YIP: VERY-SHORT ANTENNAS VIA IONIZAED PLASMAS FOR EFFICIENT RADIATION (VAIPER)
Abstract
1 Executive Summary Radio waves below 30 MHz have rather long wavelengths (15 meters at 20 MHz, and 15 km at 20 kHz), so building antennas comparable to a wavelength is difficult. As such, antennas that can generate these waves are limited in scope. Typically, there is a choice between high bandwidth, small size, and good efficiency, and at most two of the three can be achieved. On the other hand, long wavelength waves below 30 MHz (ELF/VLF/LF/MF/HF) are useful in a number of military systems, including over the horizon global communications and submerged submarine communications, over the horizon (OTH) radar, GPS-independent navigation, and subterranean mapping. As such, the limited ability to generate these waves poses a barrier to many military systems. The fundamental reason electrically short antennas are limited is that an injected voltage propagates down the antenna, reflects at the end, and returns to the feed in a time much shorter than a period. In that short echo time, the current being injected at the feed has barely changed, and so the reflected current almost entirely and immediately squelches the injected current. Consequently, electric charge is not delivered down the antenna effectively, resulting in an extremely small radiation resistance. Traditional ways of circumventing this involve antenna matching schemes, like top-hat matching, which allow the reflection to be suppressed, but these frequency domain techniques yield an antenna that is efficient in only a narrow range of frequencies. Traveling wave antennas are able to achieve both efficiency and bandwidth, but they are at least on the length scale of a wavelength. Our proposed solution overcomes this problem by allowing the current wave to propagate down the antenna, but blocking the reflected wave. This is achieved by creating an antenna not with metal but with a high-voltage ionized plasma as the conducting path. The plasma antenna consists of a series of segments, each one individually controllable. Strategically modulating the plasma conductivity in each segment controls the flow of current, and therefore charge, to the ends of the antenna. Because our approach to suppressing the reflected wave is in the time domain, in principle it works at any frequency. In practice it is limited primarily by the finite response time of the plasma. An analogy for our proposed solution to longwave generation is car traffic along a long street with many traffic lights. The traffic lights can be easily timed so that cars moving in one direction will encounter only green lights, but cars moving in the other direction will encounter many red lights. Our goal in this program is to demonstrate the feasibility of the technology in the Very Low Frequency (VLF, 3?30) or Low Frequency (30?300 kHz) range, including detection of generated signals and diagnostics of the plasma chamber. Currently, generating these waves requires large antennas (10s to hundreds of meters) that operate in a narrow frequency range. Our team consists of three senior investigators, with complementary expertise to cover the many technical challenges to simulate and then implement a proof of concept. Recognizing a long-term commitment to the PI and to this research program, Georgia Tech will contribute cost-sharing in the form of a graduate student for all three years of the program and $20k in equipment funds. We denote the new plasma antenna with the acronym VAIPER: Very-short Antennas via Ionized Plasmas for Efficient Radiation. This technique has not been tried, and many fundamental questions exist about how to implement it, so our proposal focuses on laying out the key technical challenges, rather than defining a singular approach to building it. ONR YIP 2015 1 Volume I: Technical
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 22, 2016
- Source ID
- N000141512526
Entities
People
- Morris B Cohen
Organizations
- Georgia Tech Research Corporation
- Office of Naval Research
- United States Navy