Further distribution of this work must maintain attribution to the author s and the published article's title, journal citation, and DOI. Arthur H. Firester Appl. Vijayakumar and Shanti Bhattacharya Appl. Sankha S. Sarkar, Harun H. Solak, Christian David, and J. Friso van der Veen Opt. Express 22 2 Binzhi Zhang and Daomu Zhao Opt. Express 18 12 Express 21 10 Barker, E. Norrgard, N. Klimov, J. Fedchak, J. Scherschligt, and S. Kang, K.What is Microwave Communication A communication system that utilizes the radio frequency band spanning 2 to 60 GHz.
Small capacity systems generally employ the frequencies less than 3 GHz while medium and large capacity systems utilize frequencies ranging from 3 to 15 GHz. Advantages of Microwave Radio Less affected by natural calamities Less prone to accidental damage Links across mountains and rivers are more economically feasible Single point installation and maintenance Single point security They are quickly deployed 4.
Line-of-Sight Considerations Microwave radio communication requires a clear line-of-sight LOS condition Under normal atmospheric conditions, the radio horizon is around 30 percent beyond the optical horizon Radio LOS takes into account the concept of Fresnel ellipsoids and their clearance criteria. Fresnel Zone - Areas of constructive and destructive interference created when electromagnetic wave propagation in free space is reflected multipath or diffracted as the wave intersects obstacles.
Fresnel zones are specified employing ordinal numbers that correspond to the number of half wavelength multiples that represent the difference in radio wave propagation path from the direct path The Fresnel Zone must be clear of all obstructions. Microwave Link Design Process The whole process is iterative and may go through many redesign phases before the required quality and availability are achieved Interference analysis.
Frequency Planning Rain attenuation Diffractionrefraction losses Multipath propagation Miscellaneous other losses unpredictable and sporadic in character like fog, moving objects crossing the path, poor equipment installation and less than perfect antenna alignment etc This contribution is not calculated but is considered in the planning process as an additional loss One method of calculation is based on knife edge approximation.
Gas absorption Primarily due to the water vapor and oxygen in the atmosphere in the radio relay region. The absorption peaks are located around 23GHz for water molecules and 50 to 70 GHz for oxygen molecules.
Gas attenuation versus frequency Total specific gas attenuation 23GHz 1. Attenuation due to precipitation Rain attenuation is the main contributor in the frequency range used by commercial radio links Rain attenuation increases exponentially with rain intensity The percentage of time for which a given rain intensity is attained or exceeded is available for 15 different rain zones covering the entire earths surface The specific attenuation of rain is dependent on many parameters such as the form and size of distribution of the raindrops, polarization, rain intensity and frequency Horizontal polarization gives more rain attenuation than vertical polarization Rain attenuation increases with frequency and becomes a major contributor in the frequency bands above 10 GHz The contribution due to rain attenuation is not included in the link budget and is used only in the calculation of rain fading Ground Reflection Reflection on the Earths surface may give rise to multipath propagation The direct ray at the receiver may interfered with by the ground-reflected ray and the reflection loss can be significant Since the refraction properties of the atmosphere are constantly changing the reflection loss varies.
The loss due to reflection on the ground is dependent on the total reflection coefficient of the ground and the phase shift The highest value of signal strength is o obtained for a phase angle of 0 and the lowest value is for a phase angle of o The reflection coefficient is dependent on the frequency, grazing angle angle between the ray beam and the horizontal planepolarization and ground properties The grazing angle of radio-relay paths is very small usually less than 1o It is recommended to avoid ground reflection by shielding the path against the indirect ray The contribution resulting from reflection loss is not automatically included in the link budget.
When reflection cannot be avoided, the fade margin may be adjusted by including this contribution as additional loss in the link budget Link Budget The link budget is a calculation involving the gain and loss factors associated with the antennas, transmitters, transmission lines and propagation environment, to determine the maximum distance at which a transmitter and receiver can successfully operate The gains from the antenna at each end are added to the system gain larger antennas provide a higher gain.
