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Papers by Robert Hohlfeld

Small antennas pose severe practical restrictions in tradeoffs of directivity, bandwidth, and rad... more Small antennas pose severe practical restrictions in tradeoffs of directivity, bandwidth, and radiative efficiency. Work by Chu (1948), in particular, prescnted an analytical basis for these restrictions. However, recent work on fractal elements (Cohen and Hohlfeld, 1996) suggests that such apparently complex

IEEE International Symposium on Phased Array Systems and Technology, 2003.

The Hohlfeld-Cohen-Rumsey (HCR) principle describes the necessary and sufficient conditions for a... more The Hohlfeld-Cohen-Rumsey (HCR) principle describes the necessary and sufficient conditions for all frequency independent antennas, in terms of the fractal scaling properties of a radiating configuration and the symmetries obeyed by the source currents required by Maxwell's equations. By illustration, we explicitly apply the HCR principle to the design of a frequency independent planar array covering a frequency range of

IEEE Antennas and Propagation Society International Symposium. 2001 Digest. Held in conjunction with: USNC/URSI National Radio Science Meeting (Cat. No.01CH37229)

Small antennas pose severe practical restrictions in tradeoffs of directivity, bandwidth, and rad... more Small antennas pose severe practical restrictions in tradeoffs of directivity, bandwidth, and radiative efficiency. Chu (1948) presented an analytical basis for these restrictions. We demonstrate that the applicability of Chu's analysis appears to be overgeneralized when complex (fractal) antennas are considered. We derive a spherical harmonic expansion of the current distribution of a simple fractal loop, and show that this

Fractals, 1999

Self-similarity and origin symmetry are shown to be the key geometric constraints in the determin... more Self-similarity and origin symmetry are shown to be the key geometric constraints in the determination of frequency independent properties of antennae. Fractal antennae with origin symmetry meet these criteria of the extended version of Rumsey's principle. Frequency independence is not achieved by self-similarity alone. Self-complementarity plays no role in frequency independence, but does aid in smoothing out impedance variations for coarsely iterated frequency independent antennae. New families of practical designs arise from these geometric insights, which need not follow the usual constraints of angle-defined structure of the original Rumsey's principle.

PURPOSE: To automatically structure pictures in a computer memory as the data structure of lists ... more PURPOSE: To automatically structure pictures in a computer memory as the data structure of lists connected double (or plural times) and reflected to relation among various stages of a composite picture processing task by its mutual connection. CONSTITUTION: Data structure 50 attained by a suitable high level computer language such as C includes information related to the structure 50 together with other data structure and pointers (memory addresses) to pictures related to the data structure. The relation of lists connected among pictures is constructed and maintained by these pointers. Since relation among pictures in. the connected lists is equivalent to relation among the stages of the picture processing task, the composite task for picture processing is automatically formated in the data structure of the connected lists without maintaining and managing auxiliary recording. COPYRIGHT: (C)1992,JPO

This thesis is concerned with the application of computational techniques appropriate to simulati... more This thesis is concerned with the application of computational techniques appropriate to simulation of wave propagation from radiating elements in an antenna array utilizing parallel computer architectures. The intention of this study is to implement numerical solutions of the wave equation in a parallel computer architecture to gain a computational speed advantage and to develop direct simulations techniques (based on a description dependent only on local physics) which are an accurate representation of the physical system. Energy conserving integration schemes for the wave equation represented in finite difference form have been developed and analyzed. The analysis entails first verifying the stability of these finite difference equations using von Neumann stability analysis and then finding analytic closed form solutions using discrete Laplace and Fourier transforms. Analytic results are then verified by comparison to the simulations. These integration schemes have been extended ...

Many physiological signals may be described either as isolated pulses or as quasi-periodic sequen... more Many physiological signals may be described either as isolated pulses or as quasi-periodic sequences of isolated pulses. Wavelets are a powerful tool for the representation and analysis of such physiologic waveforms because a wavelet has finite duration (compact support) as contrasted with Fourier methods based on sinusoids of infinite duration. We show two examples of physiological signal processing using wavelet bases. The first example is compression of electrocardiogram (ECG) signals using an Associated Hermite wavelet basis and the second example shows removal of artifact from non-invasive blood pressure (NIBP) measurements.

: Differential Inversion (DI) is a novel approach to the solution of the atmospheric temperature ... more : Differential Inversion (DI) is a novel approach to the solution of the atmospheric temperature sounding problem which was developed by Dr J.I.F. King of the Air Force's Phillips Laboratory. Before the present research, DI has been applied only to infrared radiance data sets, such as from TOVS/HIRS. In this report I describe the progress made in the first year of a research program to apply DI to microwave radiance data from the SSM/T sounder on the DMSP satellites. The ultimate objectives of this research are to establish effective DI sounding algorithms for the microwave spectral region and to extend DI in directions which increase its utility as a practical sounding algorithm. DI has many attractive features in this application, including its close coupling to the physical formulation of the temperature sounding problem, its freedom from the necessity of using an a priori temperature profile, and its high level of computational efficiency. At present, we have understood the ...

