Spar transmitter

A spar buoy for very low frequency (VLF) or low frequency (LF) transmission including a first portion of the spar buoy extending above a mean water line including a conductive structure including a coaxial feed, and an antenna coupled to the coaxial feed and extending above the conductive structure, and a second portion of the spar buoy below the mean water line including a transmitter coupled to the coaxial feed, an energy storage subsystem coupled to the transmitter and an electric power generation subsystem coupled to the energy storage subsystem.

TECHNICAL FIELD

This disclosure relates to very low frequency (VLF) and low frequency (LF) antennas and transmitters.

BACKGROUND

Prior art VLF transmitters used for command and control of submerged platforms are large monolithic structures, requiring massive size and operational costs to achieve their mission. These systems also typically rely on propagation off the ionosphere and consequently broadcast signals over extremely large areas, making transmit signals relatively easy to intercept.

A variety of prior art VLF transmitter architectures have been proposed and investigated. The most common type of architecture is a large ground based station, such as the Cutler station in Maine. Typically these VLF transmitters are constructed of one or a few very large top-loaded monopole structures designed to couple energy into the earth-ionosphere waveguide (EIW) and provide VLF coverage over large sections of the earth. At VLF frequencies and lower, electromagnetic waves can travel to reception depths of 10 m to 30 m unmanned underwater vehicles and submarine communications. Direct underwater VLF transmission has much greater attenuation than in-air transmission and does not have the benefit of coupling energy into the EIW to enhance the communications coverage. Therefore, transmitters with in-air antennas have much improved areal communications coverage compared to direct VLF propagation underwater. Thus, as further described below, the present disclosure describes a spar buoy with an in-air antenna for increased communications coverage.

Another prior art transmitter architecture described in U.S. Pat. No. 4,335,469, issued Jun. 15, 1982, which is incorporated herein by reference, utilizes a long wire antenna trailing behind an airplane to achieve VLF transmission from a single mobile platform. Yet another VLF transmitter architecture employs aerostats and consists of a ground based VLF source feeding a long conductor supported by a lighter than air object such as an aerostat or balloon. Yet another VLF architecture is the NASA tethered satellite system (TSS) which was intended to string a long conductor between two satellites to enable VLF/ELF transmission from orbit.

U.S. Pat. No. 9,233,733, issued Jan. 12, 2016, which is incorporated herein by reference, describes a mast stabilizing device to counteract large surface vehicle motions or rotations associated with high sea state waves and winds. It has a mass at the bottom of the mast in the water, and a spring attached to a buoy to act as a spring-mass-damper system to limit mast motions and to help the mast maintain a substantially vertical orientation desired in vertical antenna applications.

Also in the prior art are wave based power generators, such as the MARMOK-A-5, which is a device generating relatively low power (30 kW), while having dimensions of five meters in diameter and forty-two meters in height. The device weighs approximately 80 tons.

What is needed are improved VLF and LF antennas and transmitters that enable transmission of signals under the surface of water and over a contained area without needing transmitters of massive size and power. The embodiments of the present disclosure answer these and other needs.

SUMMARY

In a first embodiment disclosed herein, a spar buoy for very low frequency (VLF) or low frequency (LF) transmission comprises a first portion of the spar buoy extending above a mean water line comprising a conductive structure comprising a coaxial feed, and an antenna coupled to the coaxial feed and extending above the conductive structure, and a second portion of the spar buoy below the mean water line comprising a transmitter coupled to the coaxial feed, an energy storage subsystem coupled to the transmitter, and an electric power generation subsystem coupled to the energy storage subsystem.

In another embodiment disclosed herein a system for very low frequency (VLF) or low frequency (LF) transmission comprises a spar buoy, an air vehicle, a first portion of the spar buoy extending above a mean water line comprising a conductive structure comprising a coaxial feed, and a conductive cable coupled to the coaxial feed and coupled to the air vehicle, and a second portion of the spar buoy below the mean water line comprising a transmitter coupled to the coaxial feed, an energy storage subsystem coupled to the transmitter, and an electric power generation subsystem coupled to the energy storage subsystem.

These and other features and advantages will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features, like numerals referring to like features throughout both the drawings and the description.

