Joseph S. Colburn

Scalar and Tensor Holographic Artificial Impedance Surfaces

We have developed a method for controlling electromagnetic surface wave propagation and radiation from complex metallic shapes. The object is covered with an artificial impedance surface that is implemented as an array of sub-wavelength metallic patches on a grounded dielectric substrate. We pattern the effective impedance over the surface by varying the size of the metallic patches. Using a holographic technique, we design the surface to scatter a known input wave into a desired output wave. Furthermore, by varying the shape of the patches we can create anisotropic surfaces with tensor impedance properties that provide control over polarization. As an example, we demonstrate a tensor impedance surface that produces circularly polarized radiation from a linearly polarized source.

A Variable Negative-Inductance Integrated Circuit at UHF Frequencies

A floating variable Negative Impedance Inverter (NIl) circuit is proposed that is derived from Linvills unbalanced negative impedance converter. A negative-inductance Integrated Circuit (IC) has been realized using the IBM 8 HP BiCMOS process with an inductance that tunes from -64 to -40 nH. The IC is short-circuit-stable (which has been verified by experiment) and is dc coupled at the terminals, making it directly applicable to susceptance cancellation for slot antennas and artificial magnetic conductors. This letter reports the circuit topology, measurement technique, and measured data.

Wideband Artificial Magnetic Conductors Loaded With Non-Foster Negative Inductors

We examine how the bandwidth of artificial magnetic conductors (AMCs) can be greatly increased when loaded with negative-inductance non-Foster circuits. This increase in bandwidth is achieved by enhancing the structural inductance of the AMC by combining it in parallel with a negative inductance, thus achieving a bandwidth that exceeds what is possible with passive approaches. A prototype VHF-UHF active AMC was fabricated and measured, which achieved a bandwidth greater than 80% at a resonant frequency of 263 MHz.

A Non-Foster VHF Monopole Antenna

Non-Foster matching promises to improve the performance of electrically small antennas by tens of decibels over a decade of bandwidth, but only a few successful demonstrations have been reported in the literature. A 15-cm monopole antenna has been matched using a variable negative capacitance based on Linvills balanced negative impedance converter. The realized gain is improved by >;10 dB relative to the unmatched case over 30-200 MHz. This letter reports the design, simulation, and measurement data and gives an equivalent circuit that can be used to synthesize future designs.

Adaptive artificial impedance surface conformal antennas

Moving platforms, like manned and unmanned aerial vehicles, require adaptable directive antennas for both communication and radar applications. The integration of these antennas into the body of the platform drives significant development time and expense, and in many cases adversely affects the overall vehicle performance by adversely affecting its aerodynamics or weight. Being able to integrate the antenna function directly in the skin of the platform will result in simpler better performing vehicles. We have developed an AIB with which we have used to demonstrate 2D electronic beam steering. These results show how AIB technology can be utilized for the realization of next generation conformal, electrically steerable directive antennas.

Polarization controlling holographic artificial impedance surfaces

We have demonstrated the necessary components for designing and constructing polarization controlling holographic tensor artificial impedance surfaces. Applications using the tensor impedance holographic method and tensor impedance characterization are currently being developed.

Single-Feed Dual-Band Stacked Patch Antenna for Orthogonal Circularly Polarized GPS and SDARS Applications

We describe a single-feed stacked patch antenna design that is capable of simultaneously receiving both right hand circularly polarized (RHCP) satellite signals within the GPS LI frequency band and left hand circularly polarized (LHCP) satellite signals within the SDARS frequency band. In addition, the design provides improved SDARS vertical linear polarization (VLP) gain for terrestrial repeater signal reception at low elevation angles as compared to a current state of the art SDARS patch antenna.

Development of Reduced Order Model for Modeling Performance of Tire Pressure Monitoring System

An electromagnetic (EM) math model of TPMS calculates the data links RF pathless profile of each four wheels that was later translated into an estimate of packet errors registered by the TPM system using a higher level system simulation tool. A reduced order model (ROM) of a vehicle structure for modeling TPMS performance was developed after investigating four vehicle models having different degree of complexity and identifying effects of various vehicle structures on the pathless profile. Pathless profile of the ROM of a CTS vehicle compared very well with that of the measurement data if variances are minimal in the RF channels between the transmitter and the receiver. The higher-level system model showed the system performance of the ROM converged to that of the measurement data.

Modeling polarization mode coupling in frequency-selective surfaces

An analysis of scattering from frequency-selective surfaces (FSS) is developed that correctly accounts for coupling between scattered polarization modes. This is useful for the design of FSS structures with obliquely incident radiation, such as polarization converters (polarizers) for direct broadcast satellite reception by linearly polarized ground terminal antennas. In the past, the neglect of polarization mode coupling limited the performance range of various FSS structures, such as meander-line polarizers, to near normal incidence where mode coupling is weak. The following analysis shows how a suitable pair of polarization modes can be selected to maintain modal independence. The result is a pair of independent equivalent circuits representing scattering of the two modes, together with mode transformation networks. To illustrate the theory, a four-layer meander-line polarizer is designed for an incident angle of 45/spl deg/ off normal. The resulting axial ratio is measured to be 0.3 dB at 12.45 GHz.

A Coaxial TEM Cell for Direct Measurement of UHF Artificial Magnetic Conductors [AMTA Corner]

We have demonstrated a coaxial TEM (transverse electromagnetic wave) cell for direct measurement of artificial magnetic conductors (AMCs) in the VHF-UHF range. The advantages of the coaxial TEM cell are 1) it is a compact, bench-top device; and 2) it provides rapid, direct measurement of artificial-magnetic-conductor reflection response over a broad frequency range. We present measurements of various artificial-magnetic-conductor structures measured in the coaxial TEM cell, including 1) fixed-response, capacitor-loaded artificial magnetic conductors; 2) tunable varactor-loaded artificial magnetic conductors; and 3) wideband, tunable artificial magnetic conductors loaded with non-Foster, negative-inductance circuits. The measurements of the various artificial magnetic conductors were consistent with simulations. The coaxial TEM cell detailed here was 50 cm long with a 10.4 in diameter at the DUT, tapering to an N connector at the feed port. It was single-moded below 500 MHz. The design can be scaled to extend to other frequencies and other artificial-magnetic-conductor geometries.