The effect of longitudinal impulses on a satellite when a continuum tether from the Shuttle or Sp... more The effect of longitudinal impulses on a satellite when a continuum tether from the Shuttle or Space Station is included is investigated. It is demonstrated that, taking the continuum nature of the tether into account, the sharp onset of a forcing impulse at the Shuttle end is perceived on board the satellite. The magnitude of the satellite response is diminished from that of the forcing impulse, and the response exhibits a broadened exponential tailoff. These are due primarily to the coupling between the tether and the satellite and its influence on an impinging impulsive wave, and not on the whole system as in the spring-mass model. The full impulse response function can be quite complex, showing repeated impulses as the tether wave bounces back and forth between the satellite and Shuttle, and having a different shape at each impingement on the satellite.

: Attenuation of electromagnetic (EM) waves by Hydrometeors such as rain, snow, clouds, and fog, ... more : Attenuation of electromagnetic (EM) waves by Hydrometeors such as rain, snow, clouds, and fog, can cause serious degradation of EM wave systems. This report describes such attenuation of radio waves to X-rays, with primary applications to communications in the microwave and millimeter wave portions of the EM spectrum.

: We have demonstrated robust algorithms for generating fits in Optical Measure Theory (OMT) to r... more : We have demonstrated robust algorithms for generating fits in Optical Measure Theory (OMT) to radiance data from the TIROS Operational Vertical Sounder (TOVS). These algorithms generated physically meaningful fits on 100% of a test set of 45 TOVS radiance scans, and were successful both on short wavelength (667/cm) and on long wavelength (2250/cm) TOVS data. The resulting OMT temperature profiles exhibit meteorological characteristics and appear to be suitable for determination of atmospheric structure parameters characterizing the large-scale vertical temperature of the atmosphere, and also suitable for generation of input data for numerical weather prediction codes. Keywords: Optical data, Atmospheric sounding, Meteorological satellites, Remote sensing, Atmospheric temperature profiles.

Automatique et ingénierie système

De nombreux signaux physiologiques peuvent etre decrits soit comme des impulsions isolees, soit c... more De nombreux signaux physiologiques peuvent etre decrits soit comme des impulsions isolees, soit comme des suites quasi periodiques d’impulsions isolees. Les ondelettes constituent des outils puissants pour la representation et l’analyse de telles formes d’ondes physiologiques, car une ondelette a une duree finie (support compact), a la difference des transformees de Fourier basees sur des sinusoides de duree infinie. Deux exemples de traitement d’un signal physiologique utilisant des bases d’ondelettes seront etudies dans cet article. Le premier exemple est la compression de signaux d’electrocardiogramme (ECG) utilisant une base d’ondelettes d’Hermite associees ; le second exemple montre la suppression des artefacts presents dans des signaux de mesure non invasive de pression sanguine (PNI).

Journal of Physics: Conference Series

We use an N-body simulation, constructed using GADGET-2, to investigate an accretion flow onto an... more We use an N-body simulation, constructed using GADGET-2, to investigate an accretion flow onto an astrophysical disk that is in the opposite sense to the disk's rotation. In order to separate dynamics intrinsic to the counter-rotating flow from the impact of the flow onto the disk, we consider an initial condition in which the counter-rotating flow is in an annular region immediately exterior the main portion of the astrophysical disk. Such counter-rotating flows are seen in systems such as NGC 4826 (known as the "Evil Eye Galaxy"). Interaction between the rotating and counter-rotating components is due to two-stream instability in the boundary region. A multi-armed spiral density wave is excited in the astrophysical disk and a density distribution with high azimuthal mode number is excited in the counter-rotating flow. Density fluctuations in the counter-rotating flow aggregate into larger clumps and some of the material in the counter-rotating flow is scattered to large radii. Accretion flow processes such as this are increasingly seen to be of importance in the evolution of multi-component galactic disks.

L-11125 jJ 1.4 111n. 6 V R()COFf RlFd9LUIlON '{tT E ' 'a REPORT SECURITY CLASj AS1i CivE MARXNGS ... more L-11125 jJ 1.4 111n. 6 V R()COFf RlFd9LUIlON '{tT E ' 'a REPORT SECURITY CLASj AS1i CivE MARXNGS Unclassified __________________080'____ :a SECURITY CLASSIFiCATIC D ARBU 17 8 2 ION/ AVAILABILITY OF REPORT N/A ji7ro~ for pi~ic rol1OiasO 20 ECLASSIFICAT!ON/I OWNGRADING SCHEDULEdi.riUif

Small antennas pose severe practical restrictions in tradeoffs of directivity, bandwidth, and rad... more Small antennas pose severe practical restrictions in tradeoffs of directivity, bandwidth, and radiative efficiency. Work by Chu (1948), in particular, prescnted an analytical basis for these restrictions. However, recent work on fractal elements (Cohen and Hohlfeld, 1996) suggests that such apparently complex

IEEE International Symposium on Phased Array Systems and Technology, 2003.

