Teh-Hong Lee

Structure scattering analysis for SeaWinds Scatterometer reflector antenna using extended aperture integration and GTD

One of the major considerations of antenna design for spacecraft application is the complex structures around the antenna which can become scatterers due to significant illumination from the antenna. The resulting scattered fields can interfere with the direct antenna radiation pattern and cause degradations in the antenna directivity and sidelobe levels. A good example is the NASA SeaWinds Scatterometer designed for the Japanese ADEOS II Spacecraft. The scattering effect caused by the spacecraft structures can be analysed by using the geometrical theory of diffraction (GTD) ray tracing approach. One alternative approach to the problem of the equivalent dipole sources is to use the extended aperture integration (AIE) method for the forward scattering analysis when the structure is close to and in front of the antenna aperture. The authors determine the scattering effects to the antenna performance due to the presence of the various box structures. With the resulting information, one can determine better locations and sizes for the structures such that they will not cause significant degradation to the desired antenna performance. Results are presented for the SeaWinds reflector antenna using the AIE method and for the plate scattering analysis of the OSU Reflector Antenna Code.<<ETX>>

Spurious oscillations in high power klystrons

Spurious oscillations in high power klystrons are found to occur in the gun region, in the cavities in the main body of the tube, or in the drift tunnel. The criteria that determine whether a mode will oscillate is that its beam loading be negative, and that the power it extracts from the beam exceeds its losses to external loading and wall dissipation. Using the electromagnetic and particle-in-cell modules of MAFIA, we have devised numerical techniques with which the quality factors Q/sub b/, Q/sub e/, and Q/sub o/ can be evaluated and compared. Simulations involving a gun oscillation observed in the SLAC/DESY S-band klystron will be reported.

Multilayer R-Card fence design for ground bounce reduction

Multilayer R-Card fences have been designed and evaluated for an outdoor range in order to reduce the ground bounce effects. This design not only can reduce the specular reflection by the ground to the receiving area but also reduce the reflection back to the transmit area due to the corner reflector effects. It has been found from the previous and current studies that tapered R-Card fences can very effectively used in reducing multipath stray signals, such as the ground reflection term in an outdoor range. With available commercial resistive sheets, these fences can be built very inexpensively and has great potential in many other applications.

Design and analysis of antenna systems properly integrated into a spacecraft structure

For antennas located in a complex structural environment, such as a spacecraft, the structures can significantly impact the desired antenna pattern performance. The scattered fields from these structures due to the illumination of the antenna can create interference to the patterns of the antenna. Consequently, it is very important for antenna design engineers to be aware of the surrounding structures of the antenna and take the possible scattering effects into consideration, including the antenna and the structures. The proper use of computer codes can greatly help the engineers to achieve a much better overall design for the spacecraft in a cost effective way. The scattered field analysis approach, which uses the uniform geometrical theory of diffraction (GUTD), presented in this paper has been used to study the impact of the structure scattering in several commercial satellite systems as well as the international space station and helped the design engineers to find a better way of integrating the antennas into the spacecraft structure.

A new shaping method for circularly symmetric dual reflector antennas

This work presents a shaping method for a circularly symmetric dual reflector antenna. The shaped reflector surface is determined from the prescribed aperture field based on the principles of geometrical optics (GO), including power conservation, Snells law and ray condition. Using these principles, two first order ordinary differential equations for shaping circularly symmetric reflectors, have been formulated previously. These principles were also used to design the subreflector and consequently the main reflector surface to obtain the desired uniform aperture field distribution. However, in this research, instead of solving rather complicated partial differential equations, several nonlinear algebraic equations are formulated based on these principles and geometrical properties of conventional dual reflector antennas, and solved numerically with a certain approximation for a shaped dual reflector surface and caustic line.

Analysis of scattering from tapered and non-circular cross section struts for reflector antennas

A numerical technique used to calculate scattered fields from feed/subreflector support struts for reflector antenna system has been described in this presentation. Comparison between calculated and measured patterns, with and without the struts, indicates that the technique implemented is valid in predicting scattered fields from struts with arbitrary cross section. The numerical results presented also show that non-circular cross section struts, such as ogival ones, can provide lower scattered field levels that can be very important for reflectors with low sidelobe requirements.

Evaluation of compact range using a diagonal flat plate

The authors present a method which uses a diagonal flat plate to develop a stray signal criterion based on a tolerable RCS (radar cross section) measurement error. The scattered cross section of a 1.25 ft by 1.25 ft diagonal flat plate has been measured in a compact range with a blended rolled-edge reflector and with a knife-edge reflector. The measured RCS pattern of the plate at 6 GHz is shown, with the ideal plate RCS pattern plotted for comparison. The stray signal from one of the junctions between the rolled edge and the parabolic section is about 6.2 degrees from the axis of the plate. As a result, there should be an error in the plate RCS pattern at approximately=3.1 degrees . It is found that this error is about 0.88 dB for the blended rolled-edge reflector. The knife edge of the modified reflector is located about 12 degrees from the plate axis and, as a result, there is a significant error (5.17 dB) in the plate RCS pattern at theta =6 degrees .<<ETX>>

The software defined antenna: Microstrip antennas with gaps

The goal of this research is to develop an unconstrained reconfigurable programmable array antenna. The concept is to build microstrip patch antenna arrays using individual controllable pixels. A 16 by 16 array of mechanically actuated pixels has been constructed. Air gaps exist in the ground plane, substrate, and conductor layer. This prototype is controlled through software to build an array of microstrip antennas. A set of feed lines and antennas can be created in any pattern within milliseconds. The Software Defined Antenna was used to build a single patch antenna and an array of 2 patch antennas fed by a feed network. Radiation pattern and gain measurements were performed. The gain is consistent with that of standard microstrip patch antennas (5 to 7 dBi).

R-Card fence applications for indoor anechoic chamber

It is shown in this paper that properly designed R-Card fences can be installed in indoor measurement facility to reduce the multipath signals in the chamber. The design is especially more effective when the transmit antenna is closer to the absorber wall. The traditional foam absorber works well when the angle of incidence is less than 50 degrees if the thickness is about 2 wavelengths. The R-Card fences greatly improve the chamber performance when the angle of incidence is greater than 50 degrees. Furthermore, it may be possible to use smaller size of absorber (less than 2 wavelengths) with the R-Card fence when the overall chamber size is limited. It is then possible to measure larger size of AUT/UUT in an existing chamber as well as extend the frequency range of the chamber to the lower end.

Structure scattering of feed antenna in a dual reflector antenna system

In this paper, the impact of the structure scattering between the horn and subreflector of a Cassegrain reflector antenna system has been presented. Typical design steps involve (1) feed design that has very good VSWR and (2) use the far field pattern to compute the illumination on the subreflector and subsequently the main reflector to determine the far field pattern of the whole antenna system.