Larry W. Epp

A compact dual-polarized 8.51-GHz rectenna for high-voltage (50 V) actuator applications

This paper describes a dual-polarized rectenna capable of producing a 50-V output voltage that can be used for driving mechanical actuators. This study demonstrates a circuit topology that allows the output of multiple rectenna elements to be combined in order to step up the output voltage. In this paper, an independent rectifying circuit is used for each of two orthogonal polarizations. By proper combination, the output voltage is doubled over that of the single polarization case. Such panels are being explored for use on the next-generation space telescope to eliminate wiring between actuators and provide for true mechanical isolation.

A High-Power

A method of using low-loss waveguide septum combiners is developed into a high-power -band (31-36 GHz) amplifier producing 50 W at 33 GHz (Ka-band) using 32 low-power (>2 W) solid-state amplifier modules. By using low-loss waveguide combining and a packaged monolithic microwave integrated circuit with a low-loss microstrip-to-waveguide launcher, the output loss is minimized, allowing for the overall power-combining efficiency to remain high, 80% (average insertion loss of combiner < 0.7 dB and average insertion loss of launcher <0.3 dB) over 31-36 GHz. In the past, lower power-combining efficiencies have limited the number of modules that can be combined at -band, and hence, have limited the power output. The approach demonstrated in this paper, with high power-combining efficiency, allows a very large number (32) of solid-state amplifier modules to be combined to produce high powers. Greater than 50 W was demonstrated with low power modules, but even higher powers 120 W are possible. The current approach is based on corporate combining, using low-loss waveguide septum combiners that provide isolation, maintaining the true graceful degradation of a modular solid-state amplifier system.

Ka

A new symmetric formulation of the hybrid finite element method (HFEM) is described which combines elements of the electric field integral equation (EFIE) and the magnetic field integral equation (MFIE) for the exterior region along with the finite element solution for the interior region. The formulation is applied to scattering by inhomogeneous bodies of revolution. To avoid spurious modes in the interior region a combination of vector and nodal based finite elements are used. Integral equations in the exterior region are used to enforce the Sommerfeld radiation condition by matching both the tangential electric and magnetic fields between interior and exterior regions. Results from this symmetric formulation as well as formulations based solely on the EFIE or MFIE are compared to exact series solutions and integral equation solutions for a number of examples. The behaviors of the symmetric, EFIE, and MFIE solutions are examined at potential resonant frequencies of the interior and exterior regions, demonstrating the advantage of this symmetric formulation. >

-Band (31–36 GHz) Solid-State Amplifier Based on Low-Loss Corporate Waveguide Combining

Amplifying grid arrays, consisting of periodic unit cells loaded with active devices such as HEMTs, are currently being developed for high frequency quasi-optical use. Motivation for their development includes the inherent advantages of an approach that employs spatial power combining and spatial amplification. Thus, losses between multistage amplifiers are virtually eliminated. Also due to the spatial combining, the phase of the array is determined by the phase of the incident wave which is then amplified by the planar circuit. This eliminates the need for complex phase shifters and the associated lines for independent element control. Other advantages include the existence of graceful degradation when failures occur. Typically, a unit cell of these planar arrays has been analyzed using quasi-static transmission line approaches. This approach is used due to its simplicity and the easy addition of the port locations required by the active devices. The benefit of a grid amplifier design at high frequency is limited by this approach which may ignore strong mutual coupling or surface waves present at higher frequencies. Based on more conventional periodic array analysis, the method described extended the generalized scattering matrix approach to include the port locations of the device. This allows accurate inclusion of the effects of the mutual coupling between elements, the presence of bias lines, ground planes, and superstrates/substrates. In addition this numerically generated scattering matrix can be combined with the conventional scattering matrix of the device to form a composite matrix of a grid amplifier.

A hybrid symmetric FEM/MOM formulation applied to scattering by inhomogeneous bodies of revolution

We present results of the characterization of a nanoelectromechanical signal-processing device based on arrays of carbon nanotubes embedded in RF waveguides. The design, fabrication, and operation of the device will be discussed, including initial RF measurements. Preliminary tests suggest that transmission of an RF signal through the array is associated with the mechanical resonance of the carbon nanotubes.

