HF Frequency Research Articles

Review on HF Band Vehicular Antenna with NVIS Communication

Antennas play a fundamental role in modern communication systems by facilitating signal transmission and reception across various frequency bands. Each antenna is designed for specific applications based on its operating frequency range. Whether deployed for Car-to-Car Communication or military operations, antennas serve critical functions, with information security being paramount in military contexts. Vehicular antennas typically operate within the L Band, approximately 1 to 2 GHz, relying on satellites for signal communication. However, traditional transmission through the ionosphere faces limitations, notably the skip zone, where signal reception is hindered, leading to communication failures. To address this limitation, Near Vertical Incident Skywave (NVIS) communication has been developed. NVIS utilizes antennas with very high (> 75°) radiation angles and low HF frequencies to overcome skip zone challenges. Despite existing research efforts, achieving high radiation efficiency remains a challenge, often due to improper antenna angle settings. Additionally, previous studies have overlooked the importance of narrow beam focusing, crucial for long-range communication and direction finding. This paper aims to enhance antenna performance parameters such as efficiency and gain through the adoption of specific techniques. By optimizing antenna design and configuration, we aim to improve performance for ionospheric communication, particularly in military applications involving data and voice transmission.

Development of autonomic heart rate modulations during childhood and adolescence

Autonomic control of heart rate is well known in adult subjects, but limited data are available on the development of the heart rate control during childhood and adolescence. Continuous 12-lead electrocardiograms were recorded in 1045 healthy children and adolescents (550 females) aged 4 to 19 years during postural manoeuvres involving repeated 10-min supine, unsupported sitting, and unsupported standing positions. In each position, heart rate was measured, and heart rate variability indices were evaluated (SDNN, RMSSD, and high (HF) and low (LF) frequency components were obtained). Quasi-normalized HF frequency components were defined as qnHF = HF/(HF + LF). These measurements were, among others, related to age using linear regressions. In supine position, heart rate decreases per year of age were significant in both sexes but lower in females than in males. In standing position, these decreases per year of age were substantially lowered. RMSSD and qnHF indices were independent of age in supine position but significantly decreased with age in sitting and standing positions. Correspondingly, LF/HF proportions showed steep increases with age in sitting and standing positions but not in the supine position. The study suggests that baseline supine parasympathetic influence shows little developmental changes during childhood and adolescence but that in young children, sympathetic branch is less responsive to vagal influence. While vagal influences modulate cardiac periods in young and older children equally, they are less able to suppress the sympathetic influence in younger children.

A concise high‐efficiency broadband parallel‐circuit Class‐E power amplifier

SummaryAn analysis of a single‐ended high‐efficiency broadband parallel‐circuit (PC) Class‐E power amplifier (PA) with a single reactance compensation technique (RCT) and low‐pass (LP) Chebyshev impedance transformer is first presented under low supply voltage. Analysis of the gate and drain voltage allows to determine the required optimum voltage values for the expected PA design objectives. Derivation and design procedure of the broadband matching network (MN) is provided, which can effectively filter out higher order harmonics and reduce circuit complexity. In addition, the load adaptability for this PA is studied, achieving higher than 67.87% drain efficiency (DE) in the case of load mismatch. A PC Class‐E PA based on NMOS transistor is designed and implemented to validate the analysis over a frequency band of 7–9.4 MHz. Experimental results show that the PA delivers 36.48–38.83 dBm output power with a DE of 84.91% to 86.91%, as well as achieves 83.98% to 86.28% power‐added efficiency (PAE), 19.48–21.83 dB gain with a low supply voltage of 12 V. It is also demonstrated that the experimental results exhibit good agreement with the simulation results, which prototype the radio frequency identification transmitter with broadband high‐efficiency and system compactness. To the author's knowledge, this study represents the first approach to design broadband PC Class‐E PA under low supply voltage for Internet of Things (IoT) applications at HF frequency band, such as electronic article surveillance (EAS) system.

