HF ground-wave propagation

Top PDF HF ground-wave propagation:

A Mathematical Model of Multi Hop HF Radio Propagation

A Mathematical Model of Multi Hop HF Radio Propagation

Radio waves can travel long distances by multiple reflections off the ionosphere and off the earth with a high frequencies (HF, defined to be 3 - 30 mHz). HF ra- dio waves from the ground whose frequencies are under maximum usable fre- quency (MUF) travel further and further with each successive hop by the reflec- tions between the earth and the ionosphere again and again. And MUF has something to do with the season, time of day, and solar conditions. There is no reflection or refraction when the frequencies are over MUF. The characteristics of the reflecting surface determine the strength of the reflected wave and how far the signal will ultimately travel while maintaining useful signal integrity. The state of ocean also influences the attenuation of reflections. Ocean turbulence will affect the electromagnetic gradient of seawater, alter the local permittivity How to cite this paper: Chen, Y.R., Han,
Show more

10 Read more

Basics of Radio Wave Propagation

Basics of Radio Wave Propagation

o At MF and in the lower HF bands, aerials tend to be close to the ground (in terms of wavelength). Hence the direct wave and reflected wave tend to cancel each other out (there is a 180 degree phase shift on reflection). This means that only the surface wave remains.

10 Read more

Radio Propagation Prediction for HF Communications

Radio Propagation Prediction for HF Communications

Considering application and operational reasons, HF propagation is basically split into skywave, groundwave and near vertical incidence (NVI). Groundwave propagation is used for relatively short links over a few tens of kilometres whereas skywave propagation can be used worldwide, in suitable ionospheric conditions. Groundwave propagation is affected by the earth’s conductivity while skywave propagation is dependent on ionospheric conditions and the effects of the sun. NVI propagation is used for short range HF radio communication. The antenna plays a critical role in NVI propagation by radiating its main beam at a very high take off angle (TOA). The ionosphere is important to skywave radio propagation and provides the basis for almost all HF communications beyond LOS [7-9]. The ionosphere is also essential in optimising satellite communication systems since the satellite signals traverse the ionosphere, leading to attenuation, depolarization, refraction and dispersion as a result of scattering and frequency dependent group delay. When an HF radio wave reaches the ionosphere, it can be refracted such that it radiates back toward the Earth at some horizontal distance beyond the horizon (see Figure 1). This effect is due to refraction but it is often apparently considered to be a reflection, following Bouguer’s refraction law [10]. Figure 1 shows a simple scheme of an HF radio link demonstrating that the behaviour of different electromagnetic rays depends on the angle of elevation β. In Figure 1 rays a and b, with a small angle of incidence φ to the ionosphere, escape into space, rays c, d, e, and f, with an angle of incidence greater or equal than φ, are reflected by the ionosphere. Intercontinental broadcasting and communication on HF bands are achieved through ionospheric propagation using the skip phenomenon (as illustrated in Figure 1). Rays reflected by the ionosphere will arrive at greater distances until the elevation angle β is tangential to the ground. The skip distance, also known as the silent zone, is the minimum distance that a ray coming back from the ionosphere is reflected. Within the
Show more

8 Read more

Ultrasonic wave propagation in powders

Ultrasonic wave propagation in powders

The aim of this paper is to investigate the feasibility of using ultrasonic techniques to monitor powder caking. The theory of ultrasonic wave propagation in powder will be discussed. Two ultrasonic instruments will be described. Experimental results for monitoring caking stiffness and kinetics will be presented that illustrates the relationship between sound velocity and powder caking.