Generalized scattering matrices for unit cell characterization of grid amplifiers and device de-embedding

A new method for finding radiation patterns and the reflection coefficients associated with an axisymmetric waveguide fed horn is presented. The approach is based on a hybrid finite element method (FEM) wherein the electromagnetic fields in the FEM region are coupled to the fields outside by two surface integral equations. Because of the local nature of the FEM, this formalism allows for the presence of inhomogeneities to be included in the problem domain. The matrix equation which results from the application of this method is shown to be complex-symmetric. Comparisons of calculated and measured data for two different horns show good agreement.

High-Q mechanical resonator arrays based on carbon nanotubes

A formulation of the Finite Element Method (FEM) particular to axisymmetric problems containing anisotropic media is compared to an analytic solution. In particular, the resonant frequencies of a longitudinally biased ferrite-filled cylindrical cavity are examined. For comparison, a solution of the characteristic equation for the lossless, ferrite-filled cylindrical waveguide was modified to give the resonant frequencies of the cylindrical cavity. This analytical solution was then used to examine the error in the FEM formulation for the anisotropic case. It is noted that the FEM formulation for anisotropic material presented, based on both node and edge-based elements, is found to be free of spurious solutions. >

A hybrid finite-element method for axisymmetric waveguide-fed horns

In order to add the capability of an X-band up-link onto the NASA/JPL Deep Space Network (DSN) 70-m antenna, a new dichroic plate is needed to replace the Pyle-guide shaped dichroic plate currently in use. The replacement dichroic plate must exhibit an additional pass band at the new up-link frequency of 7.165 GHz, while maintaining a pass band at the existing down-link frequency of 8.425 GHz. Because of the wide frequency separation of these two pass bands, conventionally designed air-filled dichroic plates exhibit grating lobe problems. A new method of solving this problem using a dichroic plate with cross-shaped holes is presented and verified experimentally. Two checks of the integral equation solution are described here. One is the comparison to a modal analysis for the limiting cross shape of a square hole. As a final check a prototype dichroic plate with cross-shaped holes was built and measured. >

Determination of the eigenfrequencies of a ferrite-filled cylindrical cavity resonator using the finite element method

A generalized scattering matrix approach to analyzing quasi-optical grids used for grid amplifiers and grid oscillators is developed. The approach is verified by a novel method for de-embedding, in a waveguide simulator, the active device parameters of a differential pair high electron mobility transistor (HEMT) from the single unit cell of a grid amplifier. The method incorporates the additional ports presented to the active device into a method of moments solution of the embedding periodic array. The port(s) defined at the device or load location are within the plane of the array, and not terminated in a microstrip line with a known characteristic impedance. Therefore the generalized scattering matrix for the embedding array is normalized to the calculated input impedance(s) at these port(s). The approach described here uses a Floquet representation of the fields incident and reflected from the grid as the remaining ports in the generalized scattering matrix. The use of Floquet modes allows analysis of general geometries and nonnormal incident angles without the need for magnetic and electric wall assumptions. By developing a generalized scattering matrix for the embedding periodic array, this approach now allows conventional amplifier design techniques and analysis methods to be applied to quasi-optical grid amplifier and oscillator design. The major advantage of this unification for grid amplifier design being that the stability of the design can be predicted.

Experimental verification of an integral equation solution for a thin-walled dichroic plate with cross-shaped holes

The authors develop a preliminary model for the lump-loaded FSS (frequency selective surface) using simple representations for the lumped elements and their interconnections. The representations are derived by applying the resistive boundary condition to include a general surface impedance in the problem formulation. The surface impedance term is then incorporated in the method-of-moments solution and can be varied in each subdomain. A simple lumped-element model is obtained by considering a surface with this type of varying surface impedance to represent the loading. Comparisons of the surface and lumped-load formulations when applied to a four-legged element are shown.<<ETX>>