THE EFFECT OF POSTPARTUM PHYSICAL THERAPY ON AUTONOMIC SYMPTOMS OF POST-COVID-19 SYNDROME IN WOMEN AFTER CAESAREAN SECTION WHO EXPERIENCED CORONAVIRUS DISEASE DURING PREGNANCY

Purpose: to determine the effectiveness of restoring the state of women health with post-COVID-19 syndrome who underwent cesarean section, based on the dynamics of parameters of the autonomic nervous system.
 Methods. 44 women were examined at the beginning of the late postpartum period. The control group consisted of 15 women who gave birth vaginally and did not suffer from COVID-19 during pregnancy and observation. The comparison group consisted of 15 women after cesarean section who suffered from COVID-19 during pregnancy, recovered during pregnancy and the postpartum period according to general recommendations for the rehabilitation of patients with coronavirus infection. The main group consisted of 14 women after caesarean section, suffered from COVID-19 during pregnancy, performed a proven program of physical therapy for 2 months using breathing therapeutic exercises, therapeutic exercises with elements of yoga for the muscles of the back, abdomen, upper and lower limbs, taking into account movement limitations due to a scar of the anterior abdominal wall; massage of the chest, back, abdomen; aromatherapy sessions; autogenic training sessions. They tried to restore the state of women through a balance between the sympathetic and parasympathetic departments due to the normalizing effect of the applied agents on the hormonal background and psycho-emotional state, reducing the feeling of bodily discomfort, general strengthening of the body, accelerating the recovery of the organs of the abdominal cavity and pelvis, accelerating adaptation to changes in habitual activity due to a violation daily routine and childcare needs. The condition of the women was assessed by the results of the test with isometric load, the Kerdo index, the dynamics of 24-hour heart rate variability monitoring (power in the range of low LF frequencies and high HF frequencies, LF/HF ratio, Baevsky tension index).
 Research results. During the first examination, an imbalance between the activity of the sympathetic and parasympathetic links of the autonomic nervous system towards sympathicotonia was detected in all women, which was most pronounced in women with post-COVID-19 syndrome in all parameters. During re-examination, the best result in terms of all studied parameters was found by women of the control group, who demonstrated the dynamics of the normal course of the postpartum period after vaginal delivery and increased activity of the parasympathetic department against the background of suppression of the sympathetic department. The results of the women of the main group according to the Kerdo index, tests with isometric tension, the results of daily blood pressure monitoring, which were determined, were statistically significantly better than those of the women of the comparison group, testifying to the reduction of the imbalance of the links of the autonomic nervous system and the advantages of the rehabilitation program for the correction of the autonomic signs of post-COVID-19 syndrome, which was created taking into account the physical and psychoemotional state of women after cesarean section, in comparison with the general rehabilitation program.
 Conclusions. The use of physical therapy, taking into account the imbalance of the links of the autonomic nervous system (sympatheticotonia), is pathogenetically and practically justified in the process of physical therapy of women with post-COVID-19 syndrome in the postpartum period after abdominal delivery.

Mathematical model of the photoplethysmogram for testing methods of biological signals analysis

The purpose of this study was to develop a mathematical model of the photoplethysmogram, which can be used to test methods that introduce the instantaneous phases of the modulating signals. The model must reproduce statistical and spectral characteristics of the real photoplethysmogram, and explicitly incorporate the instantaneous phases of the modulating signals, so they can be used as a reference during testing. Methods. Anacrotic and catacrotic phases of the photoplethysmogram pulse wave were modeled as a sum of two density distributions for the skew normal distribution. The modulating signals were introduced as harmonic functions taken from the experimental instantaneous phases of the VLF (0.015...0.04 Hz), LF (0.04...0.15 Hz) and HF (0.15...0.4 Hz) oscillations in the real photoplethysmogram. The spectral power in the VLF, LF, and HF frequency ranges was calculated to compare the model and experimental data. Results. The model qualitatively reproduces the shape of the experimental photoplethysmogram pulse wave and shows less than 1% error when simulating the spectral properties of the signal. Conclusion. The proposed mathematical model can be used to test the methods for introduction of the instantaneous phases of the modulating signals in photoplethysmogram time-series.

Evaluation of Variations of Qt Interval in Hypothyroidism Through Autonomic and Vascular Profiling of Cardiac Function

Background Hypothyroidism increases the risk of cardiac disease as well as autonomic dysfunction (abnormalities of the autonomic nervous system). A prospective case-control study was conducted. The purpose of this study was to compare the QT variability and vascular stiffness of SH patients. Materials and Methods:Using ABI, we compared female SH patients to age- and sex-matched normal controls in order to better understand their condition. HF (0.15–0.4 Hz) and LF (0.003–0.04 Hz) frequencies were investigated spectroscopically. The HF represents vagal regulation, whereas the LF reflects both vagal and limbic regulation. Result In this case research 32 women participated who were 28.02+7.8 years old and 28 women in control group who were age-matched (30.2+ 7.1). Patients had greater QT variability indices (P = 0.01) than healthy controls. Cases showed a higher LF/HF ratio than controls (P = 0.03), which was statistically significant. It was shown that there was a 0.02 point difference in left-right ABI between patients and controls. Conclusion In these circumstances, sympathetic activity predominated over parasympathetic activity, with less of the latter. QTvi may be beneficial in determining the risk of cardiovascular mortality in this particular patient population.

