Showing papers in "Radio Science in 1989"

Guided electromagnetic waves in chiral media

Abstract: The eigenvalue problem for the determination of propagating modes in a waveguide filled with a lossless dielectric, homogeneous, isotropic, chiral medium is formulated. The approach is illustrated by considering in detail the case of a circular waveguide.

Aspect sensitivity measurements of VHF backscatter made with the Chung‐Li radar: Plausible mechanisms

Abstract: A new technique, using beam broadening effects, has been developed to measure the aspect sensitivity of atmospheric clear air VHF radar echoes. It uses the relatively broad antenna beam of the vertical-pointing antenna of the new Chung-Li stratospheric-tropospheric radar (25°N, 120°E). The aspect sensitivity measurement using this method is straightforward and free from convolution effects introduced by the finite width of the antenna beam pattern. The observed results agree very well with other measurements. The authors propose a turbulent layer model to explain the aspect sensitivity of the echoes. In this model, anisotropic turbulence is confined to a very thin (few meters) region at the boundary of a turbulent layer. This region is responsible for the aspect sensitivity of the echoes obtained from the vertical direction. The isotropic echoes obtained from the oblique beam arise from the isotropic turbulence embedded in the center of the layer, with 30–300 m in vertical extent. We show in an appendix that the magnitude of the partial reflection coefficient is much more sensitive to the shape, length scale, and smoothness, than to the slope of the refractive index profile. Therefore the functional shape of the refractive index profile is very important for estimating the reflection coefficient. Large errors can be made when assuming, for simplicity, nonphysical profiles. For partial reflecting mechanisms to be important, steplike discontinuities, confined within length scales of the order of a meter, would be required.

On the frequency dependence of ELF/VLF waves produced by modulated ionospheric heating

Abstract: We present results of the local field strength and, for the first time, apparent source height as a function of frequency from 200 to 6600 Hz of ELF/VLF waves produced in the auroral D region through modulated RF heating using the ionospheric heating facility near Tromso, Norway. These data show, besides the normal amplitude peaks near 2, 4, and 6 kHz, the occasional presence of peaks at intermediate frequencies, especially in the right-hand circularly polarized mode. A comprehensive model has been developed to explain the data, using a full wave solution of the ELF/VLF waves produced by the self-consistently calculated source currents, and correcting for the effects of finite source current extent and multiple Earth-ionosphere reflections. The model can reproduce the amplitudes and source heights for typical sweeps. The irregular peaks occasionally seen can also be reproduced by invoking interference between the downcoming wave and a partially reflected whistler mode wave from sharp electron density gradients in the ionosphere such as sporadic E layers. The model seems to work well above about 600 Hz, but below that it increasingly underestimates the measured amplitudes.

Effect of geomagnetic disturbances on the VHF nighttime scintillation activity at equatorial and low latitudes

Abstract: VHF nighttime scintillation observations from a chain of stations extending from the equator to 21°N magnetic latitude along 84°E longitude in the Indian zone and at Lunping (23.0°N; 122°E; magnetic latitude 14.9°N subionospheric) in the Far Eastern zone, have been examined during five individual geomagnetic storms of varying strength. The usual nighttime scintillation activity during the main phase of the storm was found to be unaffected. For cases in which recovery phase of the storm starts during midnight to dawn local time sector and where the Dst value goes below −100 nT, strong postmidnight scintillations extending well beyond sunrise hours were observed at locations from 84°E to 122°E longitudes and in a wide latitudinal belt (i.e., up to 21°N magnetic latitude) particularly in the Indian zone. Under similar circumstances ionosonde data, from the equatorial region of both the zones, show considerable postmidnight increase in h′F as well as in spread F. However, at all locations, in cases of those storms whose recovery phases start during local daytime hours, on the following night scintillation and spread F activities were found to be suppressed completely or partially depending upon the maximum negative excursion of Dst. The effect was found to be more pronounced for cases in which Dst reaches below −100 nT. The same is also true for the first two categories of storms for the following night after the recovery phase sets in. The above results were further confirmed by examining Lunping scintillation and Manila ionosonde data for another 52 cases of geomagnetic storms observed during 1977 to 1983. The results are interpreted in terms of the reversal of the equatorial electric field directions under magnetically disturbed conditions.

