Do Radio Waves Pass by and Cant Be Recorded Again?

Radio Waves

Data Transmission Media

John S. Sobolewski , in Encyclopedia of Concrete Science and Engineering science (Third Edition), 2003

Xi.B Signal Degradation

Radio waves, like lite waves, are bailiwick to reflection and refraction. They are also subject to attenuation losses due to atmospheric and natural phenomena such as pelting, snowfall, and fog. These outcome in 3 major types of signal deposition: multipath interference, fading, and attenuation losses.

Reflections from the earth'south surface, the ionosphere, natural or manmade objects, and atmospheric refraction tin can create multiple paths between the transmitting and receiving antennas. Depending on the relative path distance, the reflected wave is shifted in stage with respect to the original wave, which can crusade interference at the receiver called multipath interference. Since the amount of phase shift is frequency dependent, the combined received signal is besides frequency dependent, which can lead to serious problems in wideband transmission.

Fading is acquired by aberrant changes in the refractive index of the atmosphere. Usually, the atmosphere refracts or bends radio waves back toward the surface of the earth. However, abnormal distribution of temperature, humidity, and heavy ground fog can cause radio waves to be bent toward the surface much more than than normal so that they never attain the receiving antenna, causing changes in received betoken strength or even complete loss. Variation or, specifically, reduction of received signal force at unlike periods in time is chosen fading.

As manual frequencies increase, path attenuation losses due to the temper besides increase. More serious losses are caused past fog, snow, and especially rain, which becomes very pregnant at frequencies to a higher place 4   GHz. The effects of these losses are fading and increased fault rates. They are usually allowed for during the blueprint process past using published meteorologicial data of the region in which a radio link is located.

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Radio Propagation

Henry L. Bertoni , in Encyclopedia of Physical Scientific discipline and Engineering science (Third Edition), 2003

I Introduction

Radio waves of frequencies beneath 3  MHz take wavelength that is greater than 100   m, and hence big compared to the acme of almost buildings and small hills. Propagation of these waves over terrestrial links is influenced by the earth's ionosphere and by the electrical properties and curvature of the globe (Expect, 1996), only but weakly by buildings and vegetation (Hall and Barclay, 1989). For frequencies above 30   MHz, the wavelength is less than 10   m, and hence smaller than buildings, trees, and terrain undulation. Propagation of these high-frequency waves over short distances is therefore strongly influenced past the buildings, trees, and terrain in the vicinity of, and betwixt, the transmitter and receiver. While the ionosphere is not important for high frequencies, the variation of refractive index in the atmosphere is important over distances greater than nearly 10   km. Radio waves having frequency in the middle range between 3 and 30   MHz may be influenced past all features along the path.

In radio communications, a transmitting antenna is used to launch an electromagnetic indicate into space, which is received past a second antenna at a remote location. For antennas located in free space, such every bit for communication betwixt satellites, the propagation effects consist but of the reduction of the point with distance as a outcome of spreading the transmitted energy over an area that increases with distance. When one or both ends of the advice link are located near the ground, the temper, terrain variation, vegetation, and buildings influence the received indicate. Besides the dependence on the separation between transmitter and receiver, these features effect indicate characteristics such every bit spatial and temporal fading, frequency dependence, and echo-induced time delay spread. This affiliate describes how the individual features influence the characteristics of the received signal. Ultimately, it will be necessary to develop a description of how these features interact to simultaneously influence the radio wave.

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Electromagnetic-Wave Propagation

Revised by Douglass D. Crombie , in Reference Data for Engineers (9th Edition), 2002

Radio waves may be propagated ^* from the transmitting antenna to the receiving antenna through or along the surface of the earth, through the atmosphere, or by reflection or scattering from natural or bogus reflectors. The conductivity and dielectric constant of the ground vary considerably from those of the atmosphere. At very-low frequencies, basis waves may exist satisfactorily propagated for distances of several thousand kilometers. At high frequencies, however, the losses are then bang-up that signals can be propagated for only a few hundred kilometers past ground wave. Propagation in the medium- and high-frequency bands is chiefly by ground wave and by reflection from the ionosphere, and astringent fading is caused in these frequency bands by the interference betwixt ground and ionospheric waves.