11 Read more

Wave propagation in granular materials

Wave propagation in granular materials

Two strain-gauge pressure transducers (Honeywell AB/HP ABH006PGC1B: 0-6 psi, 1.9 cm active face) are buried in the bed at the same depth but at different distances from the piston. The near transducer is typically placed 50 mm from the piston and the far transducer is located 50 mm or more beyond this point. They are held fixed from above with lab clamps and offset laterally so that the near transducer minimally obstructs the propagation of the signal to the far transducer. Holding the transducers rigidly in this manner allows for accurate determination of their distances from the piston and maintains this distance even when the bed is agitated. Further, the location of the transducer diaphragm is held stationary as the moving piston compresses the particles between the piston and the transducers. The alternative would be to bury the transducers in the bed without any link to the container, essentially “floating” them in the granular bed. Due to concerns about the repeatability of the results in such a “floating” configuration, the transducers were held fixed in all experiments. The output from the transducers is relatively weak (0-100 mV), so the signals are amplified (Sensotec Inline Amplifier 060-6827-01) before being sent to the data acquisition system. The data acquisition system consists of a computer with a data acquisition card. In the course of the experiments, two different cards were used. The earlier experiments (sinusoidal input) made use of a high speed card (Measurement Computing PCI-DAS4020/12: Max. 20 MHz sampling rate) set to take data at 100,000 samples per second. For the later experiments (pulsed input), a more moderate speed card (Measurement Computing CIO-DAS16/M1: Max. 50 kHz sampling rate) was used. Data was taken at its maximum sampling rate. Programs were written in Labview and then Matlab to interface with the acquisition cards.
Show more

122 Read more

Wave propagation in an inhomogeneous medium

Wave propagation in an inhomogeneous medium

further away from the axis takes less time to travel from source to receiver than sound energy traveling closer to the axis,.. of sound energy in the sound channel at certain ranges.[r]

88 Read more

HF wave propagation and induced ionospheric turbulence in the magnetic equatorial region

HF wave propagation and induced ionospheric turbulence in the magnetic equatorial region

The interaction of large-amplitude high-frequency (HF) radio waves with the ionospheric plasma in the vici- nity of the dip equator is signi fi cantly different than the well-studied interactions in the polar and middle lati- tude regions. Previous theoretical study [Erukhimov et al., 1997] proposed a variety of effects including the creation of quasi-periodic structures with vertical periodicity of approximately half the wavelength of the electromagnetic (EM) wave and extending horizontally along the magnetic fi eld lines due to Ohmic heating of the plasma, as well as creation of a virtual antenna at ULF/ELF/VLF frequencies by modulation of the D/E conductivity of the equatorial electrojet similar to the modulation of the polar electrojet in the auroral region [Rietveld et al., 1984, 1987, 1989; Papadopoulos et al., 1990, 2005; Moore, 2007; Payne et al., 2007]. Advantages of the equatorial over the polar electrojet modulation were noted by Papadopoulos et al. [2005], while Erukhimov et al. [1997] emphasized the excitation of low-frequency Alfvén waves in the 1 Hz frequency range. EM waves exceeding a threshold amplitude could also drive arti fi cial ionospheric turbulence (AIT) by the nonlinear interaction of ordinary (O) mode wave with the plasma electrons. In this case, since the O mode has its electric fi eld along the ambient magnetic fi eld lines when it reaches the critical layer, it could excite AIT associated with large amplitude Langmuir waves and ion acoustic waves propagating along the magnetic fi eld. Experiments at the Large Plasma Device (LAPD) at University of California, Los Angeles (UCLA) [Van Compernolle et al., 2006; Wang et al., 2016] have also shown the formation of suprathermal electrons accom- panied by the excitation of Alfvén waves when large-amplitude microwaves are injected into the plasma perpendicular to the magnetic fi eld. Particle-in-cell simulations [Tsung et al., 2007] and subsequent laboratory experiments indicate that accelerated
Show more

16 Read more

Wave and tidal power measurement using HF radar

Wave and tidal power measurement using HF radar

Wave and tidal power installations need information about the available resource and its temporal, spatial and directional variability in order to optimise siting decisions. The same information is also needed for monitoring performance of devices and for ensuring safe installation, testing and opera- tions. Wave buoys and ADCPs provide very useful information for these applications but, unless they are deployed in large numbers, cannot provide information about spatial variability. Satellite measurements are not continuously available at indi- vidual coastal locations and anyway do not provide sufficient information for detailed resource assessment studies. X-band radars can provide some continuous spatial information over ranges up to a few km from the radar although there are some limitations when it is raining. Suitably configured, phased- array [1] HF radars can provide wave measurements to over 100km, and surface currents to over 150km from the coast with spatial resolutions of typically 3-7 km down to 1km at shorter ranges [2], [3], [4]. The most commonly used radar system is the US CODAR Seasonde [5] which provides surface currents over wide ranges but, because it uses a more compact antenna than a phased-array radar, wave measurements [6] are limited to a region close to the radar sites. HF radars also have the advantage that they are normally located on the coast and thus much more easily accessible for maintenance and repairs than an instrument deployed offshore. The ability to measure both waves and currents simultaneously provides
Show more

6 Read more

Atmospheric Millimetre Wave Propagation.