Generalized impedance/resistive boundary conditions for electromagnetic scattering problems

Abstract: Generalized impedance boundary conditions are derived for a planar, homogeneous, magnetic dielectric slab grounded by a perfect electric conducting plane and for a magnetic dielectric coated, perfect electric conducting circular cylinder. By invoking the duality principle, generalized resistive boundary conditions are also obtained for a planar, transparent dielectric slab. In the method followed here, it is not necessary to have a prior knowledge of the Fresnel reflection and/or transmission coefficients of the configuration under study to obtain the generalized boundary conditions. The boundary conditions developed here for the planar slab recover the exact Fresnel reflection and transmission (for the transparent slab) coefficients. Finally, the boundary conditions for the coated cylinder are used to obtain a solution for the scattering of an obliquely incident plane wave field, and this result is compared with the exact solution.

A two‐component rain model for the prediction of site diversity performance

Abstract: A two-component rain attenuation model is presented for the prediction of single- and joint-(diversity) path rain attenuation statistics and for the prediction of the relative measures of diversity performance, diversity gain, and diversity improvement factor (or advantage). The model is based entirely on the use of geophysical parameters for the prediction of the effects of rain. It contains no free parameters to adjust to fit the available attenuation observations. The model is general and will handle both balanced and unbalanced diversity system design for terrestrial and satellite communication systems. Model performance was evaluated by comparison with published single-path and site diversity observations. The model performs well when tested against the International Radio Consultative Committee (CCIR) site diversity data base, providing predictions of the single-path attenuation values exceeded for specified percentages of a year with a root-mean-square difference (RMSD) from the observed values of 49% and providing predictions of the minimum attenuation value simultaneously exceeded on two links for specified percentages of a year with a RMSD of 59%.

Automatic scaling of digital ionograms

Abstract: A new system has been devised to automatically scale digital ordinary-ray ionograms. The system has been developed for use at mid-latitudes and has been trained on a full set of ionograms that are typical of the region. This paper describes the stages adopted in forming, recognizing, and scaling traces from the various ionospheric layers. Examples of the output available with this system and an error analysis of autoscaled parameters are presented as evidence of the system's capabilities.

Ionospheric incoherent scatter measurements with the middle and upper atmosphere radar: Techniques and capability

Abstract: The capability of the middle and upper atmosphere (MU) radar of Japan for incoherent scatter (IS) measurements of the ionospheric plasma is reported The MU radar is a 465 MHz pulse-modulated monostatic Doppler radar with an active phased array antenna which consists of 475 crossed yagis The observational technique employed for MU radar IS measurements is described The expected accuracy of the plasma parameter measurements is examined Sample results exhibit the designed capability of the MU radar as an IS radar, and expectations indicated by numerical simulations are confirmed 15 references

Beyond the Kirchhoff approximation

Abstract: The three most successful models for describing scattering from random rough surfaces are the Kirchhoff approximation (KA), the small-perturbation method (SPM), and the two-scale-roughness (or composite roughness) surface-scattering (TSR) models. In this paper it is shown how these three models can be derived rigorously from one perturbation expansion based on the extinction theorem for scalar waves scattering from perfectly rigid surface. It is also shown how corrections to the KA proportional to the surface curvature and higher-order derivatives may be obtained. Using these results, the scattering cross section is derived for various surface models.

Experimental probing of multipath microwave channels

Abstract: It is very difficult to relate the behavior of a multipath microwave channel to its physical description. Much information, however, can be gained from radioelectrical probing of the channel, and models can be built independent of the complex underlying physical situation. In this paper we review the various experimental techniques used to investigate the channel, their successes, and their limitations. We conclude by stressing the directions in which progress has to be made.

