HF Frequency Research Articles

Single-station narrowband ranging of active storm cells without lightning-type discrimination

We present a statistical technique for ranging the edges of active storms cell using a very simple narrowband receiver tuned to 1 MHz (the MW band in radio, just below HF frequencies). We show that a principle based on the “30–30 rule” can be used to define practical warning levels. From the measurements carried out in Finland, we show that the narrowband source intensities of cloud-to-ground lightning vary log-normally; this results in a ranging uncertainty of about 20%, which can be reduced if a suitable floating average is used. Based on one storm, we suggest that the differences between intra-cloud and ground-to-cloud signals at 1 MHz are small enough to make an IC–CG discrimination. Eliminating such a discrimination allows all lightning impulses to be used in the range and improves the accuracy, since more flashes are then available as inputs into the distance-estimation algorithm. Although the system is only validated against a single storm, we provide definitions by which this and other narrowband detectors could be independently verified; existing narrowband devices have not been verified in this manner, due in part to a lack of such standardized definitions.

FEM-Based Surface Wave Multimixed-Path Propagator and Path Loss Predictions

A finite element method (FEM)-based surface wave propagation prediction simulator is developed. The simulator is tested and calibrated against analytical ray-mode models that also take into account the Millington recovery effects. It successfully calculates path losses over multimixed propagation paths at MF and HF frequency bands where the surface wave contribution is significant.

Influence of syllable complexity and frequency

Functional imaging studies have delineated a „minimal network for overt speech production“, encompassing mesiofrontal structures (supplementary motor area, anterior cingulate gyrus), bilateral pre- and postcentral convolutions, extending rostrally into posterior parts of the inferior frontal gyrus (IFG) of the language-dominant hemisphere, left anterior insula as well as bilateral components of the basal ganglia, the cerebellum, and the thalamus. In order to further elucidate the specific contribution of these cerebral regions to speech motor planning, subjects were asked to read aloud visually presented bisyllabic pseudowords during functional magnetic resonance imaging (fMRI). The test stimuli systematically varied in onset complexity (CCV versus CV) and frequency of occurence (high frequency, HF, versus low frequency, LF) of the initial syllable. A cognitive subtraction approach revealed a significant main effect of syllable onset complexity (CCV versus CV) at the level of left posterior IFG, left anterior insula and both cerebellar hemispheres. Conceivably, these areas closely cooperate in the sequencing of subsyllabic aspects of the sound structure of verbal utterances. A significant main effect of syllable frequency (LF versus HF), by contrast, did not emerge. However, calculation of the time series of hemodynamic activation within the various cerebral structures engaged in speech motor control revealed this factor to enhance functional connectivity between Broca's area and ipsilateral anterior insula.

High‐frequency magnetospheric sounding at EISCAT: Some trials and their implications

The results of some recent experiments employing the EISCAT HF “heating” facility at Ramfjordmoen, near Tromsø, Norway as a HF radar transmitter are described. The motivation for the experiments was the detection of “conjugate echoes” caused by geomagnetic field‐aligned ducting of the HF wave in the magnetosphere and reflection from the magnetically conjugate ionosphere. No such echoes were detected during the experiments, which is probably to be expected from consideration of the plasma density gradients required to sustain guidance of the waves at the low HF frequencies involved. However, echoes were obtained at ranges which could be consistent with backscattering from ionospheric irregularities in the equatorial and southern auroral regions; this is similar to spread‐Doppler clutter observed by large over‐the‐horizon radar (OTHR) systems. The experimental technique is described and the results discussed with a view toward future attempts to sound the lower magnetosphere using high‐power HF transmitters.