The free space loss of the radio signal is subtracted. The longer the link the higher the loss. These calculations give the fade margin. In most cases since the same duplex radio setup is applied to both stations the calculation of the received signal level is independent of 27 direction.This first paper will address the basic concepts of modelling and poststack migration to build a foundation of knowledge for the following topic of prestack migration and seismic inversion.
The migration algorithms will be described by the three classifications of Kirchhoff, FK, and downward-continuation.Diffraction interference patterns with phasor diagrams
To assist in describing these algorithms, a number of models will be presented that start with a known geological cross-section, which are then used to create seismic data.
These models are then used to describe features of the migration algorithms. Most of the material in this paper has been taken from my course notes that are published by the SEG. They contain an extensive list of references that have not been included in this review with the intent to keep it more readable. In addition, very little mathematics are used, which are replaced by basic physical principles that are used with the intention of providing a heuristic foundation of the migration principles.
However, many algorithms run much faster, and therefore much more economically, when the vertical axis of the migration is processed in time.
These different processes are loosely referred to as depth and time migrations. The objective of a depth migration is to position the reflected energy at its true reflecting geological location, and with an amplitude that represents the reflectivity at that location.
In contrast, the objective of a time migration is to achieve the best focusing of the energy at a relative position. In structured areas, the relative lateral position deviates from the true lateral position by distances and times that may be estimated from image-rays that leave the surface in a vertical direction.
Depth migration requires considerable effort to build an accurate velocity model with axes of horizontal position and vertical depth. In some geological areas, errors in the location of the interface or in the velocity of shallow layers will cause errors in imaging deeper layers. Therefore, a number of iterations are required to build these depth velocity models, which usually commence by estimating the structure and velocities at shallower levels and then iterate to the deeper layers.
Geological constraints should be continually applied during the development of the velocity model to ensure the structural integrity. At best, these depth migrations only produce an approximation to the subsurface with the actual horizontal and vertical positioning of reflectors being limited to some percentage of accuracy. However, in structurally complex areas, it may be too difficult to build an accurate depth model and time migration may be the only option to create a subsurface image.
Time migrations typically use a simpler velocity model with axes of horizontal position and vertical time. The velocity at a particular location may be used to focus energy at that location, and will be independent of the above or surrounding structure. A processor with limited geological input can construct these velocity models, with few iterations required. The inclusion of anisotropic velocities has allowed more accurate velocity models to be constructed with accurate geological constraints, that may produce improved focusing and more accurate positioning of the migrated image.
This pipe will reflect energy from any source point on the surface to any receiver point on the surface although at this time we are only considering the energy that returns to a single receiver located at the source point. We refer to this type of reflector as a scatterpoint. Zero offset recordings are made at various locations across the surface, with their time response plotted below each surface location as illustrated in Figure 1b. The reflection energy lies on a curve defined by the two-way traveltime T.
Consider the triangle formed from the points 0, S, P with two sides defined by T 0 and T. With the units on the three sides of the triangle the same, Pythagoras theorem is then used to define the two-way travel time T by. The equation is exact for constant velocities, and is a very good approximation for horizontally layered media when the RMS velocity V rms is used in place of V.
In structured areas, the hyperbolic assumption may not be accurate enough and ray tracing or traveltime grids may be required to estimate the traveltimes. These asymptotes are illustrated in Figure 1b by the dotted lines that intersect at the surface.
All diffractions will have similar asymptotes that also intersect at the surface; a condition assumed by most migration algorithms.Toggle navigation. Help Preferences Sign up Log in. Featured Presentations. Fresnel Biprism - Fresnel Biprism Augustin-Jean Fresnel was a French physicist who contributed significantly to the establishment of the wave theory of light and optics. Fresnel Biprism Augustin-Jean Fresnel was a French physicist who contributed significantly to the establishment of the wave theory of light and optics.
Fresnel Diffraction Near-field Fresnel-Kirchhoff diffraction formula is simplified for the limiting case of Note: Huygens-Fresnel diffraction theory is an approximation of the more Fresnel zones will be introduced to estimate the diffraction pattern.
