Afroz J. Zaman

RF MEMS phase shifters and their application in phase array antennas

Electronically scanned arrays are required for space based radars that are capable of tracking multiple robots, rovers, or other assets simultaneously and for beam-hopping communication systems between the various assets. ^Traditionally, these phased array antennas used GaAs Monolithic Microwave Integrated Circuit (MMIC) phase shifters, power amplifiers, and low noise amplifiers to amplify and steer the beam, but the development of RF MEMS switches over the past ten years has enabled system designers to consider replacing the GaAs MMIC phase shifters with RF Micro-Electro Mechanical System (MEMS) phase shifters. In this paper, the implication of replacing the relatively high loss GaAs MMICs with low loss MEMS phase shifters is investigated.

Demonstration of a X-band multilayer Yagi-like microstrip patch antenna with high directivity and large bandwidth

The feasibility of obtaining large bandwidth and high directivity from a multilayer Yagi-like microstrip patch antenna at 10 GHz is investigated. A measured 10-dB bandwidth of ~20% and directivity of ~11 dBi is demonstrated through the implementation of a vertically-stacked structure with three parasitic directors, above the driven patch, and a single reflector underneath the driven patch. Simulated and measured results are compared and show fairly close agreement. This antenna offers the advantages of large bandwidth, high directivity and symmetrical broadside patterns, and could be applicable to satellite as well as terrestrial communications

A spherical to plane wave transformation using a reflectarray

A reflectarray has generally been used as a replacement for a reflector antenna. Used in this capacity, different configurations (prime focus, offset etc.) and various applications (dual frequency, scanning etc.) have been previously demonstrated with great success. Another potential application that has not been explored previously is the use of reflectarrays to compensate for phase errors in space power combining applications such as space-fed lens and power combining amplifier. In these applications, it is required to convert a spherical wave to a plane wave with proper phase correction added to each element of the reflectarray. This paper reports an experiment to investigate the feasibility of using a reflectarray as an alternative to a lens in space power combining. The experiment involves transforming a spherical wave from a orthomode horn to a plane wave at the horn aperture. The reflectarray consists of square patches terminated in open stubs to provide necessary phase compensation. In this paper, preliminary results will be presented and the feasibility of such a compensation scheme will be discussed.

Experimental demonstration of the effects of an electric thruster plasma plume on microwave propagation

Electric thrusters are being considered for a wide variety of space missions because of the significant propellant savings that result from the use of high performance, electric propulsion technologies. The impact of electric thruster plasma plumes on microwave propagation is a key integration concern for planners of the next generation of spacecraft. Arcjets were the first electric thrusters to be considered for operational missions. The effect of arcjet plumes on propagation was studied by Ling, et al. (see IEEE Transactions on Antennas and Propagation, vol.39, no.9, p.1412-1420, 1991). Arcjets produce a lightly ionized plume and Lings analysis predicted that the plume would have a negligible effect on communication. Arcjets are now operational on the AT&T Telstar 401 satellite, and the analysis has been validated by in-space operations. However, plumes from the higher performance thrusters being developed exhibit higher ionization levels, plasma temperatures and particle velocities than arcjets. Therefore a need has risen to determine the effects of these plumes. To address this need, the authors have designed and performed an experiment to assess the attenuation and phase shift caused by these types of plasmas. The challenge with this experiment was that it was a microwave propagation experiment that had to be performed inside a metal vacuum chamber. Thus the experiment had to be designed to minimize multiple reflections within the chamber. This paper describes the experiment and presents some of the results that were obtained.

Experimental Demonstration of Microwave Signal/Electric Thruster Plasma Interaction Effects

An experiment was designed and conducted in the Electric Propulsion Laboratory of NASA Lewis Research Center to assess the impact of ion thruster exhaust plasma plume on electromagnetic signal propagation. A microwave transmission experiment was set up inside the propulsion test bed using a pair of broadband horn antennas and a 30 cm 2.3 kW ion thruster. Frequency of signal propagation covered from 6.5 to 18 GHz range. The stainless steel test bed when enclosed can be depressurized to simulate a near vacuum environment. A pulsed CW system with gating hardware was utilized to eliminate multiple chamber reflections from the test signal. Microwave signal was transmitted and received between the two hours when the thruster was operating at a given power level in such a way that the signal propagation path crossed directly through the plume volume. Signal attenuation and phase shift due to the plume was measured for the entire frequency band. Results for this worst case configuration simulation indicate that the effects of the ion thruster plume on microwave signals is a negligible attenuation (within 0.15 dB) and a small phase shift (within 8 deg.). This paper describes the detailed experiment and presents some of the results.

Miniature antennas for lunar and planetary surface communications

Surface communications assets in future NASA exploration scenarios such as robotic rovers, human extravehicular activities (EVA), and probes will necessitate small size, lightweight, low power, and robust antenna elements. With the availability of space in these devices being a major concern, miniature antennas provide a potential solution toward addressing these issues. The current problem with miniature antennas, however, is their inherently low efficiencies, which will be unacceptable for low power applications. At the NASA Glenn Research Center (GRC), prototypes of several potentially useful miniature antenna designs for lunar and planetary surface communications have been investigated. Specifically, two novel candidate designs which hold promise, the folded Hilbert curve fractal antenna (fHCFA) and the compact microstrip monopole antenna (CMMA), are described in this paper. Both demonstrate comparable performance to a quarter-wavelength monopole, but at smaller than 1/3 the size, making them more suitable for integration in communications systems with stringent size and volume requirements (i.e., wireless sensors, space suits and others)