Description:
OBJECTIVE: Develop low-profile Very-High Frequency/Ultra-High Frequency (VHF/UHF) antenna apertures using choice materials such as anisotropic/inhomogeneous magneto-dielectrics. DESCRIPTION: The advent of metamaterials has extended the design space for antenna apertures [4, p.4, (2.20)] creating the possibility for very thin (relative to wavelength) antenna structures. While basic physics limits directivity [4, p.10, (2.107)] to the available aperture size, such restrictions do not preclude antennas of reasonable directivity having thicknesses that are fractions of wavelengths. Properly designed antennas may exhibit bandwidths of an octave or more from the standpoint of the realized gain [4, p.29, (2.321)] keeping the realized gain greater than 0 dBi over one or more octaves. Specifically, the intent of this solicitation is to model and create low-profile antennas. These antennas should have a thickness that does not exceed 1/30th of the wavelength in free space for the lowest operating frequency of the antenna and the antenna should maintain a realized gain greater than 0 dBi over one or more octaves. The antenna should be realized using use magneto-dielectric layer(s) having anisotropic /inhomogeneous constitutive parameters. It is presumed, but not required, that the anisotropic /inhomogeneous (effective) constitutive parameters are achieved through the use of metamaterials. The antenna should be operational in the VHF/UHF range. Any solution that achieves these antenna goals is desired. The use of high relative permeability and permittivity materials sandwiched between the radiating element (e.g., a dipole, bow tie, Archimedean spiral, etc.) and an electric ground plane allows the radiating element to be in close proximity to a ground plane while maintaining reasonable input impedance for matching purposes [1]. A fundamental problem with such geometry is the excitation of lateral (surface waves) [3, p. 736] that may guide a significant percentage of the power to the edges of the antenna before scattering into space. Presuming the antenna structure is contained in some type of enclosure (often metallic), these lateral waves may contribute to unwanted internal resonances having a deleterious effect on both the impedance match and the antenna"s radiation pattern. Accordingly, the antenna may not perform as needed from the standpoint of efficiency or the stability of the radiation pattern broadside to the antenna. A possible solution to the aforementioned problem would employ a magneto-dielectric layer having anisotropic /inhomogeneous constitutive parameters. Properly tailored, such a layer could enhance a frequency independent antenna design while mitigating (or exploiting) the lateral wave excitations. A magneto-dielectric layer created of anisotropic metamaterials (having graded unit cell parameters) is one possible realization of such geometry. In general, the layer may have constitutive parameters that are anisotropic as well as inhomogeneous for both the permittivity and permeability. As frequency independent antennas inherently use the entire element at the lowest frequency while using less of the element as the frequency increases, the gain and input impedance remain reasonably constant [2, p. 611]. The anisotropic/inhomogeneous prescription for the metamaterial magneto-dielectric should be developed in such a way as to keep the efficiency of the antenna as high as possible while maintain a good impedance match over an octave (or more) frequency band. Clearly, the"antenna"must be considered to be the entire structure consisting of the ground plane, magneto-dielectric, and the excitation element. The realization of the anisotropic /inhomogeneous magneto-dielectric layer is further complicated by the inherent problems associated with the ferromagnetic resonance of the magnetic material and the associated magnetic loss tangent. Practical realizations of an antenna are likely to be limited to frequencies under 1 GHz. PHASE I: Demonstrate through computer simulations and/or analysis the viability of using a choice material, such as anisotropic/inhomogeneous magneto-dielectrics, to enhance absolute antenna gain. Such an analysis, over an octave bandwidth, should demonstrate stability of the input impedance (<-10 dB return loss) as well as stability of the radiation pattern across the frequency band. The realized gain (dBi or dBic) should remain positive and not exhibit significant decreases in the broadside direction within the frequency band of at least one octave anywhere in the VHF/UHF range. The demonstration should be for a low-profile antenna having a depth of a small fraction of wavelength at the lowest frequency of operation. Demonstrate the material properties simulating or measuring a small sample of inhomogeneous material to the government with accompanying measured data. Additionally, the demonstration should indicate that the antenna will be suitable for transmit (up to 50 W) as well as receive. PHASE II: Fabricate prototype antennas from the choice material, such as inhomogeneous magneto-dielectric material , demonstrated in Phase I. Measure their RF performance (reflection coefficient, radiation pattern, gain etc.) and compare measured results with commercially available antennas in the same frequency band to establish a reasonable benchmark of performance. Refine antenna fabrication to minimize the volume of magneto-dielectric material to mitigate cost while maintaining antenna performance. At least one working antenna prototype with measured data should be delivered to the government. The antenna must be suitable for both transmitting and receiving, so the ability to contend with power levels up to 50 W on transmit and low-noise temperature on receive should be demonstrated. Designs may be for linear polarization, but must not preclude the more desirable circular polarization. PHASE III: Phase III will focus on the transition of selected low-profile antenna design(s) onto military platforms such as MRAP, UAV wing, and/or onto commercial platforms such as vehicular and airborne. Address fabrication cost and volume challenges that are relevant to this platform transition. Specific RF applications that may be targeted for these antennas include terrestrial communications, tactical satellite communications, radar, GPS, and RF sensors.
