VLF Transmitter Cutler: A Navy Site with a Rich History

The VLF Transmitter Cutler is a Navy site with a rich history that spans over seven decades. Located in Maine, this site has played a crucial role in the Navy's communication systems.

Built in 1945, the VLF Transmitter Cutler was initially designed to communicate with submarines. Its unique location allowed it to transmit signals to the Atlantic Ocean, enabling the Navy to stay in touch with its underwater vessels.

The site's strategic location near the Atlantic Ocean made it an ideal spot for the Navy's communication needs.

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The Cutler VLF transmitter is a powerful radio station with a transmission power of 1800 kW, making it one of the most powerful in the world. It operates on 24.0 kHz.

Antenna maintenance is performed during the summer months, allowing the station to continue transmitting while one array is grounded for maintenance. This is crucial since the Navy closed Annapolis (NSS), the only other East Coast VLF station.

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The "bow-tie area" near the transmitter house is a region where the two arrays come close together, with two panels and three towers from each array. The fields on the grounded array are highest in this area due to proximity to the active array.

The present station operating procedure does not allow work on the bow-tie area towers or panels while transmitting either array, which would require several months of total downtime to complete the ongoing tower painting project.

The antenna arrays at Cutler are quite impressive. They consist of two separate identical arrays, designated the “north array” and the “south array”, each consisting of 13 tall metal masts connected at the top by a network of horizontal cables.

Each array is designed to function independently to allow maintenance on the other array, which is crucial since the Navy closed Annapolis (NSS), the only other East Coast VLF station. This allows continuous transmission, even during maintenance periods.

The region where the two arrays come close together, near the transmitter house, is called the "bow-tie area". This area has the highest fields on the grounded array due to proximity to the active array.

The bow-tie area is also where the ongoing tower painting project is taking place. Unfortunately, the present station policy would require several months of total downtime to complete the project.

Test transmissions have been arranged to allow painting and normal maintenance on the bow-tie area towers of an inactive array. The objective of the four-panel tests is to characterize the antenna operating parameters which had not been measured since changing to 24.0 kHz.

The antenna arrays are 6140 feet in diameter, making them quite large. Each element (“panel”) of the antenna is suspended between the central tower, two towers of the inner ring, and one tower of the outer ring.

Here's a breakdown of the antenna array structure:

The antennas have a deicing system which runs large 60 Hz electric currents through the wires, heating them, to melt the ice. This system requires 3 MW of power, higher than the transmitter output power.

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The Cutler Naval Station's transmitter is a beast, with a transmission power of 1800 kW, making it one of the most powerful radio transmitters in the world.

It operates on a very specific frequency of 24.0 kHz, which is a continuous encrypted FSK (F1B) signal at 200 baud.

The transmitter is designed to transmit coded text messages, not audio, due to its very small bandwidth.

This means the data rate is quite slow, but it's a crucial function for the station's purposes.

The callsign of the station is NAA, which is a unique identifier for the transmitter.

Bob Mhoon, the former station maintenance chief, shared valuable insights into the VLF transmitter's operation.

The Transmitter Deck, console, and amps looked similar in 1981 to how they did in 1961, with a new digital console being the only notable change.

Mhoon noted that the area behind the amplifier cabinets was filled with a cooling system, which included giant fans that drew air from outside the building through a filtering system.

These fans were massive, measuring 12 to 15 feet in diameter, and were effective in keeping the system cool, even in winter.

The system was powered by large HV AC breakers operated by a DC control voltage from banks of motorcycle batteries, made by a French company.

When the batteries finally gave out after 18 years of operation in 1979, Mhoon and his team had to get creative to find a solution.

They bought up every motorcycle battery available in the nearby fishing villages and wired them in series/parallel to match the original system.

This makeshift solution required a custom-designed cabinet and a hood and vent to prevent gassing.

The HF site, located across the road, used AN/FRT-39 and AN/FRT-40 transmitters, while the VLF site was a couple of miles off to the left.

The combiner room, where the transmission line connected to the transmitter, was a notable feature, with copper-plated walls and capacitors bolted together via flanges.

Each capacitor weighed about ten pounds and was used to filter the signal.

The tap for tuning the frequency of the system was a giant tuning coil located inside a four-story building called the helix house.

This massive coil was sealed and filled with an inert gas, making it difficult to replace when it failed, often in the winter.

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Radio signals from Earth can affect electrons in space. These signals are used by naval forces to communicate with underwater submarines.

The signals have a frequency of 20-25 kilohertz and can propagate up to 2.8 Earth radii from the planet. This is close to the distance where a sharp barrier to high-energy electrons is observed.

The signals are omnidirectional, meaning they broadcast in all directions, and can interact with local electrons in space. This interaction can deplete the population of electrons that would otherwise drive the growth of chorus waves.

Chorus waves are a type of radio wave in the VLF part of the electromagnetic spectrum, which can boost electrons to high energies. Without strong chorus wave growth, high-energy electrons are not accelerated to MeV energies within 2.8 Earth radii of the planet.

This effect can persist for days or weeks after the population of high-energy electrons beyond this distance has rebounded. It's like a "barrier" to these relativistic particles that can remain in place even after the storm has passed.

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