Geomagnetic modification of the mid‐latitude ionosphere: Toward a strategy for the improved forecasting of ƒ0F2

Abstract: The reduction of maximum usable frequency (MUF) during a geomagnetic storm represents a serious hindrance to the efficient operation of HF transmission links. This paper outlines a new approach to the modeling and forecasting of ƒ0F2 at quiet and disturbed times. Statistical analyses of ionosonde data from the Argentine Islands (65°S) have been used to define patterns for the main phase effects of mid-latitude ionospheric storms. Extended to a number of stations, these could be incorporated into algorithms to permit the forecasting of MUF for a few hours ahead and enhance the frequency management of shortwave radio communication. Data from a complete solar cycle, 1971–1981, are used to determine the errors in the forecasts and to demonstrate that a useful advantage can be attained by this method. The root-mean-square error in ƒ0F2 for 90,175 samples is 15.6%, which compares favorably with those obtained using forecasts based upon quiet time values (20.4%) or the previous day's measurements (18%). Studies of error distribution with respect to local time, season, and phase of the solar cycle highlight limitations in the technique but also indicate avenues to further improvement.

Nonlinear wave fronts and ionospheric irregularities observed by HF sounding over a powerful acoustic source

Abstract: Different wave fronts affected by significant nonlinearities have been observed in the ionosphere by a pulsed HF sounding experiment at a distance of 38 km from the source point of a 4800-kg ammonium nitrate and fuel oil (ANFO) explosion on the ground. These wave fronts are revealed by partial reflections of the radio sounding waves. A small-scale irregular structure has been generated by a first wave front at the level of a sporadic E layer which characterized the ionosphere at the time of the experiment. The time scale of these fluctuations is about 1 to 2 s; its lifetime is about 2 min. Similar irregularities were also observed at the level of a second wave front in the F region. This structure appears also as diffusion on a continuous wave sounding at horizontal distances of the order of 200 km from the source. In contrast, a third front unaffected by irregularities may originate from the lowest layers of the ionosphere or from a supersonic wave front propagating at the base of the thermosphere. The origin of these structures is discussed.

Observation of ionospheric disturbances following a 5-kt chemical explosion 2. Prolonged anomalies and stratifications in the lower thermosphere after shock passage

Abstract: We have observed unusual stratifications and prolonged spectral deformations of radio echoes after the passage through the E region of the shock wave produced by a 5-kt chemical explosion Our observations were obtained by vertical HF pulsed ionospheric sounding At the total reflection level of the sounding waves, at a horizontal distance of 30 km from the explosion, the passage of the shock wave generated a complex structure suggesting a significant deformation of the reflecting surface This effect was followed at the E region total reflection level (130 km) by a broadening of the Doppler spectra, persisting several tens of minutes Two spectral peaks appear at 05 Hz, suggesting an amplitude modulation with a period of the order of 2 s At lower virtual heights (94, 103, and 115 km), stratifications appear after the shock passage and move downward at velocities of the order of 23 m/s These layers persist for several minutes Irregularities have also been observed in the E region for over 30 min after the shock passage by a network of continuous wave HF ionospheric sounders located 250 km west of the source The origin of these structures is discussed

Direct comparison between in situ and radar measurements of temperature fluctuation spectra: A puzzling result

Abstract: Investigations of temperature fluctuation spectra of atmospheric turbulence were performed simultaneously by remote sensing and in situ measurements. A balloon borne instrumented gondola measured temperature and velocity fluctuations up to an altitude of 18 km. Spectral analysis of these data led to one-dimensional spectra. Simultaneously, the nearby stratosphere-troposphere radar “Provence” obtained vertical profiles of reflectivity for both vertical and oblique (15°) lines of sight and for various range resolutions. According to the classical interpretation, the radar echoes result from the three-dimensional spectrum of refractive index fluctuations, and its aspect sensitivity reflects the spectrum anisotropy at the half radar wavelength. We thus separately reconstructed the reflectivity profiles for the two lines of sight. For the oblique one, the in situ observed spectral level of temperature fluctuations (assumed isotropic) was converted into radar reflectivity. For the vertical measurements, a theoretical “universal” level was used, along with an isotropic conversion formula (a tentative account for the known anisotropy is given in the appendix). Comparison of the reconstructed reflectivity profiles with the observed one proves to be very puzzling. For oblique measurements, where the reconstruction process uses only widely accepted hypotheses, the observed profiles are poorly reproduced. By contrast, for vertical measurements, the proposed model leads to a good reconstruction of the shape of the reflectivity profiles. In both cases, the predicted level is always strongly overestimated. The discussion concludes that the discrepancy is real and calls for further studies of this problem.