Analysis of Relevant Energetic Proportions for Elint Activities in the HF Frequency Band

Intelligence as a warfighting function is a continuous process. It is carried out before planning the mission, during the preparation of the mission, and during its execution as well. The results of the intelligence warfighting function enable commanders to predict the course of action for potential threats and facilitate the commander´s decision-making process. If intelligence information is derived from foreign communication (Communication Intelligence) and noncommunication (Electronic Intelligence) electromagnetic radiations by other than intended recipients, we are speaking about Signals Intelligence. Communication and noncommunication foreign emitters might be also denoted as Radio-Electronic objects. This paper presents an algorithm for analysis of some relevant energetic proportions of a skywave propagation, with consideration of Electronic Intelligence in the high-frequency band. In terms of Electronic Intelligence, Radio-Electronic object is represented by the Over-The-Horizon radar.

Validation of a Pseudospectral Time-Domain (PSTD) Planetary Radar Sounding Simulator With SHARAD Radar Sounding Data

In a recent study, a 2-D pseudospectral time-domain (PSTD) full-wave simulator was developed and demonstrated to be capable of efficiently solving large-scale low-frequency (e.g., HF) electromagnetic scattering problems, for example, on the application of radar sounding simulations of planetary clutter and subsurfaces. In this article, the 2-D PSTD simulator is applied to simulate a domain as large as 4000 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula> (along-track) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 1666.67\,\,\lambda $ </tex-math></inline-formula> (cross-track) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 33.33\,\,\lambda $ </tex-math></inline-formula> (depth) with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda =15$ </tex-math></inline-formula> m at an HF frequency of 20 MHz. To accomplish the goal, the simulator is further improved to efficiently model/simulate large cross-track slices of dielectric scenes by allowing nonuniform grid sampling in horizontal (lateral) and vertical directions, and the cross-track results are then stitched together along the track to form the simulated radargram. By combining the SHAllow RADar (SHARAD) viewing geometry and Mars Orbital Laser Altimeter (MOLA) digital elevation model (DEM), we simulate SHARAD returns at three different sites on Mars: one at the North Pole and two at Oxia Planum. At all three sites, the PSTD simulated radargrams are compared with measured SHARAD radargrams. Through power-level calibration and reference time adjustment, the PSTD simulated power estimates are further validated by comparing with real power observations from SHARAD with a 5-dB uncertainty and Pearson correlation coefficient of 0.3–0.4 (a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$p$ </tex-math></inline-formula> -value on the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-9}$ </tex-math></inline-formula> ), which justifies the use of the 2-D PSTD simulator for emulating surface clutter in planetary radar sounding. This simulator is open source and can be easily modified to support radar sounding simulations in support of other planetary missions with radar sounding instruments.

A vector theory for forward propagation in a structured ionosphere with surface reflections

In a previous publication the forward propagation equation was generalized to accommodate vector propagation at HF frequencies. Solutions obtained with split-step integration were demonstrated for unbounded propagation and for a Chapman layer above a plane perfectly conducting boundary. This paper extends the solution to accommodate reflections from a smoothly varying boundary. A complete solution requires calculation of induced sources on the boundary surface. The fields from the induced sources, when combined with the incident field, satisfy the boundary conditions for the incident field vector components. Calculation of the induced sources can be incorporated in the split-step integration of the forward propagation equation. Alternatively, a variant of image theory reproduces the essential characteristics of the more computationally intensive boundary-integral methods. The paper concludes with ray-trace comparisons, which reveal a persistent but correctable bias.