The Hohlfeld-Cohen-Rumsey (HCR) principle describes the necessary and sufficient conditions for a... more The Hohlfeld-Cohen-Rumsey (HCR) principle describes the necessary and sufficient conditions for all frequency independent antennas, in terms of the fractal scaling properties of a radiating configuration and the symmetries obeyed by the source currents required by Maxwell's equations. By illustration, we explicitly apply the HCR principle to the design of a frequency independent planar array covering a frequency range of

IEEE Antennas and Propagation Society International Symposium. 2001 Digest. Held in conjunction with: USNC/URSI National Radio Science Meeting (Cat. No.01CH37229)

Small antennas pose severe practical restrictions in tradeoffs of directivity, bandwidth, and rad... more Small antennas pose severe practical restrictions in tradeoffs of directivity, bandwidth, and radiative efficiency. Chu (1948) presented an analytical basis for these restrictions. We demonstrate that the applicability of Chu's analysis appears to be overgeneralized when complex (fractal) antennas are considered. We derive a spherical harmonic expansion of the current distribution of a simple fractal loop, and show that this

Fractals, 1999

Self-similarity and origin symmetry are shown to be the key geometric constraints in the determin... more Self-similarity and origin symmetry are shown to be the key geometric constraints in the determination of frequency independent properties of antennae. Fractal antennae with origin symmetry meet these criteria of the extended version of Rumsey's principle. Frequency independence is not achieved by self-similarity alone. Self-complementarity plays no role in frequency independence, but does aid in smoothing out impedance variations for coarsely iterated frequency independent antennae. New families of practical designs arise from these geometric insights, which need not follow the usual constraints of angle-defined structure of the original Rumsey's principle.

PURPOSE: To automatically structure pictures in a computer memory as the data structure of lists ... more PURPOSE: To automatically structure pictures in a computer memory as the data structure of lists connected double (or plural times) and reflected to relation among various stages of a composite picture processing task by its mutual connection. CONSTITUTION: Data structure 50 attained by a suitable high level computer language such as C includes information related to the structure 50 together with other data structure and pointers (memory addresses) to pictures related to the data structure. The relation of lists connected among pictures is constructed and maintained by these pointers. Since relation among pictures in. the connected lists is equivalent to relation among the stages of the picture processing task, the composite task for picture processing is automatically formated in the data structure of the connected lists without maintaining and managing auxiliary recording. COPYRIGHT: (C)1992,JPO

This thesis is concerned with the application of computational techniques appropriate to simulati... more This thesis is concerned with the application of computational techniques appropriate to simulation of wave propagation from radiating elements in an antenna array utilizing parallel computer architectures. The intention of this study is to implement numerical solutions of the wave equation in a parallel computer architecture to gain a computational speed advantage and to develop direct simulations techniques (based on a description dependent only on local physics) which are an accurate representation of the physical system. Energy conserving integration schemes for the wave equation represented in finite difference form have been developed and analyzed. The analysis entails first verifying the stability of these finite difference equations using von Neumann stability analysis and then finding analytic closed form solutions using discrete Laplace and Fourier transforms. Analytic results are then verified by comparison to the simulations. These integration schemes have been extended ...

Many physiological signals may be described either as isolated pulses or as quasi-periodic sequen... more Many physiological signals may be described either as isolated pulses or as quasi-periodic sequences of isolated pulses. Wavelets are a powerful tool for the representation and analysis of such physiologic waveforms because a wavelet has finite duration (compact support) as contrasted with Fourier methods based on sinusoids of infinite duration. We show two examples of physiological signal processing using wavelet bases. The first example is compression of electrocardiogram (ECG) signals using an Associated Hermite wavelet basis and the second example shows removal of artifact from non-invasive blood pressure (NIBP) measurements.

: Differential Inversion (DI) is a novel approach to the solution of the atmospheric temperature ... more : Differential Inversion (DI) is a novel approach to the solution of the atmospheric temperature sounding problem which was developed by Dr J.I.F. King of the Air Force's Phillips Laboratory. Before the present research, DI has been applied only to infrared radiance data sets, such as from TOVS/HIRS. In this report I describe the progress made in the first year of a research program to apply DI to microwave radiance data from the SSM/T sounder on the DMSP satellites. The ultimate objectives of this research are to establish effective DI sounding algorithms for the microwave spectral region and to extend DI in directions which increase its utility as a practical sounding algorithm. DI has many attractive features in this application, including its close coupling to the physical formulation of the temperature sounding problem, its freedom from the necessity of using an a priori temperature profile, and its high level of computational efficiency. At present, we have understood the ...

