would need to be approximately 1800 kilometers. As this is clearly impractical, methods of using part of the Earth itself as an antenna were developed by both the USA and the USSR.
Communication can only be one way
As a result of the antennae being so big and so power-intensive, the only practical place for a ELF transmitter is on land, not on submarine. Because of this, communication using ELF would have to be one-way, from the transmitting station on land to the submerged submarine.
Security of the Signal
Having the ELF signal being transmitted to literally the entire world is a bad thing for covert operations, particularly for military orders. The solution to this is encryption, which inevitably adds to the length of the message and therefore to the time necessary to transmit it. This is especially problematic because the data rate of ELF and VLF is so slow.
Slow Data Rate
As far as radio communications are concerned, the higher the frequency, the more data can be transmitted in a given amount of time. This means that for very low frequencies like ELF, the data rate is extremely low, on the order of 300 bits per second. While this is enough to transmit orders to submarines, any moderate sized message would take a while to be transmitted and received.