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Detection of Explosives by Terahertz Imaging

John F. Federici , ... Zoi-Heleni Michalopoulou , in Counterterrorist Detection Techniques of Explosives, 2007

four.two About-field versus far-field

With radio wave interferometric imaging of astronomical sources, it is typically assumed that the incoming radiation consists of planar waves. In this limit, any curvature of the incoming wavefronts is negligible. This 'far-field' assumption is used for constructed imaging and simplifies the imaging reconstruction process. However, about standoff THz applications do not autumn in the 'far-field' limit, and the simplified changed Fourier transform of the electric field correlation – the far-field epitome reconstruction [99] –must be modified [100] to account for the curvature of the wavefronts in the near-field.

The condition for negligible stage shift δ tin exist approximated as Z ₀ >> b² /λ, where b is the largest baseline length of the imaging array and λis the THz wavelength. Equally an instance, assume that a ii.v-cm object needs to be imaged at various distances. The angular resolution of a planar array can be approximated as θ _mill = λ /b. At a distance Z ₀ away, the lateral spatial resolution is Δ50 _lat ≈ θ _min Z ₀ ≈ λZ ₀ /b. Using δ ≈ b ²/Z ₀ λ as an estimate for the far-field limit of a planar array, the limit can now be estimated as $\delta \approx Z_0\lambda /\Delta L_{\text{lat}}^{\text{2}}$ . To maintain a 2.5-cm lateral resolution at various distances, the maximum baseline for a planar imaging array tin exist estimated every bit b ≈ λZ ₀/Δ 50 _lat. Using 8 ~b ²/Z ₀ λ as an gauge of the far-field limit for a planar array, the limit can be estimated as $\delta \simeq Z_0\lambda /\Delta {\mathrm{Fifty}}_{\text{lat}}^{\text{ii}}$ . Table 3 summarizes the corresponding maximum baseline required and phase error. Note that, for this application, the far-field criteria of δ ≪ 1 is never satisfied. THz interferometric imaging for standoff applications must include contributions from the about-field.

Table 3. Estimated phase error δ from imaging a 2.5 cm object at various distances using 1 THz radiation. The baseline b is an estimate of the concrete size of the imaging array. Note that for distances of 5–10m, the size of the imaging array is half dozen–12 cm suggesting that a hand-held unit might be acheivable

Z 0	5 thousand	10 m	50 m	100 m	500 m	1000 one thousand
b	0.06 m	0.12 m	0.6 m	1.2 m	vi grand	12 chiliad
eight	2.4	4.8	24	48	240	480

The 'blurring' of the THz image for standoff applications tin can be greatly reduced past changing the arrangement of the detectors in the imaging array. In essence, the organisation of the detectors in the array must be modified to 'focus' the imaging array at standoff distances. In the assay, we match the curvature of the imaging array to that of the wavefront. We assume that two individual detectors of the spherical imaging array measure an electric field from an element of surface dS'. For simplicity, we presume that the detectors lay on a spherical surface whose radius of curvature R _o is centered on the origin. The correlation between the 2 wavefronts at the two detectors can be calculated from Eq. 1 with r _j expressed in spherical coordinates. 1 tin show that the coherence part becomes [72, 100]

(four) $C_{1,\mathrm{two}}\left(u,\nu \right)=\left[exp\left(i\delta \right)\right]\underset{-\infty }{\overset{\infty }{\int }}\underset{-\infty }{\overset{\infty }{\int }}{\sigma }_{\mathrm{Due\; east}}\left(\xi ,\eta \right)exp\left[-\mathrm{ii}\pi \left(u\xi +\nu \eta \right)\right]\text{d}\xi \text{d}\eta$

where $\xi ={\overline{x}}^{\prime }/R_o,\eta ={\overline{y}}^{\prime }/R_o,v=k\left({\overline{y}}_1-{\overline{y}}_2\right)/\mathrm{two}\pi$ and $u=k\left({\overline{x}}_1-{\overline{x}}_2\right)/2\pi$ . In these expressions, ${\overline{x}}_2-{\overline{\mathrm{10}}}_1=R_o\left(sin{\theta }_{2}cos{\varphi }_2=sin{\theta }_{1}cos{\varphi }_{\mathrm{ane}}\right)$ and ${\overline{y}}_{\mathrm{ii}}-{\overline{y}}_{\mathrm{ane}}=R_o\left(sin{\theta }_{2}sin{\varphi }_2-sin{\theta }_{\mathrm{ane}}sin{\varphi }_1\right)$ are the spherical coordinates of the detectors and the stage error is $\overline{\delta }=m{z}^{\prime }\left(cos{\theta }_{\mathrm{ii}}-cos{\theta }_1\right)$ .