Sheath wave propagation in a magnetoplasma

Abstract: A theory is presented for propagation of electromagnetic waves concentrated in the vacuum-gap sheath region separating a plane conductor from a cold magnetoplasma. The applied static magnetic field is parallel to the conductor and in the direction of wave propagation. The theory predicts propagation at all frequencies between zero and the upper-hybrid frequency divided by √2. Experimental results are reported in which resonances caused by the wave reflection at the end of a thin cylindrical antenna aligned with the magnetic field are used to determine the sheath-wave dispersion relation. The effect of the antenna bias, plasma density and antenna diameter on the dispersion relation are studied experimentally and the results are compared with the plane-surface theory. In spite of the difference between the theoretical and experimental geometries, qualitative agreement is noted with respect to variations in magnetic field, plasma density, sheath thickness and antenna radius. The prediction that the presence of an applied magnetic field raises the upper sheath wave cutoff frequency has been confirmed experimentally, as well as the prediction that the presence of the sheath allows propagation above the cyclotron frequency. In particular, both theory and experiment show the existence near the cyclotron frequency of a special frequency at which the sheath-wave wave number is independent of the plasma density.

On the concepts of a perfectly conducting material and of a perfectly conducting and infinitely thin screen

Abstract: The concepts of a perfectly conducting material and of a perfectly conducting and infinitely thin screen are revisited from a theoretical point of view. When both the resistivity and the thickness of a conducting film tend to zero, the properties of the infinitely thin resulting film depend on how the two vanishing parameters are linked during the limiting process.

Effects of geomagnetic pulsations on the Doppler shift of HF backscatter radar echoes

Abstract: The effects of geomagnetic pulsations on the Doppler shifts of HF backscatter radar echoes are studied. Three mechanisms are considered: (1) variation of the phase refractive index under the influence of the varying magnetic component, (2) modulation of the plasma density, and (3) electrodynamic drift of the ionosphere. It is shown that the electrodynamic drift is probably the most efficient mechanism to account for the measured Doppler shifts. In this model the Doppler shift on F propagation modes is sensitive to the eastward electric field of the wave, while E propagation modes are sensitive to the northward and eastward components with weights depending on the reflection height. Pi 2 and Pc 3 effects on the Doppler shift of backscattered echoes are presented in the experimental part. In one example of continuous pulsations it is found that the coherence of ground magnetic variations and HF echo Doppler shifts is high near specific frequencies in the spectra. The coherence of radar measurements in resolution cells about 100 km apart is nearly unity and the cross-phase spectra show very small phase differences. An example of the effects of Pi 2 pulsations on F and Es propagation modes is presented.

Spatial distribution of mid‐latitude sporadic E scintillations in summer daytime

Abstract: In the summer of 1986, 150-MHz transmissions from six US Navy Navigation Satellite System satellites were received at Kokubunji, Japan, to investigate daytime scintillations due to mid-latitude sporadic E layers From analysis of 145 satellite passes accompanied by Es scintillations, it is found that the diurnal variation of the scintillation occurrence ratio has two maxima, one before noon (1100 JST) and the other around evening (1800 JST), consistent with the diurnal variation of the occurrence probability of ƒoEs exceeding 8 MHz Some of the Es layers are not accompanied by scintillations, suggesting that they are insusceptible to the plasma instability giving rise to the irregularities Horizontal, nearly north-south scale lengths of the scintillation regions range from 20 to 600 km with a predominant peak around 50 km and have a distribution approximated by a form of L−08 (L, scale length) for L ≥ 40 km These scale lengths may represent the extents of wind shear regions producing sporadic E layers An important finding is that of the north-south asymmetry of scintillation occurrences A tilted irregularity layer model is hypothesized to account for this asymmetry

