2 MW, 14-24 kc
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 Download brochure (pdf) |
Photos and text from October 1961 QST magazine article "NAA-1961"
- thanks to ARRL
 "When operating at full power the antenna is fed by four separate 500-kw final amplifiers, each with eight ML-6697 air-cooled tubes operating in push-pull parallel. The antenna consists of some 62 miles of one-inch copper cable supported by 26 towers is a double star pattern, with the towers ranging in height from 800 to 980 feet." |
 "This is the control console for a two-megawatt transmitter. Driver stages and final amplifiers along the rear walls, with the "guts" of the units well-protected against accidental access." |
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 "Ever see a man standing inside a coax matching section? Chief Electronic Technician Swan, who is in charge of all maintenance at NAA, stands inside the copper-lined concrete tunnel mentioned in the text."
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 "Old-timers will recognize this monster as a variometer. It's used to tune the bottom end of the v.l.f. antenna, and is controlled by the operator on watch a mile away." |
 "This thousand-foot tower, guyed at three levels, supports the center of each star-shaped pattern. The "helix" house at the bottom contains the loading coils which match the coax cable to the antenna itself." |
Photos and comments from Bob Mhoon, former station maintenance chief:
Nick, that 1961 photo of the Transmitter Deck, console and amps, is just how it
looked in 1981. There is a new digital console now, but I've not seen any
photos. Also, the area behind the amplifier cabinets was filled with the cooling
system. There were giant fans, probably 12 to 15 feet in diameter and they drew
air from outside the building through a filtering system. No problem with
cooling in winter.
The system was powered via some very large HV AC breakers that were operated
with a DC control voltage from banks of what looked like small motorcycle
batteries. The battery sys was made by a French company as I remember. In 1979,
after 18 years of operation, they petered out and we were in a pickle. There
were no spares and the company was out of business. The fix was to hit town
(fishing villages) and buy up every battery available and most of the battery
cables. They were placed on freight carts (flat wagons) and wired in
series/parallel to match the original system. It took months to get motorcycle
batteries and custom design the cabinet. Because of gassing, we had to have a
hood and vent installed over the top of the banks.
"Admin and housing area of the main base. The HF site was across the road and VLF was a couple of miles off to the left in this photo." Note: VLF transmitter is AN/FRT-31. HF site transmitters were AN/FRT-39's and AN/FRT-40's. "We took delivery of diesel fuel via our own pier and had a 22K barrel storage facility. The oil was for base heating and running the power plant which had five giant diesel engines." |
"The combiner room where the transmission line connected to the transmitter and ran to the helix house. The box-like units are capacitors. The very left is the top of a chain link fence and note the room walls are covered in copper plate. When we took tour groups through we’d have people hold fluorescent bulbs that were on the fence (strictly for the show) and turn off the lights. Then they would be told to raise the bulbs overhead and they would light up the whole place from stray RF over the fence top."
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"Another photo of the combiner room. Note the bank of white capacitors. They actually bolt together via flanges and each one weighed about ten pounds." |
"The tap for tuning the frequency of the system. That is a giant tuning coil inside of a four story building called a helix house. There is a massive insulator on top of the helix house that looks like a spark plug. It was sealed and filled with an inert gas. Generally these failed in the winter and were hell to change. The helix was the center of each array." |
The antenna is tuned and matched by a set of high-Q, air-wound inductors and variometers located in the helix house. The primary tuning is done by a huge air-core inductor known as the helix. The windings of the helix are made with three pieces of 4-inch diameter Litz wire in parallel. The top of the helix is connected to each of the three feed-through bushings. The connection to the bushing on the main part of the helix house is made directly using two pieces of the 4-inch litz wire in parallel.
The connection to the bushings on the end of the two galleries is made using a large (8-inch diameter) copper bus. The helix has several taps that can be changed manually to provide coarse changes to the antenna tuning circuit. It is usually only necessary to change taps when changing frequency. Variable tuning to compensate for environmental changes is performed by a large air-core variometer, which is also wound with three pieces of 4-inch Litz wire in parallel. This tuning variometer is in series with the helix inductor in the antenna circuit.