Atmospheric Millimetre Wave Propagation.

Amplitude scintillations of millimetre radio waves depend on the fluctuations of the atmospheric refractive index which in turn has a time dependence which is a [r]

311 Read more

Effect of Partial Ground Plane Removal on the Radiation Characteristics of a Microstrip Antenna

Effect of Partial Ground Plane Removal on the Radiation Characteristics of a Microstrip Antenna

Microstrip antennas (MSAs) are used in modern com- munication systems due to its low cost, lightweight, and planar structure. One of the major concerns in practical MSA design is surface wave excitation. When an MSA is fabricated on a substrate, it shows significant perform- ance degradation by surface waves. On a finite ground plane, surface waves propagate until they reach an edge where they reflected back and diffracted by the edges. Particularly when the patch antenna is printed on high dielectric substrates, its back radiation pattern increases owing to surface wave diffraction from the edges of the antenna ground plane. Numerous efforts have been made earlier to reduce the surface waves on an MSA. One ap- proach is to construct an artificial periodic structure such as an electromagnetic band-gap (EBG) [1] or a conven- tional artificial soft/hard surface [2-5]. Both EBG and soft surfaces can be used to suppress surface wave propa- gation. The main difference is that soft surfaces exhibit band-gaps only one direction, but they offer the best performance in most applications of antennas. Another approach is to use a micro-machining technology [6], in which part of the substrate beneath the radiating patch is removed to realize a low effective dielectric constant of the substrate. This structure can be used to reduce the surface waves, it involves high fabrication cost. In recent years, methods for reducing the mutual coupling between antennas [7] and enhancing the front-to-back (F/B) ratio
Show more

8 Read more

New Method in Calculating the Trajectory of Sound Waves at Stratified Ocean

New Method in Calculating the Trajectory of Sound Waves at Stratified Ocean

At present, the trajectories calculation is carried out without regard to the reflected wave, [5], considering that its amplitude V at small gradients of sound speed in waveguides is much smaller than the amplitude of the transmitted wave W. However, it is assumed that water rays, that not touch the bottom and the surface, exist. In [5] there is adopted the following mechanism for the formation of water rays. During propagation resulting re- fraction angle between the beam and axis z, reaches a value of 90 ˚ and ray becomes horizontal, then ray turn to the opposite direction. In the literature, the beam turning is not explained, but simply postulated. Consider the possibility of change of the vertical component of the unit vector along the trajectory of the beam on the con- trary that is assumed in [5].
Show more

9 Read more

Ground Clearance for HF and Lower Frequency Antenna Installations

Ground Clearance for HF and Lower Frequency Antenna Installations

Abstract—Setting up ground based antennas for operation in the HF and longer wavelength bands frequently involves clearing large areas of land for the installation of ground mats to provide a high conductivity return path for the displacement currents. In moving from the cleared area to the virgin scrubland beyond, which is assumed to be forested with bushes and small trees, there is the possibility of an abrupt change in surface properties at the boundary resulting from the discontinuity in the vegetation which at longer wavelengths can be modelled as a change in surface impedance. By modelling the trees and bushes as point dipole moments, the aim of this paper is to estimate the significance of any such effect in terms of the gross physical properties of the vegetation. The result is to show that in normal circumstances the effect can be expected to be slight. A solution to this problem has application in helping determine the environmental footprint of the antenna installation, the amount of land that needs to be cleared to satisfactorily accommodate it.
Show more