Fresnel Diffraction - Fresnel Diffraction. Fresnel integrals: Normalization is chosen to accomodate to the fact that Fresnel Diffraction. Andersen Description: May be used for projector, or printed copies note that the blue-colored Fresnel diffraction by a screen - Fresnel approximation. Now, the expression 11 reduces to Fresnel approximation.
Plane of incidence here the xy plane is the plane that contains In a similar fashionwe may deal with the situation for. Bancroft and grad students University of Calgary. Pluton and Charon: Challenge! Future Targets. Double Stars. Procyon AB. BUP 9AB To test Fresnel Imagery with real astrophysical objects.
Zona di Fresnel Serious, intent, haunted by thoughts of an early grave, Fresnel bound himself Don't lead a low ball; expect to move it up past the datums, then work it down The evanescent wave decays exponentially in transverse direction. Areas open to a Fresnel imager The average power per unit Fresnel Equations - Fresnel Reflectance.
Equations apply for metals and nonmetals. Fresnel Reflectance.After you enable Flash, refresh this page and the presentation should play. Get the plugin now. Toggle navigation. Help Preferences Sign up Log in. To view this presentation, you'll need to allow Flash. Click to allow Flash After you enable Flash, refresh this page and the presentation should play.
View by Category Toggle navigation. Products Sold on our sister site CrystalGraphics. Title: 10'3 Fresnel diffraction. Description: Fresnel zones will be introduced to estimate the diffraction pattern. Note: Huygens-Fresnel diffraction theory is an approximation of the more Tags: diffraction fresnel huygens. Latest Highest Rated. Title: 10'3 Fresnel diffraction 1 February 27, March 2 Fresnel zones Fraunhofer diffraction is a special case of Fresnel diffraction.
The integration for Fresnel diffraction is usually complicated. Fresnel zones will be introduced to estimate the diffraction pattern. Directionality of secondary emitters Inclination factor obliquity To be proved later.
Contribution from the sources inside a slice ring dS S P x r r0 O r r dS The area of the slice ring is 3 Contribution from the l th zone to the field at P 4 Sum of disturbance at P from all zones 5 Note Huygens-Fresnel diffraction theory is an approximation of the more accurate Fresnel-Kirchhoff formula.
For the first zone Divide the zone into N subzones. The phasor chain deviates slightly from a circle due to the inclination factor. When N? The integration will be very complicated.
Poisson intended to use this unusual conclusion to deny Fresnels wave description of light, but this prediction was soon verified to be true. The spot is ironically called Poissons spot.
Microwave Link Design.ppt
May have been observed by ancient people. The spot is everywhere along the axis except immediately behind the obstacle. The irradiance is not very different from that of the unobstructed wave. Modification can be either in amplitude or in phase. Example Transparent only for odd or even zones.
The first 10 odd even zones will result in an intensity of times larger compared to the unobstructed light.Transmitted radio, sound, or light waves can follow slightly different paths before reaching a receiver, especially if there are obstructions or reflecting objects between the two.
The waves can arrive at slightly different times and will be slightly out of phase due to the different path lengths. Depending on the magnitude of the phase shift, the waves can interfere constructively and destructively. The size of the calculated Fresnel zone at any particular distance from the transmitter and receiver can help to predict whether obstructions or discontinuities along the path will cause significant interference.
In any wave-propagated transmission between a transmitter and receiver, some amount of the radiated wave propagates off-axis not on the line-of-sight path between transmitter and receiver.
This can then deflect off of objects and then radiate to the receiver. However, the direct-path wave and the deflected-path wave may arrive out of phaseleading to destructive interference when the phase difference is a half-integer multiple of the period.
The nth Fresnel zone is defined as the locus of points in 3D space such that a 2-segment path from the transmitter to the receiver that deflects off a point on that surface will be n half-wavelengths out of phase with the straight-line path. These will be ellipsoids with foci at the transmitter and receiver. In order to ensure limited interference, such transmission paths are designed with a certain clearance distance determined by a Fresnel-zone analysis.
The dependence on the interference on clearance is the cause of the picket-fencing effect when either the radio transmitter or receiver is moving, and the high and low signal strength zones are above and below the receiver's cut-off threshold. The extreme variations of signal strength at the receiver can cause interruptions in the communications link, or even prevent a signal from being received at all.
