What do you do when you need to communicate with a crew of 50 sailors submerged in a submarine in an undisclosed location somewhere in the world's oceans? That was a difficult but vital question for Navy leaders to answer in WWII.

Radio waves don't easily travel through saltwater, which meant that getting active communication with a submarine crew meant using the surface of the submarine as an antenna. This was the obvious solution, but it also turned the previously covert submarines into visible targets.

The solution to the problem

Engineers tasked with finding a more covert solution soon discovered that radio waves with low frequencies, around 10 kHz, could penetrate saltwater to depths of up to around 20 meters. They realized that if the transponders on submarines were switched to these frequency ranges, then they communicate with leadership on land.

The problem with this idea was that creating and broadcasting these low-frequency radio waves required massive antennas. Essentially, the lower the frequency of a radio wave, the longer and larger the antenna was required to be.

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Engineers eventually honed in on a range of frequencies lower than 30 kHz for submarine communication. The wavelength of these frequencies were roughly 10 kilometers or more, meaning that engineers would need massive antennas. The only way to produce these frequencies with such a high range was to use a massive system of antennas with massive amounts of power.

Nazi engineers seeking to communicate with their fleet of U-boats had the same problem. They designed the Goliath antenna network in Saxony-Anhalt, Germany. After construction, it was operated during World War II and had a transmission power of up to 1,000 kilowatts. For comparison, that's equivalent to the power used today by 500 average American households.

The Goliath Radio Transmitter

The Goliath network would regularly transmit frequencies between 15 kHz and 25 kHz. It was powerful enough to reach any German submarine located anywhere in the world submerged up to 20 meters. The only time communication was hindered was when German U-boats were navigating deep Norwegian fjords.

The Goliath antenna used three different umbrella antennas. These were essentially massive antenna towers that were draped with kilometers of guy wires radiating out from the mast. These helped not only to support the antenna tower, but they also formed part of the antenna itself.

In total, the system used three, 210 meter or 688-foot masts arranged in a triangle. The system also had cables buried around it, with a total length of 350 kilometers. Once it was completed, the system had an impressive efficiency of 50% at 15 kHz and 90% at 60 kHz.

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This massive antenna site was undoubtedly a key strategic tool used by the German Navy in communicating with their U-Boat fleet.

After the war ended, the Soviets dismantled the Goliath and shipped it to Russia. It was then re-erected near Moscow. Today, one of the original towers is still in operation, communicating low-frequency signals to submarines, and broadcasting time signals.

A deeper dive into how low-frequency networks function

Low-frequency radio frequencies are anywhere in the range of 30 to 300 kHz, and their wavelengths range from 1 to 10 kilometers.

Because their wavelengths are so long, these frequencies are the perfect tool for long-range communication networks. Low-frequency radio waves, or LF radio, is used for AM radio stations across the world, allowing them to broadcast from a centralized location across many hundreds of miles.

One of the other massive benefits of LF radio signals is the fact that their long wavelength allows them to diffract over very large physical obstacles, like mountains, or even the Earth. LF waves can follow the curvature of the Earth with ease, utilizing ground wave propagation. Low frequency waves sent through ground propagation can be clearly received more than 1,200 miles from the original source.

Another way that LF radio waves can be transmitted ultra-long distances is by intentionally reflecting the waves off of the Earth's ionosphere. This is called skip propagation or skywave, and it allows frequencies to be transmitted distances of over 190 miles from the original source. Not as far as ground wave propagation, but still an impressive distance.

The other benefit of low-frequency waves, underscored by the use of the Goliath transmitter, is the fact that low kHz low-frequency waves, under 50 kHz, can penetrate ocean depths of roughly 200 meters. As the wavelength gets longer, the penetration depth gets deeper.

Most of the world's superpowers still use some form of this LF transmission to communicate with submarines and underwater vessels today. British Royal Navy nuclear submarines stationed around the United Kingdom listen to a 198 kHz frequency for orders to launch their ballistic missiles.

The U.S. eventually built out something called the Ground Wave Emergency Network, named GWEN. It ran between the ranges of 150 kHz and 175 kHz up until 1999, when satellites far outperformed the LF network's usefulness.