If the well-nigh-field phase mistake $\overline{\delta }$ can be neglected, Eq. 4 would then relate the coherence office to the effulgence distribution of the source. Assuming that the azimuthal angles of the detectors are non equal, the magnitude of the phase error can be calculated, assuming that the largest azimuthal bending is determined past the distance to the object and the largest baseline distance. For the baseline and distances summarized in Tabular array 3, the largest possible value of cosθ_two – cosθ ₁ ≈ vii.ii × 10^–five independent of the distance to the object. If $\overline{\delta }=\mathrm{one}$ , the approximate depth of focus (range of z ') can be estimated equally that for which the phase errors are small-scale and the object is in focus in the nearly-field. For a frequency of 1 THz, the depth of focus would exist ±0.7 one thousand.

An culling configuration that eliminates the phase error is to arrange the detectors in a circle. In this configuration, all detectors have the aforementioned azimuthal bending, thereby enforcing $\overline{\delta }=\mathrm{thou}{z}^{\prime }\left(cos{\theta }_2-cos{\theta }_{\mathrm{ane}}\right)=0$ . For this circular arrangement, the first-order phase fault vanishes, implying a much larger depth of focus.

The form of the coherence part of a nigh-field imaging array approaches the far-field planar array limit when R _o → ∞because at very big radii of curvature, the surface of the spherical assortment now becomes apartment. When R _o → ∞, exp(iδ) = exp[ikz' (cosϕ₂ → cosϕ₁)] → i because the azimuthal angles are virtually zero. As Z ₀ → ∞, exp(iδ) = exp (ikb²/Z₀ ) → 1.

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ELECTROMAGENTIC Radiation

Jerry B. Marion , in Study Guide for Physics in the Modern World (2d Edition), 1981

Publisher Summary

Light, radio waves, television signals, microwaves and infrared radiation are all electromagnetic waves, but they have different frequencies. Electromagnetic waves are propagating disturbances in an electromagnetic field and are produced by the acceleration of electrical charges, such as a changing current in a wire. This affiliate explains how an approachable electromagnetic wave is produced by an antenna and detected past a receiver. All types of electromagnetic waves propagate with the speed of calorie-free, c = three.0 × 10^eight m/s, which is in vacuum. The electromagnetic waves are always transverse and they can exist polarized. Electromagnetic radiation is discrete; light waves are equanimous of tiny bundles. There are similarities and differences between mechanical and electromagnetic waves.

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Catholic Radiations and Radio Stars†

H. ALFVÉN , N. HERLOFSON , in Cosmic Rays, 1972

The normal radio wave emission from the sun amounts to 10⁻¹⁷ of the heat radiation, and increases during bursts ¹ to equally much as ten^−xiii. If a radio star, eastward.1000. the source in Cygnus, is situated at a distance of 100 light years, ^a its radio emission is of the order of 10⁻⁴ of the heat radiation of our dominicus. It is very unlikely that the atmosphere of any star could be so different from the sun's atmosphere as to allow a radio emission which is 10⁹ to 10^thirteen times greater, and it seems therefore to exist excluded that the source could be equally pocket-sized equally a star. The recent discovery ² that the intensity variations of radio stars is a "twinkling" makes it possible to assume larger dimensions.

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ELECTROMAGNETIC RADIATION

JERRY B. MARION , in Physics in the Modern World (2nd Edition), 1981

Publisher Summary

Calorie-free waves, radio waves, tv waves, infrared radiation, 10 rays, ultraviolet radiations, microwaves, and gamma (γ) rays perform a diversity of tasks. These radiation are all electromagnetic radiations, and they all have the same basic physical features. The only fundamental departure among the diverse types of electromagnetic radiations is that of frequency. The importance of a item type of electromagnetic radiation depends on the mode in which that radiation interacts with matter, and this interaction depends, in plough, on the frequency of the wave. This chapter discusses the production and the properties of electromagnetic radiation. It explains the wave character of electromagnetic radiation.

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Radio Waves from the Milky way

A.Thou. HILLAS , in Cosmic Rays, 1972

Publisher Summary

This affiliate focuses on radio waves from the galaxy. The chapter likewise discusses whether the phenomenon of catholic radiations is solar, galactic, or universal. Cosmic rays show no evident clan with any local source, but if they extend uniformly throughout all space, their generation must represent the major procedure of energy product in the universe. The trouble eases if the cosmic rays are largely bars to some smaller office of infinite. Many authors have explored the possibilities of magnetic fields for trapping and accelerating catholic rays. Their importance for the slowing down of relativistic electrons has now been recognized. Affair may easily move forth the lines of strength, simply if ionized matter moves in any other way, information technology drags the lines with it. The chapter describes the consequences of magnetic inertia and discusses the detection of radio waves coming from the Milky Mode by Jansky. It besides illustrates the polarization of radiation from a relativistic electron in a magnetic field.