Diffraction analysis of a proposed dual‐reflector feed for the spherical reflector antenna of the Arecibo Observatory

Abstract: A proposed dual-reflector feed for the spherical reflector antenna in Arecibo is presented. This is analyzed over a large frequency range: at the lower frequencies by physical optics (PO) integration, and at the higher ones by a geometrical optic (GO) ray tracing technique described in another work. The latter calculations are extended with the transition region theory (TRT) to include edge diffraction. The results clearly demonstrate the usefulness of the time efficient TRT method. However, they also show that PO integration is important, as this has detected an underillumination of the central region of the aperture. This effect is related to a similar problem with the line feeds, but can in the present case be reduced by moving the subreflectors away from the paraxial focus.

Effects of chiral microstructure on em wave propagation in discrete random media

Abstract: In this paper we examine the role of chirality or handedness on em (electromagnetic) wave propagation in a discrete random dielectric composite. Although chirality is manifested naturally by certain molecules and polymers at optical wavelengths, it must be introduced artificially at microwave frequencies. A discrete random medium is considered in which the inclusion phase is described by new constitutive equations, D = eE + βe∇×E and B = μ H + β μ ∇ × H. The new material constant β (meters), called the chirality parameter, arises due to the introduction of handedness in the material of which the inclusions are made. The influence of this parameter on the effective phase velocity and the attenuation of em waves is studied using a multiple scattering formalism that has previously been used successfully to describe dielectric composites that were nonchiral. The role of chirality in altering the propagation characteristics is readily apparent by comparing results for chiral and nonchiral media that are otherwise identical. Numerical results are presented for spherical inclusions made of a chiral material, for various concentrations and frequencies.

Simulation of HF propagation and angle of arrival in a turbulent ionosphere

Abstract: A numerical study of waves propagating in a turbulent, stratified medium, such as the ionosphere, is performed by using the phase-screen-diffraction-layer method. In this method, phase fluctuations due to irregularities and diffraction effects are computed sequentially along the ray path. Stepping in altitude, phase changes are imbedded into phase screens, while diffraction between phase screens is computed using fast Fourier transform techniques. In the reflection region, a careful examination of the propagation is required. The derivation presented in this paper leads to improvements in the computational scheme and gives some physical insights. The resulting scheme is used to compute the angle of arrival and some of its statistics as an example of practical applications.

Aspect sensitivity of 2.66‐MHz radar returns from the mesosphere

Abstract: We have measured radar returns from the mesosphere by operating a 10-antenna phase-coherent 2.66-MHz radar as an imaging Doppler interferometer. We use the resulting three-dimensional description of the scattering to characterize the aspect sensitivity of the returns for 2 hours of data taken one spring equinox afternoon. The returned power distribution is generally circularly symmetric, so that it can be described at each altitude in terms of its variation with zenith angle, or, more accurately, with the angle relative to the center of power of the distribution. The horizontal displacement from zenith of the center of power versus altitude suggests a gravity wave-like variation with a 13-km vertical wavelength and may provide an alternative to wind profiles as a way to observe propagating gravity and tidal waves. We correct the received power for antenna gain and distance and display the dependence of scattered power on the angle measured from the center of power to describe the angular dependence of the scattering processes. These results show that the scattering generally consists of two components : an anisotropic (preferentially normal) component and an isotropic component. The anisotropic component dominates within about 6° of the center of power, while the isotropic component is seen beyond 6°. The angular width of the scattering increases with altitude; above 85 km the scattering becomes almost completely isotropic.