DETAILED DESCRIPTION

The present disclosure describes a spar buoy with a coax fed monopole antenna which may have a various lengths needed for communications in the very low frequency (VLF) band (3 kHz to 30 kHz) or low frequency (LF) band (30 kHz to 300 kHz). For a mast antenna desired for transmission in the LF band, the antenna may have a length of up to 120 meters (m). In one embodiment the antenna may have a length of 20 m. The antenna may also be a lofted antenna, which is supported aloft by an air vehicle for transmission in the VLF band. If the air vehicle is an aerostat, the antenna may have a length up to 10,000 meters. If the air vehicle is an airplane, drone, parafoil, or a helicopter/quadcopter, because of the more limited weight that can be supported by such an air vehicle, the antenna may have a length up to 1,000 meters. A length of 30 km corresponds to the wavelength of the lowest very low frequency (VLF).

The antenna feed is configured to be mounted at a mean water line inside a tubular type platform or spar buoy configured for open ocean operation. The mean water line is determined by the buoyancy of the buoy. There can be a change in the mean water line of the buoy with a decrease in fuel; however, as fuel is consumed, ballast water may be added to the buoy to counteract any change in the buoyancy, which is a common technique, for example, used by submarines to maintain depth. Mounting the monopole antenna at a deeper location would aid in maintaining stability but would compromise the antenna's electrical performance.

The buoy is configured to allow the coaxial feed to extend out the top into the atmosphere. The monopole antenna may be a mast, tower, or a conductive cable supported by an aerial vehicle.

The spar-buoy has a substantially deep underwater section and a section extending above the water surface which is designed to prevent water ingress into the buoy in adverse wave and weather conditions. To help in-water stability of the buoy, and to reduce onboard fuel and power requirements, a water column oscillator power generator may be mounted at the bottom of the buoy. A non-conductive sealing cap may be located over the output of the coaxial feed to the antenna to allow the inner volume of the coaxial feed to be evacuated or filled with a gas to prevent electrical breakdown in the feed and improve power handling. A horn may be used for the antenna feed to smooth the impedance transition into the air and help suppress voltage breakdown. The depth of the antenna feed decreases somewhat, but not significantly, the overall exposed length or height of the antenna above the water-line.

To assist in the ballasting of the spar, a tethered mass or an equipment torus/donut may be placed low on the spar buoy to stabilize the buoy in adverse open ocean conditions and stabilize the antenna in a preferred vertical orientation. As discussed above, a traditional ballasting method can be used to maintain a specific water-line.

The spar buoy deep-draft floating chamber, or hollow cylindrical hull, may be encircled with spiraling strakes to add vertical stability in open ocean conditions.

Unlike, traditional VLF transmitters, which use a small number of very large transmitters, the present invention describes a transmitter architecture which may be employed in an alternative architecture with a large number of distributed, coordinated, and electrically small or relatively low power transmitters on independent autonomous platforms. Unlike the prior art, this approach enables mobility and improved control of VLF coverage as well as reduced transmitter power.

FIG.1shows a spar buoy10with an antenna12, which in the embodiment ofFIG.1is a conductive mast. The spar buoy may be used for communications to and from a submarine200, as shown inFIG.1.

To achieve vertical stability for the tall antenna12, the bulk of the mass of the spar buoy10is preferably concentrated in the lower half of the spar buoy10, which is mostly below the water line24. In the configuration of the spar buoy shown inFIG.1, this lower half of the spar buoy10may include the following subsystems: an energy storage subsystem14, which may include a battery and fuel, which may be used to power electronics and for propulsion; a transmitter and electronics subsystem16, which may include a transmitter, position and motion sensors, and signal control and communication electronics; and an electric power generation subsystem18, which may generate power from ocean waves or generate power in other ways, such as an electricity generator that uses fuel stored in the energy storage subsystem14. The position of these subsystems as shown inFIG.1is only an example. The subsystems are preferably arranged so as to lower the center of gravity of the spar buoy10for stability.

As shown inFIG.1, the feed location20(as detailed hereafter, the “feed location”20can also be called a “VFL transmitter and RF transmitter feed”, a “feed location” and an “antenna feed”) for the antenna12is at the mean water line24of the ocean or water surface. A conductive outer structure22extends above the mean water line24and surrounds the part of the antenna12that is within the conductive outer structure22. The conductive outer structure22is conductive to provide an electrical grounding connection to the surrounding water and to prevent water ingress to the high voltage feed35, shown inFIG.2, which could cause shorting and flash-over damage to the transmitter and other electronics in the buoy.