SC-FDE Layer for Sensor Networks in Remote Areas Using NVIS Communications

Despite high costs and lengthy deployments, satellite communications have traditionally been used to provide coverage in remote areas. However, given the fact that there is no radio infrastructure available in these areas, Near Vertical Incidence Skywave (NVIS) technology has positioned itself as an attractive alternative to communicate with low-power nodes in remote areas. This type of communication works in the HF frequency range complying with STANAG and MIL-STD standards, which define a physical layer for scenarios that differ from NVIS and low-power communication. The purpose of this paper was to present the definition of a new communication physical layer based on single-carrier frequency-domain equalization (SC-FDE) based on these standards but adapted to the ionospheric communication channel. This physical layer was compared to an OFDM-based layer from a previous study. The experiments performed show that this new approach achieves better results than OFDM in terms of a higher signal quality with a higher specific BER probability. Finally, this layer was also used in the theoretical design of an NVIS gateway to link sensor network devices spanning large-scale remote areas in a secure manner in the context of ubiquitous sensor networks (USN).

Low Heart Rate Variability Predicts Stroke and Other Complications in the First Six Postoperative Months After a Hip Fracture Operation.

Background: One-year mortality after hip fractures is underestimated and is reported as 25%. An improved risk stratifying could contribute to a better follow up of these patients. Heart Rate Variability (HRV) is an easy point-of-care investigation and is been used in cardiology, endocrinology, and perioperative care. This observational study intended to explore relevant associations between HRV parameters and 6-months mortality and morbidity after a hip fracture.Methods: One hundred and sixty-five patients admitted to two hospitals were included, and short-time HRV measurements (5 min, and 10 min at the two hospitals, respectively) were obtained. Mortality data were gathered by means of the Norwegian central address register. Patients, close relatives of patients, and in some cases their general physicians or nursery home physicians were interviewed 6 months postoperatively regarding the incidence of pneumonia, cardiac events, or stroke.Results: One and hundred fifty-seven (95.2%) patients were followed up after 6 months post-surgery. Twenty-one (13%) died during this period. Twenty patients (13%) developed pneumonia, eight (5 %) stroke, and four (2%) myocardial infarction. No HRV parameter was associated with 6-month general mortality. However, patients who developed stroke had significantly lower High Frequency Power (HF, p < 0.001) and lower Very Low Frequency Power (VLF, p = 0.003) at inclusion compared to patients without complications. Patients who developed pneumonia had at the inclusion lower root mean square of successive differences (RMSSD, p = 0.044). Patients with a history of coronary heart disease (n = 41) showed a mortality of 7%. Mortality in this group was associated with standard deviation of beat-to-beat intervals (SDNN, p = 0.006), Total Power (TP, p = 0.009), HF (p = 0.026), and Low Frequency Power (LF, p = 0.012). Beta-blocker intake was associated with lower heart rate, but not with differences in HRV parameters.Conclusion: In this exploratory study, we present for the first-time significant associations between different preoperative HRV parameters and stroke, myocardial infarction, and pneumonia during a 6-month period after hip fracture. HRV might be a simple and effective tool to identify patients at risk that would warrant better follow-up.

Peculiarities of Heart Rate Variability Changes in Random-Bred Male Rats during Transition into Anesthetic Sleep under Stimulation of Central Neurotransmitter Systems.

General anesthesia with Nembutal (40 mg/kg) dramatically decreased the power of all waves of HRV spectrum in rats, especially in LF and VLF frequency bands, but the HR and respiration rate were little changed. At this, individual spectral peaks in HF range were observed at the same frequencies (1.3-1.5 Hz), which are characteristic of the wakeful state. Preliminary stimulation of noradrenergic system with maprotiline (10 mg/kg) increased the power of HF waves and elevated the respiratory rate in narcotized rats in comparison with the control values, although it did not shift the spectral peak at 1.5 Hz in frequency axis. Preliminary stimulation of cholinergic system with galantamine (2 mg/kg) somewhat decreased the power of HF waves and respiratory rate in narcotized rats (in comparison with the control values); additionally, it shifted HF peak to 1.1-1.4 Hz. Activation of serotonergic system with 5-hydroxytryptophan (50 mg/kg) and fluoxetine (3 mg/kg) decreased the HR, the power of HF waves, and respiratory rate in narcotized rats. It also shifted the spectral peak of HF waves to 0.9-0.95 Hz. Preliminary stimulation of dopaminergic system with L-DOPA (20 ml/kg) and amantadine (20 ml/kg) increased the power of VLF waves in narcotized rats in comparison with the control values. Numerous peaks appeared in HF (1.1-1.2 Hz) and VLF frequency bands. Generally, preliminary stimulation of serotonergic or dopaminergic systems markedly affects the neural activity under following general anesthesia: first aggravates the effect of anesthesia on vital centers in CNS, whereas second weakens the effect of anesthesia at the suprasegmental level of neural control.