10 Read more

Wave propogation scattering due to defect on thin composite plates

Wave propogation scattering due to defect on thin composite plates

The engineering structures which based on laminated composites, have a high probability of unexpected damage development during services. The damage formations must be monitored from the beginning before it headed towards structural failure which could result in substantial damage. This lead to the necessity of Structural Health Monitoring (SHM) system to be installed during the construction of laminated composite structures. However, an understanding of damage area detection and damage characteristics is essential, before a SHM system can be integrated into the structures. This article presents the effects of propagating wave propagation through an existing damage on composite plates. Theoretically, a propagating wave that started from any source will vary when crossing an area with damage. This study shows a high frequency wave propagation (kHz range) show different reactions when passing through the damaged area, compared with the low frequency wave propagation. Results of the study will lead to good damage detection method, which utilizing the available vibration source; especially for the condition monitoring of thin laminated composite structures. Keywords: SHM; wave propagation; wavelet analysis; Gl/epoxy composites.
Show more

9 Read more

Wave Propagation in Nanocomposite Materials

Wave Propagation in Nanocomposite Materials

Electromagnetic wave propagation is first analyzed in a composite material mde of chiral nano-inclusions embedded in a dielectric, with the help of Maxwell-Garnett formula for permittivity and permeability and its reciprocal for chirality. Then, this composite material appears as an homo-geneous isotropic chiral medium which may be described by the Post constitutive relations. We analyze the propagation of an harmonic plane wave in such a medium and we show that two different modes can propagate. We also discuss harmonic plane wave scattering on a semi-infinite chiral composite medium. Then, still in the frame of Maxwell-Garnett theory, the propagation of TE and TM fields is investigated in a periodic material made of nano dots immersed in a dielectric. The periodic fields are solutions of a Mathieu equation and such a material behaves as a diffraction grating.
Show more

7 Read more

Reconfigurable antennas and radio wave propagation at millimeter-wave frequencies

Reconfigurable antennas and radio wave propagation at millimeter-wave frequencies

Next, the performance of the conformal antenna arrays is investigated. The characteristics of the developed antenna prototypes are validated by the measurements using a probe fed antenna measurement system. The radiation patterns of the antenna can be changed by placing the antenna on objects with different curvature radii. Based on these results, several antenna arrays have been designed together with the switching network. Conformal antenna structures using beam switching technology can be beneficial for high-capacity communication systems. If the line-of-sight (LOS) link is blocked, the main beam direction can be controlled in order to get the highest level of the received signal through a reflection. Future work may contain studies on the antenna packaging and integration of the conformal antennas with the devices, such as smart watches, mobile phones, or radio access points. Conformal antenna design may be improved as well, by characterizing different substrate materials at mm-wave frequencies.
Show more

62 Read more

Influence of tidal sand wave fields on wind wave propagation

Influence of tidal sand wave fields on wind wave propagation

This MSc project is conducted to assess the influence of sand wave fields on wind waves. The influence on wind waves is made insightful and quantifiable by assessing the spatial variability of amplification factors of the wind wave amplitude and the corresponding near-bed orbital flow velocities inside the domain. Subsequently, a base configuration with certain parameter settings is established. From there a parameter is changed indi- vidually and the change of spatial variability due to this change in parameter is assessed. The base case is chosen based on a high influence situation, which is when the sand wave crests were orientated perpendicular to the wind wave crests. This orientation however causes a symmetric situation and might therefore be a somewhat risky base case. Also, the results may describe very case specific behaviour and therefore the generality of the results may be questioned. To come to more generic conclusions, combinations of param- eters should be assessed for their influence on wind waves and this method of assessment might therefore not be that suitable. From the results in this thesis, it became for ex- ample evident that the orientation angle has a significant influence on the effect of other input parameters (e.g. sand wave height and sand wave length), such that research to the influence of changing combinations of these parameters may result in more in-depth knowledge about the relations of parameters to the effect size. in Section 4.4 however, the influence of various combinations of orientation angles and sand wave lengths were explored, which resulted in local maxima of influence (likely Bragg Resonant cases). As this shows that local maxima exist, it may be required to enhance full exploration of the influence on wind waves by all combinations of parameters: simply assuming linear or exponential behaviour between two parameter values is therefore not sufficient. To prevent extremely numerous of combinations and with that very high computational cost, the parameter space can be reduced from currently 7 to 4 parameters by a scaling procedure of the Mild-Slope Equation to its dimensionless form. The formulation of this dimensionless Mild-Slope Equation is introduced in Appendix D.
Show more