A buoyant cable antenna26, such as that described in U.S. Pat. No. 8,593,355, Nov. 26, 2013, which is incorporated herein by reference, may be connected to the transmitter and electronics subsystem16on the spar buoy10to enable a remote operator or system to send, via a long haul communication link28or similar communication links, commands and messages to the spar buoy10. The messages may then be transmitted via the antenna12. The buoyant cable antenna may also be used to allow the spar buoy10to function in a communication network as a relay or router of commands and messages to other spar buoys10or to other devices.

FIG.2shows elements of the subsystems in the spar buoy10. The control and communications electronics29may be coupled to the buoyant cable antenna26for receiving commands and messages via the long haul communications link28or via some other local communications link. The commands and messages may be processed by the control and communications electronics to derive a signal30for transmission. The signal30may be modulated by modulator32for VLF or LF transmission and driven by radio frequency (RF) driver33via matching network34and coaxial feed35into the antenna12(as detailed hereafter the “antenna”12can be called a “rigid mast”, or “conductive mast”, or “mast antenna”, or “mast”). A high voltage is generated in the coaxial feed35. The transmitter may include the modulator32, the RF driver33and the matching network34. The energy source36and power plant38operate to provide electric power to the transmitter. The energy source36and power plant38may include a battery, stored fuel, an electric power generator, or a device for generating power from ocean waves. The power provided or generated provides power for the transmitter and electronic subsystem16in the spar buoy10. The power may also be used for propulsion of the spar buoy10, which may be used for station keeping or for repositioning of the spar buoy10. As shown inFIG.2, the spar buoy10may include motion and position sensors31, which may include global positioning satellite (GPS) receivers and gyroscopes, to assist in station keeping and/or repositioning of the buoy. As also shown inFIG.2, control electronics can be coupled to the transmitter and to the energy storage subsystem; and communications electronics can be coupled to the control electronics for receiving commands and messages from a communications link or for communications with another spar buoy.

As shown inFIG.1, the antenna12may be a monopole antenna and may be a conductive rigid tower or rigid mast12. A similar antenna12is also shown for the spar buoy configurations shown inFIGS.4and6. The antenna12is preferably supported by the spar buoy10above the water line24when the water surface is calm. The antenna12is conductive and may be metallic to provide structural integrity. The output37of the coaxial feed35to the antenna12is preferably located above the water line24. However, the output37can be located below the water line24at the expense of reduced antenna efficiency due to conductive losses in the radiating structure.

FIG.3Ashows an alternative to the mast antenna12shown inFIG.1. InFIG.3A, the antenna is a lofted antenna, which is a conductive cable40tethered to and supported by an air vehicle42. The air vehicle42may be a manned or autonomous airplane, a drone, an aerostat, a parafoil or a helicopter/quadcopter. The air vehicle42may be powered and controlled via a power cable44and a control cable46, respectively, which run from the spar buoy10to the air vehicle42. The power cable44and control cable46may be inside the conductive cable40, as shown inFIG.3B, or alongside the conductive cable40. The air vehicle42trajectory is preferably controlled so that the antenna cable40is kept as straight and as vertical as possible given different environmental and wave conditions.

The conductive cable40and the power44and control46cables may be 1000 meters to 10,000 meters long. If the air vehicle is an aerostat then the conductive cable may be 10 m000 meters long, because an aerostat can support the weight of a long conductive cable40, and the power44and control46cables. However, if the air vehicle42is an airplane, drone, parafoil or helicopter/quadcopter, and the spar buoy10supplies the power to the air vehicle, then because the power required is large and the added current that must be sent on the power cable44is high, the weight of the conductive cable40and the power44and control46cables limits the length of the cables. For a configuration with an air vehicle42that is an airplane, drone, parafoil, or a helicopter/quadcopter, the maximum length for the conductive cable40, the power cable44and the control cable46is estimated to be 1000 meters. Cable handling equipment, such as winches, may efficiently stow the cables in a compact volume inside the spar buoy.

An isolation transformer may be included at the base48of the conductive cable40for direct current isolation of the VLF transmitter and RF transmitter feed20from the aerial vehicle42. Also, another isolation transformer may be used to isolate direct current on the power cable44and the control cable46from the aerial vehicle42.

