Robert B. Dybdal

Radar cross section measurements

The progress in radar cross section measurements is strongly related to the progress in radar technology. The recent acceleration in radar technology and processing techniques has generated a corresponding acceleration in interest for radar cross section measurements. Historically, early radar cross section measurements were performed to determine the detection range of radar systems, a fundamental objective that still exists. Later measurements, coupled with analytic techniques and computer codes, were performed to extend our understanding of the radar scattering process. At the present time, the availability of broad-band electronics, signal processing techniques, and digital technology results in radar cross section measurement programs which are directed toward exploring the performance of operational waveforms and processing, target discrimination, target detectability in clutter, and radar scattering control. The fundamentals of radar cross section measurements are reviewed. Measurement facilities, including the present research activities on compact range techniques, are then described. Instrumentation radars have benefited from both wide-bandwidth electronics and digital processing capabilities; Fourier transform techniques, in particular, provide both additional information on target scattering, and increase measurement accuracy by isolating the target from radar returns from the measurement facility. The frequency coverage has also extended to include millimeter-wave frequencies. Achievable accuracy is important in any measurement program, and those factors that limit the accuracy of radar cross section measurements are discussed.

Defocusing loss for a long periodic-fed reflector

Log periodic designs are a well established wide-band antenna technology, but one in which the phase center travels along the structure as the frequency varies. When a log periodic antenna is used as a feed for a reflector antenna, the phase center location cannot be maintained at the reflector focus over the frequency range and defocusing results. The purpose of this analysis is to quantify the defocusing loss for a log periodicfed reflector. The analysis presented compares favorably with measured results.

Millimeter Wave Antenna Technology

Millimeter wave antenna technology has had a long history of development, and as millimeter wave systems evolve through planning to implementation, a significant amount of additional development work will be required. Millimeter wave antennas play a key role in the rationale for millimeter system designs because high spatial resolution can be achieved with modest physical dimensions. Reflector, lens, array, and horn technologies are surveyed. Multiple beam designs and adaptive processing antennas are described because these technologies afford high leverage opportunities to enhance electronic survivability and to extend communication capabilities. Ancillary components, such as radomes, are a necessary part of practical antenna designs and are discussed in some detail.

Experimental evaluation of a circularly polarized metallic lens antenna

The performance of a 3-foot-diameter circularly polarized metallic lens antenna at X -band frequencies is described. The measured antenna gain, efficiency, half-power beamwidth, sidelobe level, and optimum focal length were determined for both on-axis and scan conditions. The effect of different lens illuminations for the on-axis case was ascertained.

Multiple polarization communications

If two independent signals are communicated using orthogonal polarizations, the communication throughput is doubled compared to a single polarization. Polarization reuse superimposes two independent signals in the same bandwidth and requires design attention to obtain adequate isolation between the signals to avoid cochannel interference. Polarization reuse is limited to doubling the communication throughput because any polarization has only one orthogonal polarization. A new technique uses multiple non-orthogonal polarizations and signal processing techniques to separate and combine multiple signal components and can more than double the communication throughput.

Coherent RF Error Statistics

RF error statistics for power, voltage, and phase are presented for an error component which is coherently related to a desired signal. The error component is assumed to have a constant magnitude and a phase distribution which is equally likely and uniformly distributed from 0 to 360°. The statistics which result have non-zero mean values for power and voltage errors and the standard deviation of the errors differ significantly from those projected from Gaussian statistics.

Monopulse resolution of interferometric ambiguities

Practical interferometric systems capable of high angular accuracy must resolve ambiguities which result from large baseline dimensions. A new technique is described which resolves ambiguities based on monopulse measurements made with interferometric elements.

Millimeter wavelength radar cross section measurements

Radar cross section (RCS) measurements performed at 93 GHz are reported for flat plate targets and a scaled sphere-cone target. RCS predictions based on common asymptotic methods agree well with measured results for these targets. This frequency range is shown to be useful in making scaled model measurements on targets which are large in terms of wavelengths.

VHF to EHF performance of a 90-ft quasi-tapered anechoic chamber

This paper reports the measured reflection characteristics of a 90-ft anechoic chamber operated by The Aerospace Corporation. The design of this chamber, described as quasi-tapered, is unique in that it tapers from a large rectangular test region to a smaller square transmitting end. This design incorporates the performance advantages of a fully tapered chamber at low frequencies with some of the flexibility of a rectangular chamber when used at higher frequencies. The performance of this chamber is reported from 100 MHz to 93 GHz.

System measurement of antennas

Antenna measurements are typically performed at a component level establishing the gain, pattern, polarization, and impedance characteristics. Clearly, the need for these measurements continues, but additional antenna-related measurements are required in system applications to verify compliance with system requirements. The trend of integrating antennas with system electronics also requires such characterizations to evaluate antenna system performance since an antenna port per se for measurement does not exist. Examples of antenna-related system measurements are described.