83 Read more

Electromagnetic Wave Propagation into Fresh Water

Electromagnetic Wave Propagation into Fresh Water

Underwater communications have attracted significant interest in recent years since they have a wide range of applications including coastline protection, underwater environmental observation for exploration, off-shore oil/gas field monitoring, oceanographic data collection, autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs), etc., [1]. Reliable data moni- toring and transmission in shallow fresh water (e.g., riv- ers and lakes) have also received growing interest since they can provide information that is crucial for the local economies as well as the environment. Traditionally, underwater communications have been done through acoustic and optical systems that have certain advantages and disadvantages. For example, acoustic communica- tions are widely used in underwater environment but their performance in shallow waters is severely affected by multipath propagation. Also, the channel latency caused by the low propagation velocity in water is an- other limitation of acoustic communications. On the con- trary, laser based optical systems have significantly higher propagation speed than underwater acoustic waves. However, strong backscattering caused by sus- pended particles in water always limits the application of optical systems to very short distances.
Show more

6 Read more

Influence of high latitude anomalies on tropical climate phenomena and global climate

Influence of high latitude anomalies on tropical climate phenomena and global climate

Analysis of the energy budget has clarified the mechanism. The increase in salinity caused by the anomaly drives deep convective overturning in the Weddell Sea and Drake Passage, creating strong kinetic and potential energy anomalies. Interaction between the convection and the bottom topography allows energy exchange between the baroclinic and barotropic modes of propagation, which excites a barotropic wave response that propagates to the western Pacific. Energy continues to be transferred to the barotropic mode some time after removing the anomaly, and peaks 50 days into the integration. Upon reaching the western Pacific, interaction of the barotropic signal with the boundary and topography transfers energy to the baroclinic modes, generating an equatorward baroclinic Kelvin wave response. At the equator the signal then propagates eastwards across the Pacific. Poleward propagating Kelvin waves are formed at the eastern boundary, which in turn form westward propagating Rossby waves which carry energy westward into the basin interior. The equatorial response is very rapid (order of days) due to the involvement of the barotropic mode. Initially the amplitude of the response is weak, but the cumulative effect of a succession of barotropic waves bringing energy to the western boundary results in strong changes in SST. The strongest SST anomaly in the equatorial Pacific occurs 6 months into the model run, with an amplitude of 2 ◦ C.
Show more

166 Read more

Effect Of Terrain Features On Wave Propagation

Effect Of Terrain Features On Wave Propagation

Path-loss prediction algorithms for advanced wireless communication system planning have long considered the effect of electromagnetic propagation over buildings between the base station and subscriber. It briefly discusses the theory of reflection, diffraction and scattering over urban, suburban and rural area. This research is to present the implementation of different models for predicting the path loss between a base station antenna and mobile station antennas. The prediction of path loss is a very important step in planning a mobile radio system and accurate prediction methods are needed to determine the parameters of a radio system which will provide efficient and reliable coverage of a specified service area. The significant of this research is to determined the best emperical method for predicting the path loss between base station antenna and mobile station. This research describes three chosen path loss models which is Hatta-Okumura model, Stanford University Interim (SUI) model and Ericson model. In this research, the analyzing model above in different distance range, height of base station and height of mobile station in urban, suburban and rural environment. As a conclusion, in order to build an effective communication link the precise path loss prediction is very important. For each type of terrains (urban, suburban and rural) Hatta-Okumura model are the best path loss prediction model.
Show more

24 Read more

Directional spectra comparisons between HF radar and a wave model

Directional spectra comparisons between HF radar and a wave model

Wyatt, L.R., Green, J.J. and Middleditch, A. (2008) Directional spectra comparisons between HF radar and a wave model. In: Proceedings of the IEEE/OES/CMTC 9th working conference on Current Measurement Technology. IEEE/OES/CMTC 9th working conference on Current Measurement Technology, March 17-19 2008, Charleston, South Carolina, USA. Institute of Electrical and Electronics Engineers , pp. 211-216. ISBN 978-1-4244-1485-7

7 Read more

Show all 10000 documents...