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DYNAMIC TUNER FOR NARROW-BAND VLF SUBMARINE COMMUNICATION TRANSMITTING SYSTEM

Description:

OBJECTIVE: To provide a means for high power Very Low Frequency (VLF) transmitting systems to broadcast more efficiently and effectively in spite of the very narrow band antennas. They can do this by dynamically tuning the antenna(s) in sync with the modulated signal frequency shifts. DESCRIPTION: Communication to submarines while at speed and depth utilizes high power VLF RF signals because they are able to penetrate the ocean to depths that can be received by a submerged antenna. The shore-based transmitting systems that broadcast the RF signals require very large antennas to launch the long wavelength signals, and operate with RF input power levels of several hundred kilowatts to 2 mega-watts. But in spite of their size the antennas are electrically small and have very narrow bandwidths, often less than the modulation bandwidth. As a consequence, the lack of bandwidth reduces the efficiency of the transmitter and causes a reduction in receive detectability due to slow frequency transitions of the broadcast signal. Typically, the antennas are tuned with series inductors that are several hundred micro-henries and are linear for RF currents up to 3,000 amperes. There is a critical need for a dynamic tuner that can rapidly tune an antenna (in less than 2 milliseconds) in sync with the 200 baud MSK modulation, thereby simulating a very broadband VLF transmitting antenna, which will dramatically increase the transmit efficiency and the detectability of the radiated signal. Key Challenges to achieving the above goals are: 1) Developing an architecture that is compatible with existing VLF transmitter tuning circuits 2) Identifying new or advanced core materials that minimize core losses in an inductor while simultaneously operating at high flux density (if a non air-core inductor is used). 3) Avoiding non-linear behavior so that the transmitting system does not generate high order harmonics during the tuning transition between the two modulation frequencies. 4) Avoiding abrupt step tuning because this re-generates the undesirable modulation sidebands, causing adjacent channel interference. 5) Managing the high voltages and currents associated with volt-ampere requirements exceeding 4 MVA. 6) Offering accurate control of the inductance by minimizing the influence or compensating for the high RF current in the inductor from the transmitter. 7) The architecture and implementation must be robust, able to withstand lightning induced voltage transients in the antenna tuning circuit. 8) Developing a high level of confidence in the success of the approach when scaled up to the extremely high power levels that the existing VLF transmitters operate, especially due to the size and cost of a full scale deployment. The circuits are electrically simple but physically large. The dynamic tuner must meet all requirements for any operating frequency from 15 kHz through 30 kHz. PHASE I: Explore and define an architecture for incorporating dynamic antenna tuning into each of the five VLF transmitting systems operated by the U.S. Navy. The contractor shall perform detailed analysis and modeling of the characteristics of the dynamic tuner, including time domain and frequency domain analysis for predictions of harmonic generation and modulation sidebands. This analysis and modeling should substantiate any recommendations made and show that the chosen approach will meet the objectives and criteria set forth herein. The design concept must address the following risks: - Compatibility with existing antenna tuning systems - Core and winding heating - RF voltage breakdown - Lightning induced voltage transient breakdown - Overheating from RF current - Generation of harmonic energy exceeding the specification threshold - Sideband generation that exceeds the adjacent channel interference specification - Development of a reliable mechanism for accurate control of the dynamic tuner PHASE II: Finalize and optimize the design(s) chosen in phase I, and build and test a one tenth to full scale prototype of the dynamic tuner. The contractor shall include a test plan that exercises the prototype dynamic tuner to its full rated KVA. This test plan may include coordination and use of one or more U.S. Navy facilities in order to obtain the high KVA required. The testing shall demonstrate to the maximum extent possible compliance with all of the goals outlined above. PHASE III: Transition technology to the U.S. Navy FVLF In-Service Engineering Agent. Provide units for use in a U.S. Navy FVLF transmitter. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: More and more commercial applications are calling for electrically small antennas. There are various techniques for effectively receiving with electrically small antennas but transmitting efficiently is difficult. The technology being developed will apply to efficiently transmitting on electrically small antennas. The approach is independent of frequency REFERENCES: 1. VLF Radio Engineering; by Arthur D. Watt; Pergamon Press 1967 2. VLF/LF High-Voltage Design and Testing, SSC San Diego TECHNICAL REPORT 1904; by Hansen, Peder & A. D. Watt, September 2003 3. Johannessen, Paul R.,"Automatic Tuning of High-Q Antenna for VLF FSK Transmission", IEEE Transactions on Communications Systems Vol CS-12, No 1, March 1964
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