Space weather effects on radio propagation: study of the CEDAR, GEM and ISTP storm events

Abstract. The impact of 14 geomagnetic storms from a list of CEDAR, GEM and ISTP storms, that occurred during 1997–1999, on radio propagation conditions has been investigated. The propagation conditions were estimated through variations of the MOF and LOF (the maximum and lowest operation frequencies) on three high-latitude HF radio paths in north-west Russia. Geophysical data of Dst, Bz, AE as well as some riometer data from Sodankyla observatory, Finland, were used for the analysis. It was shown that the storm impact on the ionosphere and radio propagation for each storm has an individual character. Nevertheless, there are common patterns in variation of the propagation parameters for all storms. Thus, the frequency range Δ=MOF−LOF increases several hours before a storm, then it narrows sharply during the storm, and expands again several hours after the end of the storm. This regular behaviour should be useful for the HF radio propagation predictions and frequency management at high latitudes. On the trans-auroral radio path, the time interval when the signal is lost through a storm (tdes) depends on the local time. For the day-time storms an average value tdes is 6 h, but for night storms tdes is only 2 h. The ionization increase in the F2 layer before storm onset is 3.5 h during the day-time and 2.4 h at night. Mechanisms to explain the observed variations are discussed including some novel possibilities involving energy input through the cusp.

Instrumentation and observation target of the Lunar Radar Sounder (LRS) experiment on-board the SELENE spacecraft

Abstract The Lunar Radar Sounder (LRS) on-board the SELENE lunar orbiter is currently being equipped to provide the data of subsurface stratification and tectonic features in the shallow part (several km deep) of the lunar crust, by using an FM/CW radar technique in HF (∼5 MHz) frequency range. Knowledge of the subsurface structure is crucial to better understanding, not only of the geologic history of the Moon, but also of the Moon’s regional and global thermal history of the Moon and of the origin of the Earth-Moon system. In addition to the subsurface radar experiment, LRS will provide the spectrum of plasma waves and solar and planetary radio waves in a wide frequency range from 10 Hz to 30 MHz. This paper provides the basic function parameter of the LRS system based on the final function test and proposes observation targets and data analysis that will provide important information leading to a greater understanding of the tectonics and thermal history of the Moon.

Wave and plasma measurements and GPS diagnostics of the main ionospheric trough as a hybrid method used for Space Weather purposes

Abstract. The region of the main ionospheric trough is a unique region of the ionosphere, where different types of waves and instabilities can be generated. This region of the ionosphere acts like a lens, focusing a variety of indicators from the equator of plasmapause and local ionospheric plasma. This paper reports the results of monitoring the mid-latitude trough structure, dynamics and wave activity. For these purposes, the data gathered by the currently-operating DEMETER satellite and past diagnostics located on IK-19, Apex, and MAGION-3 spacecraft, as well as TEC measurements were used. A global-time varying picture of the ionospheric trough was reconstructed using the sequence of wave spectra registered and plasma measurements in the top-side ionosphere. The authors present the wave activity from ULF frequency band to the HF frequency detected inside the trough region and discuss its properties during geomagnetic disturbances. It is thought that broadband emissions are correlated with low frequency radiation, which is excited by the wave-particle interaction in the equatorial plasmapause and moves to the ionosphere along the geomagnetic field line. In the ionosphere, the suprathermal electrons can interact with these electrostatic waves and excite electron acoustic waves or HF longitudinal plasma waves. Furthermore, the electron density trough can provide useful data on the magnetosphere ionosphere dynamics and morphology and, in consequence, can be used for Space Weather purposes.

On the Efficiency of Load Modulation in RFID Systems Operating in Real Environment

The performance of UHF-RFID systems are particularly exposed to environmental effects, such as multipath propagation and electromagnetic coupling. This sensitivity to the electromagnetic properties of the environment is peculiar of systems operating in the UHF band and therefore has not been completely investigated, since many RFID systems still work at HF frequencies. In this letter, the possible impairments due to the most usual environmental effects are considered, with particular reference to the impact on the bit error rate (BER) at the reader side in the Uplink stage of the RFID system.

Observations of structured optical emissions and particle precipitation equatorward of the traditional auroral oval

High‐sensitivity all‐sky images from south‐central Alaska show the common occurrence of slow‐moving, faint optical emissions at 557.7 and 427.8 nm well equatorward of the discrete aurora. These emissions appear over a continuum of forms ranging from homogenous longitudinal bands to bands with irregular structure on their poleward edge to widely distributed arrays of vortex‐like curls and widely spaced spots on the order of 10 km in diameter. These forms appear to correspond to various stages in the temporal evolution of nearly corotating precipitation regions populated by particles injected from more distant areas of the magnetosphere and may exhibit morphological control by an instability operating on cold plasma near the plasmapause. Although these phenomena are most common in the evening hours and typically persist for hours at a time, one case demonstrates that the features can remain throughout the night on occasion. These faint optical features appear to be colocated with regions of enhanced background counts in DMSP particle measurements and to be associated with the ring current/outer radiation belt region equatorward of subauroral polarization streams (SAPS). The features can occur in conjunction with HF and MF radio frequency absorption, at levels which are often too low to show up on routine absorption instruments such as the riometer. This has practical implications on the operation and interpretation of ionospheric interaction experiments carried out in the subauroral region.