In each helix house there is a large ferrite core inductor known as a saturable core reactor. The inductance of this reactor can be rapidly varied electronically over a finite range of values. The reactor can be connected in parallel with a portion of the helix and/or the tuning variometer. It is used to tune the antenna in synchronism with the two frequencies of the minimum shift keying (MSK) modulation. The MSK waveform consists of two frequencies selected to transmit marks and spaces. The reactor driver receives an antenna tune signal from the modulator, which enables it to tune the antenna synchronously with the mark and space frequencies. The saturable core reactor provides a method of increasing the effective bandwidth of the antenna (bandwidth enhancement). When the reactor is operating, it resonates the antenna circuit at both the mark and space frequency, and the impedance reflected to the transmitter is nearly pure resistance.
This reduces the stresses on the transmitter, transmission line, and matching components. These stresses are greater for larger values of Q, which occur at lower frequencies for Cutler. In fact, the reactor is necessary to radiate full power at the lower frequencies at Cutler. For six-panel operation on 24.0 kHz, the reactor is not necessary to radiate full power. However, for four-panel operation, the antenna bandwidth is reduced and the reactor is needed. The voltage on the reactor depends upon its helix tap connections. In four-panel operation, the helix taps must be changed to keep from exceeding the reactor voltage limits.
A coupling variometer converts the series resonant antenna to a parallel resonant impedance and to change the impedance at mark and space frequencies to match the transmission line impedance of 100 ohms. This coupling variometer, known as the triple deck, consists of three single variometers in parallel, each wound with 4-inch diameter Litz wire. The coupling variometer is connected from the tuning variometer to ground. The 100-ohm transmission line from the transmitter building is also connected to the top of the coupling variometer.
The transmitter is located in a building approximately halfway between the center of each array (figure 1). The transmission line and other cables for power, monitoring, and control are routed to each helix house through a tunnel large enough to walk through.
 Figure 1 - Cutler antenna arrays |
 Figure 2 - Cutler topload panel |
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Photo from Bob Mhoon - "Counterweight. If deicing didn’t work, the arrays had the counterweight towers and the weight of the ice would cause the array to lower to the ground while pulling up the rollers. Those latched and you had to deice manually with baseball bats. Normally you would switch out the RF connection and connect to the AC power plant to warm the array and melt the ice." |
Antenna insulator update 2009 - click for magazine article |
 post card
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The Cutler VLF transmitter. located in Washington County, ME, became operational on 4 January 1961. The Cutler antenna consists of two arrays. each having six diamond-shaped topload panels made up of cables hoisted by halyards that are attached to 13 towers. Each panel has eight active cables. called conductors. that carry the radio frequency (RF) current. One support catenary cable crosses the eight conductors in the center of the diamond. The RF conductors in these topload panels are specially designed with low enough resistance to have acceptable losses for VLF radiation. but enough resistance to enable deicing with 60-Hz current during winter months. Most of the cables consist of a 1-inch-diameter strand of a special alloy called Calsun bronze. However in order to provide corona-free operation at the high-radiated power levels, some sections of the conductors are 1.5 inches in diameter. The 1.5-inch diameter conductors were specially made with hollow center conductors covered by Everdure alloy exterior wires in order to meet the size. Strength. resistance. and weight requirements for use in the antenna. These cables. known as hollow core cables. make up part of the outer two cables on each panel. The hollow core sections on the cables inside of the catenary are 225 ft long, while those on the outside of the catenary are 775 ft long.
ANTENNA DESCRIPTION
The US Navy VLF transmitting station at Cutler, ME is the "flagship" of the Navy's fixed very low frequency (FVLF) transmitting sites and has been operational since 4 January 1961. The station is located in Washington County, ME on a peninsula near the small town of Cutler.
This site normally operates with a radiated power level of 1-million watts, termed "full power", and at times as high as 1.8-million watts radiated, termed "maximum power. In order to radiate power levels of this magnitude in the VLF band. an enormous antenna system is required. The Cutler VLF antenna consists of two separate arrays (north and south), each consisting of 13 towers. Every array has a center or zero tower called N0 (for the north array) and S0 (for the south array). which are 997.5 ft tall. Each array has six middle towers 575.0 ft tall. which are located with equal spacing on a circle of radius 1825 ft centered on the zero tower. Each array also has six outer towers 799.0 ft tall. also equally spaced on a circle of radius 3070 ft centered on the zero tower. A plan view of this antenna is given in figure 1. Every array is over 1 mile across and together they cover almost the entire peninsula. This antenna system is one of the largest in the world.
