The ELF system used by submarines operates at which frequency range?

The ELF system used by submarines operates at which frequency range?

• A. 40-80 Hz
• B. 4-8 Hz
• C. 400-800 Hz
• D. 0.4-0.8 Hz

Answer: (A)

Submarine ELF communications rely on very low frequencies because seawater greatly attenuates higher-frequency signals. To reach submerged submarines, the signal must travel deep into water, which is only feasible with frequencies in the tens of hertz range and with enormous land-based antennas that can generate sufficient energy. The range around 40 to 80 Hz achieves the best balance: it penetrates seawater better than higher frequencies and can be radiated by practical, though still massive, antennas, allowing reception on submarines at depth, albeit with very low data rates. Other options are not practical for submarine use: frequencies much lower (like 0.4–0.8 Hz or 4–8 Hz) would require even larger, more challenging antennas and infrastructure, making deployment impractical; and frequencies above a few hundred hertz attenuate too quickly in seawater to be useful for long-range submerged communication.

Submarine ELF communications rely on very low frequencies because seawater greatly attenuates higher-frequency signals. To reach submerged submarines, the signal must travel deep into water, which is only feasible with frequencies in the tens of hertz range and with enormous land-based antennas that can generate sufficient energy. The range around 40 to 80 Hz achieves the best balance: it penetrates seawater better than higher frequencies and can be radiated by practical, though still massive, antennas, allowing reception on submarines at depth, albeit with very low data rates.

Other options are not practical for submarine use: frequencies much lower (like 0.4–0.8 Hz or 4–8 Hz) would require even larger, more challenging antennas and infrastructure, making deployment impractical; and frequencies above a few hundred hertz attenuate too quickly in seawater to be useful for long-range submerged communication.