Africa's progress against COVID-19 has become another reason to respect ...
In 5G communication systems, the phased-array antenna is one of the lead front-end components that defines massive multiple-input, multiple-output (MIMO) performance.
... In addition, systems that operate at millimeter-wave (mmWave) frequencies offer benefits that include small antenna sizes and more available bandwidth.. .
A waveguide is an electromagnetic feed line that is used for high frequency signals. Waveguides conduct microwave energy at lower loss than coaxial cables and are used in microwave communications, radars and other high frequency applications.
The waveguide must have a certain minimum cross section, relative to the wavelength of the signal to function properly.
If wavelength of the signal is too long (Frequency is too low) when compared to the cross section of the waveguide, the electromagnetic fields cannot propagate.
The lowest frequency range at which a waveguide will operate is where the cross section is large enough to fit one complete wavelength of the signal.
Geometrically speaking there are three types of waveguides – Rectangular Waveguides, Double Rigid Waveguides and Circular Waveguides.
A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction.
Without the physical constraint of a waveguide, wave amplitudes decrease according to the inverse square law as they expand into three dimensional space.
There are different types of waveguides for different types of waves. The original and most common meaning is a hollow conductive metal pipe used to carry high frequency radio waves, particularly microwaves.
Dielectric waveguides are used at higher radio frequencies, and transparent dielectric waveguides and optical fibers serve as waveguides for light.
In acoustics, air ducts and horns are used as waveguides for sound in musical instruments and loudspeakers, and specially-shaped metal rods conduct ultrasonic waves in ultrasonic machining.
•distances with low attenuation and a wide usable range of wavelengths.
•In a microwave oven a waveguide transfers power from the magnetron, where waves are formed, to the cooking chamber.
In a radar, a waveguide transfers radio frequency energy to and from the antenna, where the impedance needs to be matched for efficient power transmission (see below).
•Rectangular and circular waveguides are commonly used to connect feeds of parabolic dishes to their electronics, either low-noise receivers or power amplifier/transmitters.
•Waveguides are used in scientific instruments to measure optical, acoustic and elastic properties of materials and objects.
The waveguide can be put in contact with the specimen (as in a medical ultrasonography), in which case the waveguide ensures that the power of the testing wave is conserved, or the specimen may be put inside the waveguide (as in a dielectric constant measurement[3]),
so that smaller objects can be tested and the accuracy is better.
•Transmission lines are a specific type of waveguide, very commonly used.
Waveguides can be constructed to carry waves over a wide portion of the electromagnetic spectrum, but are especially useful in the microwave and optical frequency ranges.
Depending on the frequency, they can be constructed from either conductive or dielectric materials. Waveguides are used for transferring both power and communication signals.
Waveguides used at optical frequencies are typically dielectric waveguides, structures in which a dielectric material with high permittivity, and thus high index of refraction, is surrounded by a material with lower permittivity.
The structure guides optical waves by total internal reflection. An example of an optical waveguide is optical fiber.
Other types of optical waveguide are also used, including photonic-crystal fiber, which guides waves by any of several distinct mechanisms. Guides in the form of a hollow tube with a highly reflective inner surface have also been used as light pipes for illumination applications.
The inner surfaces may be polished metal, or may be covered with a multilayer film that guides light by Bragg reflection (this is a special case of a photonic-crystal fiber).
One can also use small prisms around the pipe which reflect light via total internal reflection [2]—such confinement is necessarily imperfect,
however, since total internal reflection can never truly guide light within a lower-index core (in the prism case, some light leaks out at the prism corners).
An acoustic waveguide is a physical structure for guiding sound waves. A duct for sound propagation also behaves like a transmission line. The duct contains some medium, such as air, that supports sound propagation.
Sound synthesis uses digital delay lines as computational elements to simulate wave propagation in tubes of wind instruments and the vibrating strings of string instruments.
•Circular polarization
•Electromagnetic wave
•Linear polarization
•Orthomode transducer
An orthomode transducer (OMT) is a waveguide component. It is commonly referred to as a polarisation duplexer. Orthomode transducers serve either to combine or to separate two orthogonally polarized microwave signal paths.
One of the paths forms the uplink, which is transmitted over the same waveguide as the received signal path, or downlink path.
Such a device may be part of a VSAT antenna feed or a terrestrial microwave radio feed; for example, OMTs are often used with a feed horn to isolate orthogonal polarizations of a signal and to transfer transmit and receive signals to different ports.
For VSAT modems the transmission and reception paths are at 90° to each other, or in other words, the signals are orthogonally polarized with respect to each other.
This orthogonal shift between the two signal paths provides approximately an isolation of 40 dB in the Ku band and Ka band radio frequency bands.
Wherever there are two polarizations of radio signals (Horizontal and Vertical), the transmitted and received radio signal to and from the antenna are said to be “orthogonal”.
This means that the modulation planes of the two radio signal waves are at 90 degrees angles to each other.
The OMT device is used to separate two equal frequency signals, of high and low signal power.
The feedhorn also selects the polarization of the waves to be received, which helps to attenuate unwanted signals from adjacent channels and transponders, and from other communications satellites at nearby orbital positions.
This can be either horizontal or vertical if the polarization is linear, or clockwise or counterclockwise (also called left- and right-handed) if it is circular.
A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam.
A cubit [shall be] the length thereof, and a cubit the breadth thereof; foursquare shall it be: and two cubits [shall be] the height thereof: the horns thereof [shall be] of the same.
And I will bring the blind by a way [that] they knew not; I will lead them in paths [that] they have not known: I will make darkness light before them, and crooked things straight. These things will I do unto them, and not forsake them.
"These things will I do unto them, and not forsake them."