The effect of longitudinal impulses on a satellite when a continuum tether from the Shuttle or Sp... more The effect of longitudinal impulses on a satellite when a continuum tether from the Shuttle or Space Station is included is investigated. It is demonstrated that, taking the continuum nature of the tether into account, the sharp onset of a forcing impulse at the Shuttle end is perceived on board the satellite. The magnitude of the satellite response is diminished from that of the forcing impulse, and the response exhibits a broadened exponential tailoff. These are due primarily to the coupling between the tether and the satellite and its influence on an impinging impulsive wave, and not on the whole system as in the spring-mass model. The full impulse response function can be quite complex, showing repeated impulses as the tether wave bounces back and forth between the satellite and Shuttle, and having a different shape at each impingement on the satellite.

: Attenuation of electromagnetic (EM) waves by Hydrometeors such as rain, snow, clouds, and fog, ... more : Attenuation of electromagnetic (EM) waves by Hydrometeors such as rain, snow, clouds, and fog, can cause serious degradation of EM wave systems. This report describes such attenuation of radio waves to X-rays, with primary applications to communications in the microwave and millimeter wave portions of the EM spectrum.

: We have demonstrated robust algorithms for generating fits in Optical Measure Theory (OMT) to r... more : We have demonstrated robust algorithms for generating fits in Optical Measure Theory (OMT) to radiance data from the TIROS Operational Vertical Sounder (TOVS). These algorithms generated physically meaningful fits on 100% of a test set of 45 TOVS radiance scans, and were successful both on short wavelength (667/cm) and on long wavelength (2250/cm) TOVS data. The resulting OMT temperature profiles exhibit meteorological characteristics and appear to be suitable for determination of atmospheric structure parameters characterizing the large-scale vertical temperature of the atmosphere, and also suitable for generation of input data for numerical weather prediction codes. Keywords: Optical data, Atmospheric sounding, Meteorological satellites, Remote sensing, Atmospheric temperature profiles.

Automatique et ingénierie système

De nombreux signaux physiologiques peuvent etre decrits soit comme des impulsions isolees, soit c... more De nombreux signaux physiologiques peuvent etre decrits soit comme des impulsions isolees, soit comme des suites quasi periodiques d’impulsions isolees. Les ondelettes constituent des outils puissants pour la representation et l’analyse de telles formes d’ondes physiologiques, car une ondelette a une duree finie (support compact), a la difference des transformees de Fourier basees sur des sinusoides de duree infinie. Deux exemples de traitement d’un signal physiologique utilisant des bases d’ondelettes seront etudies dans cet article. Le premier exemple est la compression de signaux d’electrocardiogramme (ECG) utilisant une base d’ondelettes d’Hermite associees ; le second exemple montre la suppression des artefacts presents dans des signaux de mesure non invasive de pression sanguine (PNI).

Journal of Physics: Conference Series

We use an N-body simulation, constructed using GADGET-2, to investigate an accretion flow onto an... more We use an N-body simulation, constructed using GADGET-2, to investigate an accretion flow onto an astrophysical disk that is in the opposite sense to the disk's rotation. In order to separate dynamics intrinsic to the counter-rotating flow from the impact of the flow onto the disk, we consider an initial condition in which the counter-rotating flow is in an annular region immediately exterior the main portion of the astrophysical disk. Such counter-rotating flows are seen in systems such as NGC 4826 (known as the "Evil Eye Galaxy"). Interaction between the rotating and counter-rotating components is due to two-stream instability in the boundary region. A multi-armed spiral density wave is excited in the astrophysical disk and a density distribution with high azimuthal mode number is excited in the counter-rotating flow. Density fluctuations in the counter-rotating flow aggregate into larger clumps and some of the material in the counter-rotating flow is scattered to large radii. Accretion flow processes such as this are increasingly seen to be of importance in the evolution of multi-component galactic disks.

L-11125 jJ 1.4 111n. 6 V R()COFf RlFd9LUIlON '{tT E ' 'a REPORT SECURITY CLASj AS1i CivE MARXNGS ... more L-11125 jJ 1.4 111n. 6 V R()COFf RlFd9LUIlON '{tT E ' 'a REPORT SECURITY CLASj AS1i CivE MARXNGS Unclassified __________________080'____ :a SECURITY CLASSIFiCATIC D ARBU 17 8 2 ION/ AVAILABILITY OF REPORT N/A ji7ro~ for pi~ic rol1OiasO 20 ECLASSIFICAT!ON/I OWNGRADING SCHEDULEdi.riUif