Proposal for ionospheric tomography technique using a new HF radio source

Measurements of the line integral of the electron density along satellite-to-ground ray paths (i.e. TEC) using differential phase or Doppler of two coherent VHF/UHF signals transmitted from NNSS or GPS satellite networks have been used in the ionospheric tomography for mapping large-scale ionospheric images over region of interest. In this paper, we present theoretical studies of using a new signal source HF frequency in tomographic imaging. The initial phase problem inherent in the phase measurement can be eliminated by measuring Faraday rotations. Relative rotation on two adjacent HF frequencies is used to solve the ambiguity problem. A second-order approximation of the Faraday rotation incorporated with ray-tracing technique improves the reconstruction degradation due to ionospheric refractions. CASSIOPE is a multi-purpose small satellite that receives HF signals from ground radar facilities and it is scheduled for launch in early 2008. Simulations have demonstrated the potential applications of the ionospheric tomography in CASSIOPE/e-POP satellite experiment.

Assessment of Ventilatory Thresholds from Heart Rate Variability in Well-Trained Subjects during Cycling

The purpose of this study was to implement a new method for assessing the ventilatory thresholds from heart rate variability (HRV) analysis. ECG, VO2, VCO2, and VE were collected from eleven well-trained subjects during an incremental exhaustive test performed on a cycle ergometer. The "Short-Term Fourier Transform" analysis was applied to RR time series to compute the high frequency HRV energy (HF, frequency range: 0.15 - 2 Hz) and HF frequency peak (fHF) vs. power stages. For all subjects, visual examination of ventilatory equivalents, fHF, and instantaneous HF energy multiplied by fHF (HF.fHF) showed two nonlinear increases. The first nonlinear increase corresponded to the first ventilatory threshold (VT1) and was associated with the first HF threshold (T(RSA1) from fHF and HFT1 from HF.fHF detection). The second nonlinear increase represented the second ventilatory threshold (VT2) and was associated with the second HF threshold (T(RSA2) from fHF and HFT2 from HF.fHF detection). HFT1 , T(RSA1), HFT2, and T(RSA2) were, respectively, not significantly different from VT1 (VT1 = 219 +/- 45 vs. HFT1 = 220 +/- 48 W, p = 0.975; VT1 vs. T(RSA1) = 213 +/- 56 W, p = 0.662) and VT2 (VT2 = 293 +/- 45 vs. HFT2 = 294 +/- - 48 W, p = 0.956; vs. T(RSA2) = 300 +/- 58 W, p = 0.445). In addition, when expressed as a function of power, HFT1, T(RSA1), HFT2, and T(RSA2) were respectively correlated with VT1 (with HFT1 r2 = 0.94, p < 0.001; with T(RSA1) r2 = 0.48, p < 0.05) and VT2 (with HFT2 r2 = 0.97, p < 0.001; with T(RSA2 )r2 = 0.79, p < 0.001). This study confirms that ventilatory thresholds can be determined from RR time series using HRV time-frequency analysis in healthy well-trained subjects. In addition it shows that HF.fHF provides a more reliable and accurate index than fHF alone for this assessment.