Fresnel zones are seen in opticsradio communicationselectrodynamicsseismologyacousticsgravitational radiationand other situations involving the radiation of waves and multipath propagation. Fresnel zone computations are used to anticipate obstacle clearances required when designing highly directive systems such as microwave parabolic antenna systems.
Although intuitively, line-of-sight between transmitter and receiver seems to be all that is required for a strong antenna system, because of the complex nature of radio waves, obstructions within the first Fresnel zone can cause significant weakness, even if those obstructions are not blocking the line-of-sight signal path.
For this reason, it is valuable to do a calculation of the size of the 1st, or primary, Fresnel zone for a given antenna system. Doing this will enable the antenna installer to decide if an obstacle, such as a tree, is going to make a significant impact on signal strength. Fresnel zones are confocal prolate ellipsoidal shaped regions in space e.
The first region includes the ellipsoidal space which the direct line-of-sight signal passes through. The effect regarding phase-shift alone will be minimal. Therefore, this bounced signal can potentially result in having a positive impact on the receiver, as it is receiving a stronger signal than it would have without the deflection, and the additional signal will potentially be mostly in-phase.GPR is an electromagnetic EM geophysical method for high-resolution detection, imaging and mapping of subsurface soils and rock conditions.
Radar short for radio detection and ranginga system that uses short EM pulses, was fully developed in Britain for defense against enemy planes during the Second World War, although several such systems did exist in Britain, France, Germany and the USA before the War.
In addition to its numerous military and civil applications, radar is now a very important tool in ground investigations, normally from the near surface to a depth of several tens of meters. During more than two decades of development, GPR systems have become the geophysical tools that provide the subsurface window for a variety of geological, engineering, environmental and archaeological applications: determining the thicknesses of soil horizons and depth to water table; detecting air-filled subsurface cavities, buried channels and tunnels; mapping contamination plumes; investigating the condition of dam cores, masonry structures and bridge piers; detecting buried objects in archeological surveys; finding ice or permafrost thicknesses; studying the condition of the asphalt layer on roads, etc.
A typical GPR system has three main components: Transmitter and receiver that are directly connected to an antenna, and a control unit timing Fig. The transmitting antenna radiates a short high-frequency EM pulse into the ground, where it is refracted, diffracted and reflected primarily as it encounters changes in dielectric permittivity and electric conductivity.
The propagation of a radar signal depends mainly on the electrical properties of the subsurface materials see AppendixA for the electrical properties of geological media. Figure 2: Comparison of actual and GPR-imaged subsurface profiles, demonstrating the capabilities of the GPR system as an imaging tool of the shallow subsurface.
The cliff shown is in the site of Nesher Ramle quarry. Horizontal scales indicate positions of measurement stations along mapped profile; vertical scale indicates depth to the reflectors. Figure 2a: Photograph of mapped quarry cliff showing complex structure in chalky-limestone layers.
Area imaged by GPR is shown inside the dashed frame. Figure 2b: Plot image display of black-filled wiggle traces variable area, VAR. Figure 2c: Variable-intensity plot image, displayed in grey levels.
Note that most of the signals in the profiles are reflections exceptfor the two topmost events, which are two direct waves from the transmitter to the receiver, one in the air and the other one in the ground.
The amplitude of these two topmost events is considerably strong: they generally appear as two thick reflections that reduce our ability to visualize the shallowest part of the profile. The most common operation mode of GPR is the Reflection mode, whereby traces of returned waves are collected either continuously or in stations along a line, thus creating a time cross-section or a profile image of the subsurface.
Several types of radar such as frequency-modulated sine-wave radar or holographic radar are in use, but for ground investigations, the most commonly used i. GPR is the pulse radar. GPR data are displayed on printer paper or on a computer screen during acquisition i. For a given transect, the data consist of a cross-section of signal amplitudes intensities versus location along the two-way time axis and the horizontal axis.
The plot is referred to as a normal-incidence time section when the transmitter-receiver offset is negligible relative to the investigated depth and at a monostatic configuration.