Eberhardt Heidrich

Wide-band multiport antenna characterization by polarimetric RCS measurements

A theoretical description of the scattering of multiport antennas is derived in accordance with the polarimetric radar target characterization. For two-port antennas with dual linear orthogonal polarization, a procedure for the determination of the antenna mode scattering parameters and the structural scattering parameters of the complete polarimetric antenna scattering matrix [S] is given. A measurement setup and experimental results of wide-band polarimetric radar cross section (RCS) antenna measurements are presented. For two-port antennas, in total, 24 unknowns have to be determined. The load variation method is used. The polarimetric RCS antenna measurement is especially advantageous over a wide bandwidth; for example 2-40 GHz with only one single calibration. From the antenna scattering matrix [S], the antenna parameters like polarimetric gain, optimum gain, gain bandwidth, wave admittance, structural scattering, etc., are derived. Results are shown for an X-band horn antenna and a 2-18-GHz dual linearly polarized horn. The measurement technique and evaluation gives the most complete and very accurate information about antenna radiation, antenna characteristics, and scattering obtainable with one single measurement setup. Some quantities determined here, like the maximum available gain and the polarization characteristics, are difficult to access with standard transmission measurements. No cable from the antenna to the receiver is necessary. This helps avoid some difficulties usually experienced at low signal levels.

Dual polarized multilayer aperture-coupled patch antennas for spaceborne application in C- and X-band

The paper discusses the element and subarray requirements and designs for advanced dual polarized future spaceborne SAR antennas using aperture-coupled microstrip patch elements. Special emphasis is given to achieve high polarization purity, low loss, good port-to-port isolation, high bandwidth and high aperture efficiency. Two design examples are shown for frequencies in C-band (5.3 GHz) and X-band (9.6 GHz) having a port-to-port-isolation of 25 dB and a polarization isolation better than 28 dB.<<ETX>>

Reduction and minimization of antenna scattering

The authors analyze the radar cross section of antennas under various load conditions over a wide frequency range. Special emphasis is given to the polarization characteristics of radiated and scattered fields and their influence on the total scattered antenna field. Several techniques for reduction and minimization of antenna scattering are discussed. It is demonstrated how the total scattered field of an antenna can be modified and minimized using appropriate load reflection coefficients. However, in the general case this minimization is only realizable in a narrow frequency range, for certain aspect angles and polarizations.<<ETX>>

Design considerations for comb line microstrip travelling wave antennas

An efficient design procedure for microstrip stub traveling wave arrays is presented. The design frequency of 24.125 GHz and the desired sidelobe suppression of more than 20 dB demand an accurate array synthesis and a high fabrication accuracy. Special attention is given to the influence of the T-junction which connects the stub to the microstrip feed line and its overall effect on the radiation mechanism. To demonstrate the quality of the design approach several arrays are shown having one or two excitation ports and main beams 45 degrees off broadside. Considering the design frequency of 24.125 GHz, the scan angle of more than 40 degrees off broadside, and the achieved sidelobe suppression, a considerable improvement has been reached compared to the state of the art.<<ETX>>

Microwave Front End for True Ground Speed Measurements

A dual channel microwave Doppler-Sensor-Module for 24 GHz is presented. This front end is well suited for true ground speed and distance measurements in all kinds of automotive applications. The microwave components such as oscillator, mixer, antenna and their characteristics in the MIC are discussed. The influence of the antenna pattern and the road surface on the Doppler spectrum is treated in a 3D-field theoretical simulation. This simulation takes the antenna nearfield and the distributed scattering of the road into account

The signal flow error cube for polarimetric antenna and RCS measurements

It is demonstrated that signal flow graphs for circuits implying free-space transmission and polarization properties, as applicable to antenna and radar cross-section measurements, can be represented in a three-dimensional error cube. Although these graphs are rather complicated, they are a very useful tool for understanding the physical system behavior. In addition, mathematical formulations for error correction and calibration can easily be derived. High-level calibration techniques containing all systematic errors can advantageously be derived and also illustrated by means of signal flowcharts. Moreover, they clearly show the schematic of the system and the points of influence for errors such as spurious signals, reflections or coupling.<<ETX>>

Theoretical and Experimental Determination of the Polarimetric Antenna Radar Cross Section

ABSTRACT The total radar cross section (RCS) of antennas is the vector sum of structural and antenna mode scattering. These RCS contributions can be described completely by their polarimetric scattering matrices. The paper introduces a new polarimetric network description for three and four port antennas. The total scattering matrix is separated into the individual contributions with a new measurement procedure, based on complex load variations. Two waveguide and two planar antennas are chosen for demonstration and verification. By load variation the antenna mode scattering can be manipulated in phase and amplitude. It can especially be steered to compensate for the structural and aperture contributions. The possibilities for RCS minimization under polarimetric aspects are shown to be limited in bandwidth, polarization and aspect angle.