Effects of low-dose norethindrone acetate plus ethinyl estradiol (0.5 mg/2.5 μg) in women with postmenopausal symptoms: Updated analysis of three randomized, controlled trials

Based on the potential risks of post-menopausal hormone therapy (HT) found by the Women's Health Initiative, guidelines for HT now recommend use of the lowest effective dose and shortest treatment duration consistent with individual treatment goals. Current (2003) guidance established by the US Food and Drug Administration (FDA) recommends that clinical assessments of HT include women with more frequent and more intense vasomotor symptoms than previously studied. Therefore, this analysis was conducted to further assess the efficacy of a low-dose combination of norethindrone acetate and ethinyl estradiol (NA/EE) previously assessed in dose-ranging studies, while meeting conservative FDA trial design and analysis criteria. The aim of this post hoc analysis and overview was to present data on the efficacy and tolerability of a low-dose combination-NA/EE 0.5 mg/2.5 microg-in the treatment of postmenopausal symptoms, based on data from previously published studies of NA/EE. In addition, the effects of low-dose NA/EE on bone and endometrium are briefly reviewed. Data from 3 previously published randomized, placebo-controlled trials were analyzed using current FDA guidance for the assessment of HT in postmenopausal women. Studies 1 and 2 assessed the efficacy of NA/EE at various doses, including 0.5 mg/2.5 microg, in vasomotor symptom (hot-flush [HF]) relief over 16 and 12 weeks, respectively, using self-reporting of symptom frequency and intensity (scores: 0=none; 1=mild; 2=moderate; and 3=severe) in daily diaries. Study 3 assessed the effects of NA/EE at various doses, including 0.5 mg/2.5 microg, on bone and endometrium, using quantitative computed tomography of the lumbar spine at 12 and 24 months and endometrial biopsy at 6, 12, 18, and 24 months of treatment. In all 3 studies, women were asked to record vaginal bleeding and spotting in diaries. Any adverse events were recorded in diaries and/or at clinic visits. Physical and gynecologic examinations and standard clinical laboratory testing were conducted at baseline and at appropriate follow-up visits in all 3 studies. Studies 1, 2, and 3 enrolled 219, 266, and 1265 women, respectively. Overall, in studies 1 and 2, 91% of women were white, the mean age was approximately 52 years, and mean time since last menstrual period was approximately 24 months. In study 1, NA/EE 0.5 mg/2.5 microg was associated with significant reductions from baseline in mean weekly total HF frequency from week 4 (63.6%) through week 16 (73.7%) (all, P<0.05). In study 2, the frequency of moderate or severe HFs was decreased by 61.1% at week 4 (P<0.05) and by 82.2% at week 12 (P<0.001) with NA/EE 0.5 mg/2.5 microg, and the mean intensity score was significantly lower than that with placebo at weeks 8 and 12 (both, P=0.001). In study 3, cumulative amenorrhea rates were approximately 90% in the NA/EE 0.5-mg/2.5-microg and placebo groups at 12 months. Lumbar spine bone mineral density (BMD) was maintained at 24 months with NA/EE 0.5 mg/2.5 microg but was significantly decreased from baseline, by 7.4%, in the placebo group (P<0.001). Endometrial hyperplasia was not observed in the group receiving NA/EE 0.5 mg/2.5 microg over 24 months. The tolerability of NA/EE was similar to that of placebo. The most common adverse events experienced with NA/EE were headache (15.2%), abdominal pain (10.2%), and breast pain (9.0%). The results from this post hoc analysis and overview of 3 previously published studies suggest that NA/EE 0.5 mg/2.5 microg was associated with decreased frequency and intensity of vasomotor symptoms. This dose of NA/EE was also associated with maintenance of BMD over 24 months, a significant positive effect on BMD compared with placebo. Low-dose NA/EE was also associated with cumulative amenorrhea rates comparable to those of placebo and was not associated with endometrial hyperplasia. This dose was well tolerated, with rates of adverse events generally similar to those of placebo.

Heart rate variability during high‐intensity field exercise in female distance runners

The purposes of this study were to demonstrate the transition of heart rate variability (HRV) during trials in the field and to examine the relationship between peak frequency of high-frequency band (HF) and stride frequency. Ten healthy long-distance college female runners (age 19-21 years) performed a 3000 m realistic time trial. The time-series power spectrum analysis by maximum entropy method was used to evaluate cardiac autonomic nervous activity during the race. Cross-correlation coefficients were calculated to estimate the degree of linear co-ordination between the central peak frequency of HF and stride frequency. Just after starting, the decrease in HF (0.15-1.00 Hz) and a transient increase of low-frequency band (LF)/HF were found. After that, the HF remained at a low level and LF/HF decreased sharply. These findings suggested that the parasympathetic activity was suppressed and sympathetic activity increased just after starting, and the sympathetic activity reached the saturated level according to continuation of high-intensity exercise. In spite of the significant decrease of HRV during trials, peak frequency of HF could be differentiated clearly. The cross-correlation coefficient of peak frequency of HF and stride frequency was from 0.703 to 0.868. This finding indicated that exercise rhythm reflected HRV during high-intensity running in the field.

