extremely low frequencies

2026-05-09 The submarine is a surprisingly ancient technology—at least in its early, primitive forms. The idea is quite simple, that a well-enough-sealed boat ought to be able to submerge and resurface. It's the practicalities that make the whole thing difficult. It is generally considered that the US Civil War was the first use of submarines in combat; these were primitive machines with very limited operating endurance and navigational capabilities. These submarines were more like torpedoes: you pointed them in the right direction and hoped they went straight. The First World War benefited from tremendous advances in submarine technology. A number of experimental designs during the 19th century had built practical experience, especially in Germany, and the Germans apt use of the first modern "U-boats" had a significant military impact. British and US designs made similar advances, and submarine warfare was born. The chief advantage of the submarine is its ability to submerge and maneuver while hidden. WW1 submarines were diesel-electric or gasoline, so their submerged endurance was limited by the power supply stored onboard. Still, these submarines could operate underwater longer than any before, long enough to establish the submarine sneak attack as a key part of naval warfare. It was also long enough to expose one of the trickiest challenges of underwater defense: communications. Water, especially seawater, is dense and conductive. This is very bad for radio wave propagation: by the first world war it had already been discovered that seawater effectively blocked radio communications. HF radio, the main form of communications at sea (and, in the WW1 era, in general) might only penetrate seawater for a few meters in real-world That meant that submarines had to surface in order to communicate, another de facto limitation on their endurance while submerged. The Navy had been evaluating electronic communication aboard ships since 1887, when they demonstrated a simple and "radio-adjacent" technology using conduction of waves through the seawater itself. This scheme never worked very well, but was saved by the development of modern wireless transmitters late in that century. Marconi himself demonstrated radio to the Navy in 1899, and in 1903 the Navy bought its first radio sets. Tactical reports from conflicts elsewhere on the globe, like the Russo-Japanese war, reinforced the idea that radio would serve a key role in naval combat.

When C-class submarines Stingray and Tarpon, and D-class Narwhal, launched in 1909, they were immediately given duties including the evaluation of radio equipment. In a classic tale of early technology, the evaluations went poorly. Tarpon ran into mechanical trouble that prevented its planned trial voyage, so the radio set was never installed. Stingray received a cutting-edge quenched spark gap transmitter and receiver set, but the transmitter turned out to be DOA. Still, Stingray was able to demonstrate its receivers, copying a message from the nearby Boston Navy Yard while surfaced. Narwhal's mission was more ambitious: underwater communication. A test was made on the same direct conduction technology, using brass plates suspended below the ships, demonstrated in 1887. It similarly failed to perform. A repetition of those experiments, done the next year and with improved equipment aboard Narwhal's sister ship Grayling, produced better results. The system provided reliable communications with the "antenna" plates submerged as much as two feet below the water... and no deeper. Frustrated Navy engineers concluded that it was possible to get radio signals through seawater, but not practical. Through the First World War and following decades, engineers focused on ways to get the antenna to the surface without having to bring up the entire submarine. Around 1915, the Navy adopted a floating antenna buoy that a submarine could "winch up" towards the surface on a cable. Putting anything at the surface was less than ideal, but the anti-submarine technology of the era the small antenna buoy was still very difficult to detect at long range. Submarines just had to make sure it was retracted back to the submarine's deck before attempting anything where stealth was key. These floating buoys were not reliable during WW1, but they could work, and the technology has continued to develop to this day. Still, there were other ideas about underwater communications. The most important development came from two engineers of the National Bureau of Standards (NBS), or at least, that's what a court ruled after a patent dispute between two sets of supposed inventors. John Willoughby was employed by the NBS, which would later be known as the National Institute of Standards and Technology (NIST), to investigate new types of radio receivers.