A vector theory for forward propagation in a structured ionosphere

The scalar forward propagation equation, most often with the parabolic approximation, has been used extensively for simulating ionospheric radio propagation. More recently, the formalism has been applied at HF frequencies where external magnetic field effects must be accommodated. This paper presents a generalization of the forward propagation equation to accommodate vector fields with forward-marching integration. The ramifications for characteristic mode identification are explored.

ICHealth: large-scale digital health data on evidence-based cardiovascular medications, hospitalizations and epidemiological characteristics of heart failure patients in Brazil

Abstract Introduction Heart failure (HF) is one of the main causes of morbimortality in Brazil. However, little is known about the characteristics of the Brazilian HF population assisted at the community level, including the use of HF medications and frequency of hospitalizations. ePHealth is an app-based large-scale digital data collection platform that currently has health data on more than 1.5 million people. Purpose To assess the use of evidence-based HF medications, hospitalizations and epidemiological characteristics of HF patients in Brazil. Methods This is an observational retrospective study of the digital health data collected using the ePHealth platform on individuals, who self-reported a diagnosis of HF. Data collected included sociodemographics, clinical data, risk factors, comorbidities, medications and hospitalizations. Results Data collected at more than 40,000 home visits on 5907 individuals with HF were analysed. Majority of them were female, aged 55 to 75 years and brown (Figure 1). About 36% were married, 20% were illiterate, 65% were retired and 66% earned ≤2 minimum wages. Mean BMI was 26.7 kg/m2 (SD 5.9), risk factors and comorbidities were frequent (Figure 1). The use of HF medications was very low (Table 1). There were 575 hospitalizations (9.7%), due to the main following reasons: probable or definite heart failure decompensation (89, 15.5%), heart attack (60, 10.4%), cardiovascular procedures (54, 9.4%) and stroke (42, 7.3%). Conclusion This data suggests that community-level use of evidence-based cardiovascular medications in a population of individuals with HF in Brazil is very low and that hospitalizations are frequent. This study also provides a better understanding of the characteristics of a population of HF individuals, using large-scale real-world data collected on a community-level via an entirely digital platform. ePHealth is a disruptive platform able to provide data on the burden of HF and other cardiovascular diseases, informing decisions on implementation of prevention and management programmes. Figure 1. HF population characteristics Funding Acknowledgement Type of funding source: Private company. Main funding source(s): ePHealth

Assessing the detectability of Europa’s eutectic zone using radar sounding

Radar sounding is a geophysical method capable of directly imaging subsurface interfaces within the ice shell of the icy moons, including Jupiter’s moon, Europa. For this reason, both the European Space Agency’s JUpiter ICy moons Explorer and the National Aeronautics and Space Administration’s Europa Clipper missions have ice penetrating radar sounders in their payloads. In addition to the ice–ocean interface and shallow water lenses, liquid water in the eutectic zone of Europa’s ice shell could also be a target for radar sounding investigations. However, the wide range of possible configurations for eutectic-zone water bodies and the overlying ice make their absolute echo strength difficult to predict. To address this challenge, we employ a suite of simple water configurations and scattering models to bound the eutectic detectability in terms of its effective reflectivity. We find that, for each configuration, a range of physically plausible eutectic parameters exist that could produce detectable echoes, with effective reflectivity values greater than -50 dB at HF or VHF frequencies.

Oxybutynin vs Placebo for Hot Flashes in Women With or Without Breast Cancer: A Randomized, Double-Blind Clinical Trial (ACCRU SC-1603).