Anharmonic vibrational spectroscopy calculations for (NH3)(HF) and (NH3)(DF): fundamental, overtone, and combination transitions

In order to study the effects of hydrogen bonding on the spectroscopic properties of (NH3)(HF) and (NH3)(DF) complexes, vibrational spectra (including fundamental, overtone and combination transitions) were calculated using the vibrational self consistent field (VSCF) method. This ab initio VSCF method accounts for both one-dimensional anharmonicity and pair-wise mode-mode couplings for all vibrational modes of the molecule, using points on the potential energy surface (at the MP2/TZP level of theory in this study). An analysis of the coupling strength shows surprisingly important coupling effects from pair-wise interactions not expected to be major. This indicates the benefits of including all pair-wise mode-mode couplings for weakly bound systems. Hydrogen bonding induces approximately 20% red shifts for the HF and DF stretch frequencies. The corrections due to anharmonicity for these modes are -6% and -5%, respectively. The anharmonic corrections for the intermolecular stretch of (NH3)(HF) and (NH3)(DF) are each about -5%. The NH3 umbrella motion has virtually no anharmonic correction in the complex, whereas free ammonia experiences a -15% correction. Also, the closing motion as well as the opening motion is restricted. The 1 + 1 combination transition of the proton stretching and intermolecular stretching modes has remarkably large intensity, larger even than the intensities for the first overtone of the proton stretching modes. The anharmonic frequency for the fundamental HF stretch, 3268 cm(-1), is in good agreement with the experimental gas phase result, 3215 cm(-1). A comparison to solid rare-gas matrix data shows that the VSCF frequencies are a consistent improvement over the harmonic approximation. The experimental data also support the use of the MP2 level of theory for the associated electronic structure calculations.

Effects of Nonpeptide and Selective V1 and V2 Antagonists on Blood Pressure Short-Term Variability in Spontaneously Hypertensive Rats

Effects of V1 (OPC-21268) and V2 (OPC-31260) vasopressin antagonists on blood pressure (BP) short-term variability were investigated in adult spontaneously hypertensive rats (SHR) under basal conditions and after the stimulation of vasopressin release by hemorrhage. BP was recorded intra-arterially and sampled at 20 Hz to be analyzed on a personal computer. BP time spectra were calculated on 30 stationary overlapping 2048 point-time series. Spectral power was estimated in total (0.00976 – 3 Hz), very low frequency (VLF: 0.00976 – 0.195 Hz), low frequency (LF: 0.195 – 0.605 Hz), and high frequency (HF: 0.8 – 3 Hz) regions. Under basal conditions a V1 antagonist (5 mg/kg, i.v.) decreased BP without affecting BP variability, while combined (V1 +V2) blockade or V2 blockade (1 mg/kg, i.v.) alone did not affect cardiovascular parameters. Mild hemorrhage (5 ml/kg per min) increased HF-BP variability, while moderate (10 ml/kg per min) and massive (15 ml/kg per min) hemorrhage did not affect it. In V1, but not V2, antagonist pre-treated SHR HF-BP increased significantly after moderate and massive hemorrhage. V1 or V2 antagonist pre-treatment also enhanced VLF-BP variability during massive hemorrhage. Moreover V1 blockade prevented hemorrhage-induced bradycardia, while V2 blockade potentiated it. It follows that in adult SHR, vasopressin buffers BP oscillations in HF and VLF frequency domains only in hypovolaemic conditions and that the modulation of the autonomic adjustment of the HR to hemorrhage by vasopressin is preserved.