Enhanced backscattering phenomenon in a random continuum

Abstract: The enhancement effect of the backscattered intensity in a random continuum under the forward scatter approximation is studied. This effect is attributed to the gain of coherence recovered from random decorrelation when the backward scattered ray passes through the same random structure as the incident ray. This effect can be completely described by the enhancement factor defined as the ratio of the average backscattered power over that in the absence of multiple scattering. In this paper, this enhancement phenomenon is investigated for two kinds of sources: a point source and a beam wave, and two kinds of power spectral functions: a Gaussian spectrum and a power law spectrum. For the case of a point source two ranges of scattering strength are considered: weak scattering and strong scattering. In the limit of weak scattering, the supereikonal approximation is applied. On the other hand, in the limit of strong scattering, the statistical saturation is assumed to be reached. For the case of a beam wave, only strong scattering is considered. Computations have been made as the beam size is varied. It is found that the enhancement effect subsides as the beam size is increased.

The second-order multiple scattering theory for the vector radiative transfer equation

Abstract: Vector radiative transfer (RT) and first-order multiple scattering (FOMS) theories are often used for analyzing the depolarization effect by the random medium. However, the numerical solution for the RT theory is limited to moderate-sized particles because of the numerical stability. In order to analyze the depolarization effect by large particles near the backscattering direction, we obtained the second-order multiple scattering (SOMS) theory for the vector RT theory. The derivation was based on the second-order solution of each Fourier component of the Stokes vector. The numerical results were compared with FOMS and RT theories. It was shown that the SOMS theory was most useful for large particles and near the backscattering direction. Experimental results for large spherical particles were compared with the SOMS theory. The second-order ladder term which was included in the SOMS theory was not sufficient to explain the sharp peak observed in the backscattering direction in the depolarized intensity. The peak appears to be caused by the backscattering enhancement effect.

Natural and man‐made emissions at 1.0–5.6 MHz measured between 10 and 18 RE

Abstract: A survey has been undertaken of the 1.0- to 5.6-MHz electric field spectrum measured with the Active Magnetosphere Particle Tracer Explorers/Ion Release Module spacecraft at distances of 15–18 RE(100,000–120,000 km) from the Earth. Data from one full year of operation were considered in order to obtain complete local time coverage at the ecliptic plane. The data were further binned according to the presence or absence of intense auroral kilometric radiation (AKR) emissions at lower frequencies, in order to resolve emissions processes or interference sources that might be related to the level of AKR. For the times of intense AKR, the principal features found in this survey are (1) the occurrence statistics of the intense AKR with local time generally confirm previous AKR surveys using, for example, the ISEE satellite; and (2) the only 1- to 5.6-MHz emissions unique to the times of intense AKR are the occasional occurrences of signals at the first harmonic of the AKR. The latter observation seems likely to be natural rather than artificial, and it is interesting because so few examples of harmonic AKR have previously been published that the effect has been controversial. For the times of weak or absent AKR, the results of this survey may be summarized as follows: for times of roughly 0800–1600 LT (dayside), a nearly constant level was recorded which at 1–3.2 MHz was equal to the galactic background level, and at 3.2–5.6 MHz was above the galactic backgound level and probably was due to spacecraft-generated noise; at other local times, the same nearly constant level was sometimes recorded, but frequently bursts of noise at 3–5.6 MHz were observed. These bursts seem more likely to represent ionospheric breakthrough of man-made radiation rather than natural emissions. A comparison with previous satellite observations implies that the terrestrial background level in this frequency range may have increased considerably between 1970 and 1985.

A method for real height analysis of oblique ionograms

Abstract: A method is presented for the conversion of an HF oblique ionogram to an ionospheric density true height profile, within the usual approximations of spherical symmetry and no magnetic field. It is a lamination method for inversion of the equations for group path delay and sounder range, which are treated on equal footing, similarly to an earlier method by Rao. A reentrant procedure has been added to alleviate the problem of valley ambiguity. Use of the method and its clear improvement over other methods, which invoke further approximations, are demonstrated by calculations on oblique ionograms from known, model daytime profiles. Future directions are indicated. 6 refs.