BackgroundHot flashes (HFs) negatively affect quality of life among perimenopausal and postmenopausal women. This study investigated the efficacy of oxybutynin vs placebo in decreasing HFs.MethodsIn this randomized, multicenter, double-blind study, women with and without breast cancer with 28 or more HFs per week, lasting longer than 30 days, who were not candidates for estrogen-based therapy, were assigned to oral oxybutynin (2.5 mg twice a day or 5 mg twice a day) or placebo for 6 weeks. The primary endpoint was the intrapatient change from baseline in weekly HF score between each oxybutynin dose and placebo using a repeated-measures mixed model. Secondary endpoints included changes in weekly HF frequency, HF-related daily interference scale questionnaires, and self-reported symptoms.ResultsWe enrolled 150 women. Baseline characteristics were well balanced. Mean (SD) age was 57 (8.2) years. Two-thirds (65%) were taking tamoxifen or an aromatase inhibitor. Patients on both oxybutynin doses reported greater reductions in the weekly HF score (5 mg twice a day: −16.9 [SD 15.6], 2.5 mg twice a day: −10.6 [SD 7.7]), placebo −5.7 (SD 10.2); P < .005 for both oxybutynin doses vs placebo), HF frequency (5 mg twice a day: −7.5 [SD 6.6], 2.5 mg twice a day: −4.8 [SD 3.2], placebo: −2.6 [SD 4.3]; P < .003 for both oxybutynin doses vs placebo), and improvement in most HF-related daily interference scale measures and in overall quality of life. Patients on both oxybutynin arms reported more side effects than patients on placebo, particularly dry mouth, difficulty urinating, and abdominal pain. Most side effects were grade 1 or 2. There were no differences in study discontinuation because of adverse effects.ConclusionOxybutynin is an effective and relatively well-tolerated treatment option for women with HFs.

An Enhanced Multiplication of RF Energy Harvesting Efficiency Using Relay Resonator for Food Monitoring.

Recently, radio frequency (RF) energy harvesting (RFEH) has become a promising technology for a battery-less sensor module. The ambient RF radiation from the available sources is captured by receiver antennas and converted to electrical energy, which is used to supply smart sensor modules. In this paper, an enhanced method to improve the efficiency of the RFEH system using strongly coupled electromagnetic resonance technology was proposed. A relay resonator was added between the reader and tag antennas to improve the wireless power transmission efficiency to the sensor module. The design of the relay resonator was based on the resonant technique and near-field magnetic coupling concept to improve the communication distance and the power supply for a sensor module. It was designed such that the self-resonant frequencies of the reader antenna, tag antenna, and the relay resonator are synchronous at the HF frequency (13.56MHz). The proposed method was analyzed using Thevenin equivalent circuit, simulated and experimental validated to evaluate its performance. The experimental results showed that the proposed harvesting method is able to generate a great higher power up to 10 times than that provided by conventional harvesting methods without a relay resonator. Moreover, as an empirical feasibility test of the proposed RF energy harvesting device, a smart sensor module which is placed inside a meat box was developed. It was utilized to collect vital data, including temperature, relative humidity and gas concentration, to monitor the freshness of meat. Overall, by exploiting relay resonator, the proposed smart sensor tag could continuously monitor meat freshness without any batteries at the innovative maximum distance of approximately 50 cm.

Improvement of intermodal frequency stability of two-mode HeNe laser

A method of improving intermodal frequency stability of two-mode helium-neon laser has been developed. The approach is based on using in the stabilization circuit a signal processor for real-time analysis of data from an LF amplitude and an HF frequency detector. We propose to use the signal from the LF detector in the initial phase of the stabilization process and the signal from the HF detector afterwards. Experiments proved that with the suggested design it is possible to obtain frequency stability of the intermodal frequency better than 100 Hz or 2*10-10 for 1000s averaging. In the paper we also show the influence the intermodal frequency stability on the absolute stability of the laser frequency.

Effects of HF frequency on plasma characteristics in dual-frequency helium discharge at atmospheric pressure by fluid modeling

On the basis of the fluid theory and the drift–diffusion approximation, a numerical model for dual-frequency atmospheric pressure helium discharge is established, in order to investigate the effects of the high frequency source (HF) on the characteristics of dual-frequency atmospheric pressure helium discharge. The numerical results showed that the electron heating rate increases with enhancing HF frequency, as well as the particles densities, electron dissipation rate, current density, net electron generation and bulk plasma region. Moreover, it is also observed that the efficient electron heating region moves when the HF frequency has been changed. The plasma parameters are not linear change with the HF frequency linearly increasing.

A HF Frequency Prediction Method of Engineering Calculation in the Asia Pacific Region

On the premise of comprehending the ionosphere prediction method in the Asia Oceania region, HF frequency prediction method recommended by the International Telecommunication Union (ITU) is improved in this paper, and the engineering calculation method of predicting HF frequency in the Asia Pacific region is concluded and summarized. Comparing with the ITS (the Institute for Telecommunication Science) software’s predicting result, the method is improved to some extent in time availability and SNR in the receiving point.