Each array consists of six diamond-shaped panels made up of cables supported from the towers by insulated halyards leading to permanent winches located at the bottom of each tower. A top view of one panel is given in figure 2. Each panel has eight active cables called conductors that carry the RF current. One support catenary cable crosses the eight conductors in the center of the diamond. The RF cables in these topload panels are specially designed to have low enough resistance to have acceptable loss for VLF radiation, but enough resistance to enable deicing by running 60-Hz current through them when needed during the winter.
Most of the conductor cables consist of 1-inch-diameter wire made from a special alloy called Calsun bronze. However, in order to provide corona-free operation at the high-power levels, some sections of the cables are 1.5 inches in diameter. These cables, specially made with hollow center conductors covered by exterior wires, were made of Everdure alloy in order to meet the size, strength, resistance, and weight requirements. The cables, known as hollow core cables, make up part of the outer two cables on each panel. The hollow core sections on the cables inside of the catenary are 225 ft long, while those on the outside of the catenary are 775 ft long.
The halyards are insulated from the panels by a string of 16 Lapp compression cone fail-safe insulators with large grading rings on each end (figure 2). Each individual fail-safe insulator weighs 750 lbs and the complete insulator string, plus hardware, weighs more than 6 tons. One insulator string is on each panel corner and the total weight of insulators on each panel exceeds 24 tons.
DEICING
The weather conditions along the coast of Maine are such that severe icing occurs during the winter months. The original requirement for the VLF Cutler transmitter called for continuous operation in all weather conditions. In order to survive severe icing, the antenna halyards are led through a counterweight system so that as the ice buildup increases the panel weight the counterweights let the halyards out, lowering the panel. The counterweight system is designed to allow the panels to lower all the way to the ground, if necessary. During installation, this actually happened. As the ice melts the counterweights hoist the panels back to their original position: thus, the arrays will survive no matter how large the amount of ice buildup.
However, as the ice builds up and the panel lowers, the antenna capacitance increases and the antenna must be retuned. The tuning range is limited and the limit eventually reached whereby the antenna can no longer be tuned and transmission ceases. The solution to this problem is to de-ice the antenna system by heating the wires with 60-Hz current. Constructing a deicing system that would allow simultaneous transmission and deicing would have been prohibitively expensive. Instead, two arrays have been built that allow transmission on one array while the other is deicing. This approach allows ice to build up on the transmitting array while the other array is deicing. When the one array is sufficiently deiced, the roles are reversed. This continues as long as necessary. Obviously, for this approach to allow continuous transmission, the deicing system must completely remove ice from one array in, at most, the amount of time it takes to reach the tuning limit on the other array The design value for heating chosen to accomplish this was 1.64 Watts per square inch of surface area, which corresponds to approximately 500 kW per panel or 3 MW for the entire array. The Cutler deicing system has the capability of operating at up to four times this much heating. Note that deicing power significantly exceeds transmit power.
The topload panels are fed by a four-wire cage made up of 1-inch copper cables. For transmitting, eight topload panel cables are all fed in parallel, one pair fed by each of the cage wires. For deicing. the topload cable pairs are fed in series with 60-Hz current. To provide the correct amount of heating with reasonable 60-Hz current, the topload cables need to have an appropriate resistance. For a given current, the heating in watts per square inch should be essentially the same for all cables. The deicing system is configured such that each 1-inch-diameter copper cable in the feed cage carries the full deicing current. This current is divided between two of the 1-inch-diameter topload conductors. Since heating is proportional to current squared, these topload cables must have about four times the resistance of the feed cage cables to provide the same heating This was accomplished by making the 1-inch topload cables out of Calsun bronze, which has a conductivity equal to 19% of copper.
The heating in the 1.5-inch-diameter portion of the cables must be 50% greater than in the 1-inch diameter cable because the surface area is proportional to the diameter. Consequentially, the larger diameter sections must have more resistance. which is contrary to the normal variation of resistance with diameter. This was accomplished by making a composite cable known as hollow core by using hollow copper tubes in the inner portion and wires of a copper alloy called Everdure which has a conductivity equal to 7.75% of copper for the outer portion. Mechanical connections of the topload conductor cables are made using swage-type end fittings combined with clevis shackles. Electrical connection is insured by crossing the mechanical connections with a 1-inch-diameter copper jumper cable clamped to the cables on both sides.