noun: hem; plural noun: hems

1the edge of a piece of cloth or clothing which has been turned under and sewn.

"the hem of her dress"









knowledge; plural noun: knowledges


facts, information, and skills acquired through experience or education; the theoretical or practical understanding of a subject.

"a thirst for knowledge"
A brain abscess is a pus-filled swelling in the brain. It usually occurs when bacteria or fungi enter the brain tissue after an infection or severe head injury.
•viral infections – several common viruses can spread to the brain and cause encephalitis in rare cases, including the herpes simplex virus (which causes cold sores and genital herpes) and the chickenpox virus
•a problem with the immune system, the body's defence against infection – sometimes something goes wrong with the immune system and it mistakenly attacks the brain, causing it to become inflamed
•bacterial or fungal infections – these are much rarer causes of encephalitis than viral infections
One early strategy was to deliberately cause a mild infection with the unmodified pathogen.
Infection is the leading cause of death in the human population.
This was the principle of variolation, in which the inoculation of a small amount of dried material from a smallpox pustule would cause a mild infection followed by long-lasting protection against reinfection.
However, infection following variolation was not always mild: fatal smallpox ensued in about 3% of cases, which would not meet modern criteria for safety.
Jenner's achievement was the realization that infection with a bovine analogue of smallpox, vaccinia (from vacca—a cow), which caused cowpox, would provide protective immunity against smallpox in humans without the risk of significant disease.
He named the process vaccination,
and Pasteur, in his honour, extended the term to the stimulation of protection to other infectious agents.
Humans are not a natural host of vaccinia, which establishes only a brief and limited subcutaneous infection but contains antigens that stimulate an immune response that is cross-reactive with smallpox antigens and thereby confers protection from the human disease.
This established the general principles of safe and effective vaccination, and vaccine development in the early part of the 20th century followed two empirical pathways.
The first was the search for attenuated organisms with reduced pathogenicity that would stimulate protective immunity;
the second was the development of vaccines based on killed organisms and, subsequently, purified components of organisms that would be as effective as live whole organisms.
Killed vaccines were desirable because any live vaccine, including vaccinia, can cause lethal systemic infection in the immunosuppressed.
Immunocompromised or immunosuppressed means having a weakened immune system.
Immunocompromised patients have a reduced ability to fight infections and other diseases. This may be caused by certain diseases or conditions, such as AIDS, cancer, diabetes, malnutrition, and certain genetic disorders.

1lack of proper nutrition, caused by not having enough to eat, not eating enough of the right things, or being unable to use the food that one does eat."nearly 67% of the country's population suffers from malnutrition"
poor diet
inadequate diet
unhealthy diet
lack of food
Titanium is a chemical element with the symbol Ti and atomic number 22. It is a lustrous transition metal with a silver color, low density, and high strength. Titanium is resistant to corrosion in sea water, aqua regia, and chlorine.
It is paramagnetic and has fairly low electrical and thermal conductivity compared to other metals.