In the summer of 1917, he was arranging various types of coil antennas at a receiver test site on the Chesapeake Bay when he accidentally dropped one of the antennas into the water. Strangely enough, the radio receiver connected to the antenna continued to provide good reception even as it sank into the bay. NBS management was not especially enthusiastic about this accident, but Willoughby was. He knew that the Navy was investigating means of communication with submarines, and that seawater seemed to block radio waves, all of which suggested that he might have stumbled on an important discovery. Lacking NBS support for further research, he took the idea to gifted radio inventor and NBS colleague Percival Lowell 1. In a fine tradition of innovation, the two took to Willoughby's basement for a series of experiments that illuminated the underlying phenomenon: Willoughby had been experimenting with unusually low radio frequencies, below 30kHz where wavelengths become too long for most antenna designs and coils become the best receivers. These lower frequencies were significantly less affected by water than higher, more conventional frequencies, and Willoughby and Lowell built a successful prototype for what they called "long-wave" radio between two coils. The NBS remained surprisingly uninterested, but Willoughby had a contact in the Navy who felt quite differently. In 1918, Willoughby and Percival joined LtCmd H. P. LeClair, then running the Navy's experimental radio program, at submarine base New London (so named after New London, Connecticut, across the Thames River (Connecticut) from the base). They made a hurried and rough installation of their equipment on submarine D-1 and a surface support vessel. Not everything went perfectly, but they proved the idea: Willoughby, Lowell, and LeClair listened attentively to their radio sets as the D-1 submerged and continued to come in loud and clear. Within a matter of a few years, the Navy accepted long-wave radio as a standard technology for submarine communications. The various jury-rigged installations at New London showed that coil antennas could easily be integrated into a submarine's rigging, and even better, the Navy had found that long-wave radio propagated over the surface as well as under it. Long-wave communications would serve the entire Navy, and a transmitter site was already underway.

Long-range communications had become a top concern throughout the military in the early 20th century, and a series of meetings between US military branches and between the US and UK lead to a scheme of "High Power" radio stations. The first of these, NAA, went up near Arlington, Virginia in 1913. Over the following years, similar stations were built in the US and Europe, facilitating the first direct communications between the two and the first transatlantic voice communication in 1915. The construction and operation of these stations also lead to considerable advances in radio technology generally, especially powerful transmitters. NAA was one of the early stations to be equipped with Poulson arc transmitters, almost two times more efficient than earlier designs and well-suited to long-wave operation. Around the same time as the Willoughby/Lowell experiments, Navy engineer LtCdr Albert Taylor found similar results with long-wire antennas shallowly under the water. These experiments offered another design for concealed submarine antennas (which could be stored onboard in reels and let out with floats that kept them just under the surface), and also demonstrated that long-wire antennas could be buried for transmit use. Five years later, in 1918, construction was underway on NSS—a new high power station in Annapolis, Maryland. Unlike those before, NSS was specifically designed for long-wave signals. Two 500 kW Poulson arc transmitters driving an antenna 400' square and suspended between four 500' tall towers 2. The long-wave capability at Annapolis was not originally intended for submarine communications, but it quickly fell into that niche. During the 1920s, NSS became a key station for submarine command and control of submarines. NSS itself remained in service until 1996, and it was joined by VLF transmitters at Cutler, Maine; Jim Creek, Washington; Lualualei, Hawaii; LaMoure, North Dakota; and Aguada, Puerto Rico; besides sites in Europe operated with allied militaries. Each of these stations is its own interesting story. The 1,205' VLF antenna tower at Aguada remains the tallest structure in the Caribbean. LaMoure was originally built in the 1960s for a long-wave navigation system called Omega, and was repurposed for submarine C2.

Jim Creek went into service in 1952 as the most powerful radio transmitter in the world, using a fascinating antenna that draped from one ridge to another across a mountain valley. Let's focus, though, on Cutler. VLF Transmitter Cutler is the spiritual descendant of the Navy's original High Power program, symbolized in its inheritance of the callsign NAA. Cutler was part of a Cold War expansion of the VLF system, going into service in 1961. Many other VLF sites received upgrades around the same period, but Cutler was a completely new design. Cutler's two antennas, for redundancy, are each supported by 13 towers. The center tower is about 1,000' tall, and the other 12 make up two concentric rings of about 900' height. The complete antenna is over 6,000' across, or nearly 2 km. Between the tower tops stretches a web of tight horizontal wires, each 1" copper, that form an enormous capacitor. The capacitor's other plate is the ground, electrically reinforced by many miles of buried groundplane wires. The radiating elements are vertical wires, hanging down from the upper horizontal mesh. In Maine's harsh winters, the wires accumulate ice until their weight threatens the towers. Each antenna is alternately switched into a deicing mode in which it is turned into a 3 MW heating element... just for long enough that the ice melts off. Outer towers are supplemented by short, stout structures that allow the 220 ton tension weights to move up and down on tracks. "Helix houses" at the feedlines of the two antennas sheltered enormous inductors; walls lined with copper served as insulation and to ground the occasional arcs that made the helix houses and transmitter rooms unsafe to enter during operation. The two antennas were driven by a transmitter complex designed and built by Continental Electronics. The 11 MW on-site power plant supplied the AN/FRT-31 transmitter, custom to this installation, consisting of four parallel units of eight ML-6697 transmitter tubes. The transmitter's control room rivaled that of many power plants, as did its output: the military required at least 1 MW, Continental rated the transmitter for just over 2 MW, and it still operates today at powers as high as 1.8 MW.