Applications of the IRI in Southern Africa

The IRI forms the basis of the Single Site Location Direction Finding networks of the South African Defence Force as well as theNational Intelligence Agency. It is also used in “Path Analysis” applications where the possible transmitter coverage is calculated. Another application of the IRI is in HF frequency predictions, especially for the South African Defence Force involved in peace keeping duties in Africa. The IRI is either used independently or in conjunction with vertical ionosondes. In the latter case the scaled F2 peak parameters (foF2, hmF2) are used as inputs to the IRI. The IRI thus gets “calibrated” to extend the area covered by the ionosonde(s). The IRI has proved to be a very important tool in South Africa and Africa in the fight against crime, drug trafficking, political instability and maintaining the peace in potentially unstable countries.

Simulations and measurements of a radar cross section of a Boeing 747‐200 in the 20–60 MHz frequency band

HF and VHF low frequency bands provide a promising way to perform radar target recognition. At these frequencies, Radar Cross Section (RCS) behavior is not well known because the scattered field is due to a complex phenomenon where the interactions between the different parts of the structure have a significant contribution, which makes the prediction difficult. A wire model of a commercial Boeing 747‐200 aircraft, developed to be used with the Numerical Electromagnetic Code (NEC), is presented. The reliability of this model has been assessed by comparing the results given by NEC with the measurements made in an anechoïc chamber with a scaled aircraft, and a relatively good agreement was observed between simulations and measurements. The RCS variations of an aircraft along different flight routes have been investigated, and it is shown that it is necessary to know the flight route of the airplane to envisage target identification in spite of the use of the low frequency band.

Dynamics of hydrogen–HF complexes in helium nanodroplets

High-resolution infrared spectra are reported for ortho- and para-H2, ortho- and para-D2, and HD binary complexes with HF, solvated in helium nanodroplets. These rotationally resolved spectra provide detailed information on the rotational and vibrational dynamics of these systems. The para-H2–HF complex is observed, indicative of the unusual growth conditions present in the droplets. In the case of the HD–HF complex, extensive line broadening is observed, which we tentatively explain in terms of the near resonance between the HF and HD vibrational frequencies, which presumably facilitates vibrational relaxation.

Scale factors for CO vibrational frequencies in organometallic complexes

Corrective scale factors for computed harmonic CO vibrational frequencies for 31 organometallic complexes have been determined at the HF and B3LYP levels. The relativistic effective core potentials, SBKJC-21G (B1) and Lanl2DZ (B2), and their associated basis sets were used on the metals and the 6-31G(d) basis set was used on ligands. The optimized scale factors are 0.8516 for HF/B1, 0.8487 for HF/B2, 0.9550 for B3LYP/B1, and 0.9521 for B3LYP/B2. Whereas the generic scale factor available in the literature [J. Phys. Chem. 100 (1996) 16502] for B3LYP/6-31G(d) worked well for the metal carbonyl frequencies, the Hartree–Fock scale factors are substantially lower than earlier proposed general values. Furthermore, the scaled B3LYP frequencies exhibit a greater reliability than do HF frequencies. Both of these features can be attributed to the ability of the former method to account for the high degree of electron correlation in triply bonded CO and its variation with bonding environment.

Self-demodulation acoustic signatures for nonlinear propagation in glass beads

The present Note describes some experimental work related to the nonlinear propagation of acoustic waves in granular media such as unconsolidated glass beads. The studied nonlinear effect is a self-demodulation process performed with the operation of the so-called parametric transmitting antenna. The pump (or carrier) wave is generated by a high power ultrasonic broad-band transducer (100 kHz central frequency) which is LF (low frequency, i.e., a few kHz) amplitude modulated. As the attenuation of acoustic waves increases with frequency, only the LF demodulated wave can be transmitted. A parametric study is performed where the HF central frequency is monitored between 60 and 300 kHz. The LF demodulation profile versus the HF frequency is modified, its shape being temporally derived almost twice. A numerical analysis of the order of temporal derivation is done in the Fourier domain, its value varying from 1.25 to 2.7. Qualitative agreement with current theoretical models is described, and an advanced theoretical analysis by the same authors [Phys. Rev. E 66 (2002) 041303], taking into account absorption, nonlinearity, dispersion and scattering, is briefly discussed. To cite this article: V. Tournat et al., C. R. Mecanique 331 (2003).