FIG.4shows another embodiment of a spar buoy50with a number of optional elements that may enhance the functionality or robustness of the VLF or LF transmitter.

A high voltage is generated in the coaxial feed35and therefore may create an impedance discontinuity at the feed location20, which may be a challenge for the matching network34. In the embodiment ofFIG.4a horn52with flared edges is used instead of the conductive outer structure22shown inFIG.1. The horn52flares from the spar at the feed location20into the atmosphere to smooth the impedance transition into the air and help suppress voltage breakdown. At the feed location20, the diameter of the horn52may be the same as the diameter of the spar buoy at that location, then the diameter of the horn52flares to a larger diameter.

To further improve voltage breakdown suppression, a a non-conductive sealing cap54may sealed over the top of the horn52in order to contain an inert gas in the volume55between the antenna feed20and the top of the horn52. The gas may be SF6, dry N2, or a vacuum, which each have a higher dielectric/voltage breakdown strength compared to air. Instead of a gas, a dielectric liquid, such as silicone oil or mineral oil, may be used in the volume55between the antenna feed20and the top of the horn52. The non-conductive sealing cap54includes a mechanism, such as a valve53, to allow gas or liquid to be installed or evacuated from the volume55. The non-conductive sealing cap54also helps prevent water ingress into the spar buoy.

To assist with the proper vertical mass distribution in the spar buoy, equipment or fuel, including the transmitter and the energy storage subsystem, may be placed in a torus56around the perimeter57of the spar buoy. The torus56may be a sealed compartment removably secured to the spar buoy with a power cable feed into the spar buoy, or the torus may be permanently attached or welded to the spar buoy.

One or more propulsion devices or propulsors58may be attached to a side wall or the spar buoy or a truss appendage of the spar buoy. The propulsion devices58may be configured to perform station keeping in order to resist motion and rotation under wave and wind action. Further the propulsion devices58can be oriented in the direction of water current with control surfaces and propulsors oriented to minimize spar buoy yaw and pitch motions. The propulsion device58may also be used to geographically reposition the spar buoy.

As shown inFIG.4, the spar buoy may also include a ballast mass68capable of offsetting the high mass moment of inertia of the tall mast antenna12. The ballast mass68may be deployable on a truss or a cable69that can unspool from the submerged spar buoy base59.

A wave energy generator70, which may be a column oscillator power generator including a planform71, may be mounted at the base59of the spar buoy or below the ballast mass68to provide power. Alternatively, power may be provided by a free floating wave energy generator72adjacent to the spar buoy with an electrical power conduit74connecting to the spar buoy, as shown inFIG.4.

FIG.5shows another embodiment showing a conductive mast12with guys13attached between points on the antenna12and the edge of the horn52. The guys13are insulated from the antenna12and from the horn52, and provide additional support for the antenna12especially in windy conditions and ice loadings on the mast12. Guys13can also be used in the configuration shown inFIG.7to support the added weight of the capacitively top-loaded antenna100.

FIG.6shows a preferred embodiment of the spar buoy80in which all the essential and any optional elements are sized to fit within a set outer diameter33of the spar buoy80for ease of packing, transport and deployment. The spar buoy80may be generally tubular in shape, as shown inFIG.6. As shown inFIG.6, in this embodiment of the spar buoy80, the nonconductive sealing cap85fits over the conductive outer structure22, and both may have a disk type shape with a diameter that is less than or equal to the set outer diameter83of the buoy. The spar buoy80may include a propulsion device82, a wave energy converter84, and ballast86, as shown inFIG.6, which are all configured to fit within the set outer diameter83of the spar buoy80.

FIG.7shows another embodiment of the spar buoy80in which all the essential and any optional elements are sized to fit within a set outer diameter83of the buoy spar80, which may be generally tubular in shape, for ease of packing, transport and deployment. As shown inFIG.7, this embodiment of the spar buoy80has a capacitively top-loaded antenna100, which is one example of a capacitively top-loaded antenna. A capacitively top-loaded antenna may have many configurations. The capacitively top-loaded antenna100may be stowed within the buoy80and then deployed by extending antenna support arms102. The capacitively top-loaded antenna100significantly decreases the required antenna height compared to configurations without a capacitively top-loaded antenna100.