Numerical investigations of two‐frequency mutual coherence functions of an ionospheric reflection channel

Abstract: The ionospheric communication channel is characterized by its transfer function. Each frequency component of the transfer function is given by the ratio of the wave field received to that transmitted. Many works exist which deal with the computation of the ionospheric transfer function when electron density varies smoothly. As frequently happens, the ionosphere is permeated by random irregularities; this paper is concerned with the computation of transfer functions in such a complex ionosphere. Each frequency component of the transfer function can be computed by using the phase-screen-diffraction-layer method, also called the split-step algorithm, in underwater acoustics. Stepping along the ray path, the random phase fluctuations in each slab are lumped into a phase screen. Diffractions between the phase screens are taken into account by assuming the waves are propagating in the background medium. Since different frequencies follow different paths between the transmitter and the receiver, care must be exercised by taking proper correlations between the different ray paths. The simulation scheme presented briefly in this paper introduces the concept of correlated phase screens to achieve proper correlations between ray paths at different frequencies. Although this concept is not limited by the channel bandwidth, the computations presented here are restricted to a bandwidth of the order of the coherence bandwidth of the channel. The result of the simulation is used to compute the two-frequency mutual coherence function, which is widely used to characterize a random communication channel. One very important parameter of the two-frequency mutual coherence function is the coherence bandwidth. This parameter can also be derived from a much simpler treatment of the phase screen theory. Comparisons are made and show the need for further investigations.

An aerostat-supported ELF/VLF transmitter

Abstract: A demonstration of an aerostat-supported extremely low frequency/very low frequency (ELF/VLF) transmitting antenna was performed. At ELF the vertical electric dipole (VED) antenna radiated at least 100,000 times more power than would a horizontal electric dipole having the same moment. That efficiency was achieved with an altitude of 12,500 feet (3810 m). Calculations show that the radiated power will increase as the fourth power of aerostat altitude. The tether antenna exhibited a corona onset voltage of 180 kV and was resistant to the degrading effects of ELF corona. Prolonged in-corona operation is therefore possible. The antenna was continuously tuned, despite changes in height and capacitance caused by the aerostat flight dynamics. The huge 300-H ELF tuning inductor posed no problem. Enhanced VED moments were achieved at ELF by operation at voltages up to 260 kV, 40% above the corona onset voltage. At VLF the antenna emulated a monopole that had a radiation efficiency greater than 90%. The measured bandwidths were large: 1.5 kHz at 23 kHz and 3.5 kHz at 34 kHz. The antenna height exceeded one-quarter wavelength at VLF, so the antenna could be tuned capacitively and required relatively low base voltages. At both VLF and ELF the measured fields agreed closely with predictions.

Maximum entropy estimation of Doppler shift and spectral width of VHF radar signals

Abstract: Empirical investigations show that at low and moderate signal-to-noise ratios, maximum entropy (ME) Doppler shift and spectral width estimates of VHF radar signals have significantly higher accuracies than conventional periodogram estimates with noise thresholding. The variances of the ME estimates decrease with decreasing spectral width and clearly indicate a limiting signal-to-noise ratio below which the Doppler shift estimates are dominated by cosmic and instrumental noise rather than fluctuating radar signals. Two criteria are derived empirically that yield estimates of the optimum ME prediction error filter lengths for computing the Doppler shift and spectral width of individual radar signals. At small signal-to-noise ratios the Doppler shift criterion produces variances that are close to the minimum variance bounds of spectral methods. Fast ME algorithms for computing signal power, Doppler shift, and spectral width are described. At large signal-to-noise ratios the ME Doppler shift estimator is faster than the corresponding periodogram estimator based on a fast Fourier tranform, whereas at low signal-to-noise ratios, it is slower. For computing a typical height profile of the mean radial velocity in the troposphere and lower stratosphere, the ME estimator is as fast as the periodogram estimator, whereas for a height profile of the mean spectral width, it needs approximately 3 times as much computation time as the periodogram estimator.

A space domain integral equation approach in the analysis of dielectric‐covered slots

Abstract: Coupling through dielectric-covered slots is becoming a very important excitation mechanism for antennas and arrays in microwave and millimeter wave frequencies. The characteristics of these antennas can be analyzed by solving appropriate integral equations. A space domain method of moments applied for solving these equations may complicate the existing Sommerfeld integrals even more. In this paper a combination of analytical and numerical techniques has been developed which reduces the complexity of the integrals and improves efficiency and accuracy in their computation.