Titanium is superconducting when cooled below its critical temperature of 0.49 K.
Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field.
Magnetoencephalography (MEG) is a functional neuroimaging technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain, using very sensitive magnetometers.
Arrays of SQUIDs (superconducting quantum unit interference devices) are currently the most common magnetometer, while the SERF (spin exchange relaxation-free) magnetometer is being investigated for future machines.
Applications of MEG include basic research into perceptual and cognitive brain processes, localizing regions affected by pathology before surgical removal, determining the function of various parts of the brain, and neurofeedback.
This can be applied in a clinical setting to find locations of abnormalities as well as in an experimental setting to simply measure brain activity.
Electric current produces a magnetic field.
is that paramagnetism is (physics) the tendency of magnetic dipoles to align with an external magnetic field; materials that exhibit this tendency become temporary magnets while superparamagnetism is (physics) paramagnetism that occurs below the curie temperature or the.
Temporary magnets can vary in composition, as they are essentially any material that behaves like a permanent magnet when in the presence of a magnetic field.
In vivo and in vitro toxicological studies confirm that for low solubility, low toxicity materials such as TiO2, carbon black and iron oxide, ultrafine particles are more toxic and inflammogenic than fine particles [reviewed in [7]].
In such studies the NPs generate reactive oxygen species (ROS) to a greater extent than larger particles leading to increased transcription of pro-inflammatory mediators via intracellular signaling pathways including calcium disturbances and oxidative stress [7]
Hepatitis C virus (HCV) replication is associated with the endoplasmic reticulum (ER), where the virus causes stress.
Cells cope with ER stress by activating an adaptive program called the unfolded protein response (UPR), which alleviates this stress by stimulating protein folding and degradation in the ER and down-regulating overall protein synthesis.
Recent work suggests that HCV also alters ER calcium homeostasis, inducing oxidative stress. Future progress in understanding the control that HCV exerts over the ER will provide insight into viral strategies for pathogenesis and persistence in chronically infected patients.
Autism is a severe developmental disorder with poorly understood etiology.
Oxidative stress in autism has been studied at the membrane level and also by measuring products of lipid peroxidation, detoxifying agents (such as glutathione), and antioxidants involved in the defense system against reactive oxygen species (ROS).
Lipid peroxidation markers are elevated in autism, indicating that oxidative stress is increased in this disease.
Biochemical studies have shown that nanoparticles influence the antioxidant enzyme activities, photosynthetic processes, oxidative stress and DNA expression in plants.
Oxidative stress occurs when there is an imbalance in any cell compartment between reactive oxygen species (ROS) and antioxidant defense.
un: fiber

a thread or filament from which a vegetable tissue, mineral substance, or textile is formed.

"the basket comes lined with natural coco fibres"

dietary material containing substances such as cellulose, lignin, and pectin, that are resistant to the action of digestive enzymes.



fibrous material
Polymer optical fibers have generated increasing interest in recent years.
Because of low cost, ease of handling, long wavelength transparency, great flexibility, and biocompatibility, polymer optical fibers show great potential for short-distance networking, optical sensing and power delivery.
Electrospun nanofibers are particularly well-suitable for optical sensors because sensor sensitivity increases with increasing surface area per unit mass. Optical sensing works by detecting ions and molecules of interest via fluorescence quenching mechanism.
Wang et al. successfully developed nanofibrous thin film optical sensors for metal ion (Fe3+ and Hg2+) and 2,4-dinitrotoluene (DNT) detection using the electrospinning technique.[17]
Quantum dots show useful optical and electrical properties, including high optical gain and photochemical stability. A variety of quantum dots have been successfully incorporated into polymer nanofibers.[78]
Meng et al. showed that quantum dot-doped polymer nanofiber sensor for humidity detection shows fast response, high sensitivity, and long-term stability while requiring low power consumption.
Kelly et al. developed a sensor that warns first responders when the carbon filters in their respirators have become saturated with toxic fume particles. The respirators typically contain activated charcoal that traps airborne toxins.
As the filters become saturated, chemicals begin to pass through and render the respirators useless. In order to easily determine when the filter is spent,
Kelly and his team developed a mask
equipped with a sensor composed of carbon nanofibers assembled into repeating structures called photonic crystals that reflect specific wavelengths of light. The sensors exhibit an iridescent color that changes when the fibers absorb toxins.
An optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair.
Optical fibers are used most often as a means to transmit light[a] between the two ends of the fiber and find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths (data transfer rates) than electrical cables.
Fibers are used instead of metal wires because signals travel along them with less loss; in addition, fibers are immune to electromagnetic interference, a problem from which metal wires suffer..
Fibers are also used for illumination and imaging, and are often wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of a fiberscope.
Specially designed fibers are also used for a variety of other applications, some of them being fiber optic sensors and fiber lasers
Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) as it travels through the transmission medium.
Attenuation coefficients in fiber optics usually use units of dB/km through the medium due to the relatively high quality of transparency of modern optical transmission media. The medium is usually a fiber of silica glass that confines the incident light beam to the inside.
Attenuation is an important factor limiting the transmission of a digital signal across large distances. Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal.
Empirical research has shown that attenuation in optical fiber is caused primarily by both scattering and absorption.
The propagation of light through the core of an optical fiber is based on total internal reflection of the lightwave. Rough and irregular surfaces, even at the molecular level, can cause light rays to be reflected in random directions.
This is called diffuse reflection or scattering, and it is typically characterized by wide variety of reflection angles.
Light scattering depends on the wavelength of the light being scattered.
Thus, limits to spatial scales of visibility arise, depending on the frequency of the incident light-wave and the physical dimension (or spatial scale) of the scattering center, which is typically in the form of some specific micro-structural feature.
Since visible light has a wavelength of the order of one micrometer (one millionth of a meter) scattering centers will have dimensions on a similar spatial scale.
Thus, attenuation results from the incoherent scattering of light at internal surfaces and interfaces.
In (poly)crystalline materials such as metals and ceramics, in addition to pores, most of the internal surfaces or interfaces are in the form of grain boundaries that separate tiny regions of crystalline order.
It has recently been shown that when the size of the scattering center (or grain boundary) is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent.
This phenomenon has given rise to the production of transparent ceramic materials.
Similarly, the scattering of light in optical quality glass fiber is caused by molecular level irregularities (compositional fluctuations) in the glass structure. Indeed, one emerging school of thought is that a glass is simply the limiting case of a polycrystalline solid.
Within this framework, "domains" exhibiting various degrees of short-range order become the building blocks of both metals and alloys, as well as glasses and ceramics.
Distributed both between and within these domains are micro-structural defects that provide the most ideal locations for light scattering. This same phenomenon is seen as one of the limiting factors in the transparency of IR missile domes.
At high optical powers, scattering can also be caused by nonlinear optical processes in the fiber.[
In addition to light scattering, attenuation or signal loss can also occur due to selective absorption of specific wavelengths, in a manner similar to that responsible for the appearance of color. Primary material considerations include both electrons and molecules as follows:
At the electronic level, it depends on whether the electron orbitals are spaced (or "quantized") such that they can absorb a quantum of light (or photon) of a specific wavelength or frequency in the ultraviolet (UV) or visible ranges. This is what gives rise to color.
At the atomic or molecular level, it depends on the frequencies of atomic or molecular vibrations or chemical bonds, how close-packed its atoms or molecules are, and whether or not the atoms or molecules exhibit long-range order.
These factors will determine the capacity of the material transmitting longer wavelengths in the infrared (IR), far IR, radio and microwave ranges.
A fiber laser (or fibre laser in British English) is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and holmium.
They are related to doped fiber amplifiers, which provide light amplification without lasing. Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing can also provide gain and thus serve as gain media for a fiber laser.
Other applications of fiber lasers include material processing, telecommunications, spectroscopy, medicine, and directed energy weapons.
Unlike most other types of lasers, the laser cavity in fiber lasers is constructed monolithically by fusion splicing different types of fiber; fiber Bragg gratings replace conventional dielectric mirrors to provide optical feedback.
They may also be designed for single longitudinal mode operation of ultra narrow distributed feedback lasers (DFB) where a phase-shifted Bragg grating overlaps the gain medium. Fiber lasers are pumped by semiconductor laser diodes or by other fiber lasers.
Many high-power fiber lasers are based on double-clad fiber. The gain medium forms the core of the fiber, which is surrounded by two layers of cladding. The lasing mode propagates in the core, while a multimode pump beam propagates in the inner cladding layer.
The outer cladding keeps this pump light confined. This arrangement allows the core to be pumped with a much higher-power beam than could otherwise be made to propagate in it,
and allows the conversion of pump light with relatively low brightness into a much higher-brightness signal. There is an important question about the shape of the double-clad fiber; a fiber with circular symmetry seems to be the worst possible design.
The design should allow the core to be small enough to support only a few (or even one) modes. It should provide sufficient cladding to confine the core and optical pump section over a relatively short piece of the fiber.
Tapered double-clad fiber (T-DCF) has tapered core and cladding which enables power scaling of amplifiers and lasers without thermal lensing mode instability.
The ring resonator is fabricated by coiling the tapered fiber, which is firstly made by heating and stretching a piece of optical fiber,
The key
The G hit.

A circular polarization of light.…
God bless you all.
Ultralow-threshold lasing has been achieved in fused-silica microspheres coated with semiconductor quantum dots. The cw Ti:sapphire pump input and laser output are coupled to and from whispering-gallery modes by tapered fibers.
When excess light energy is absorbed by plants during photosynthesis, harmful reactive oxygen species are produced. ... When leaves are irradiated by excess light energy, the chloroplasts that carry out photosynthesis can easily suffer oxidative stress and stop photosynthesizing.

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