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III-Nitride Electronic Devices

Keisuke Shinohara , in Semiconductors and Semimetals, 2019

ane Introduction

Millimeter wave is a radio wave with a frequency between 30 and 300   GHz. Millimeter wave radio offers several advantages over lower frequency microwave radio. Its wide bandwidth enables a very high data transmission rate of upward to 80   Gbit/s or even higher. The directional and narrow radiation pattern from millimeter moving ridge radios allows many radios to be deployed without causing mutual interference. Its short wavelength (ane–ten   mm) reduces the size of antennas and thus enables to build compact apparatus. In addition, the curt wavelength enables to obtain higher resolution images in millimeter-moving ridge imaging systems. Furthermore, atmospheric absorption by oxygen and water exists in various parts of the millimeter wave spectrum, allowing a repeated apply of radios in a short range.

Past taking advantage of these unique characteristics of millimeter-moving ridge radios, a broad range of applications has emerged. These include satellite communications (35, threescore, 94   GHz), wireless LAN (60   GHz), point-to-point backhaul system (lxx–80   GHz), x   Gbit/due south wireless link for uncompressed HDTV indicate manual (120   GHz), body scanners for drome security (24–30   GHz), automotive radars (77, 79   GHz), passive imaging system for aircraft safe landing (94   GHz), radio astronomy, ecology remote sensing, and drone-to-ground communications (94   GHz). Upcoming 5G mobile network is expected to support high information-rate instantaneous communications, low latency, and massive connectivity, enabling unprecedented applications for mobile, health, democratic vehicles, smart cities, smart homes, and the Internet-of-Things (IoT). The planned 5G spectrum allocation includes sub-6-GHz and 28   GHz, and fifty-fifty college millimeter-wave frequency bands such as 40, threescore, and 71–86   GHz are under evaluation. Consequently, the demand for compact, low-cost, and high-performance millimeter-wave components is significantly increasing.

Since the showtime demonstration of GaN-based MESFETs in 1993 (Khan et al., 1993) and HEMTs in 1994 (Khan et al., 1994), a tremendous progress on GaN-based transistor and MMIC technologies take been made in a broad range of technical areas. They include substrate and epitaxial materials, devices, MMICs, and packaging technologies. Progress in GaN-based epitaxial material growth techniques such as molecular axle epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) enabled various III-N HEMT epitaxial designs consisting of AlGaN/GaN, AlN/GaN, InAl(Ga)N/GaN, and most recently ScAlN/GaN hetero-structures. They are grown on a nonnative substrate such as SiC, Si, and sapphire. Optimization of the growth weather condition reduced a defect density in the HEMT epitaxial materials, resulting in an increased electron mobility of the two dimensional electron gas (2DEG) and a higher critical electric field in the HEMT structures. High thermal electrical conductivity of the SiC substrate is advantageous to reduce a need for cooling when transistors are operated at high power densities and the channel temperature is increased due to self-heating. Advances in device designs and fabrication process technologies enhanced high frequency performance of GaN-based HEMTs, enabling millimeter-moving ridge GaN ability amplifier (PA) MMICs that have significantly higher output power and power density than is available from amplifier circuits based on other textile systems such as Si, GaAs or InP.

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Tuners and radio receivers

John Linsley Hood , in Audio Electronics (Second Edition), 1998

Critical frequency

The style in which radio waves are refracted past the ionosphere, shown schematically in Fig. 2.3, is strongly dependent on their frequency, with a 'disquisitional frequency' ('F_c') dependent on electron density, per cubic metre, according to the equation

Fig. 2.3. The refraction of radio waves by the ionosphere.

${\mathrm{F}}_{\text{c}}=9\surd {\mathrm{North}}_{\text{max}}$

where N _max is the maximum density of electrons/cubic metre within the layer. Too, the penetration of the ionosphere past radio waves increases equally the frequency is increased. So certain frequency bands will tend to exist refracted back towards the earth'southward surface at different heights, giving different transmitter to receiver distances for optimum reception, equally shown in Fig. 2.4, while some will non be refracted plenty, and volition proceed on into outer space.

Fig. 2.4. The influence of frequency on the optimum transmitter to receiver distance – the 'skip distance'.

The dependence of radio transmission on ionosphere conditions, which, in turn depends on time of mean solar day, time of year, geographical latitude, and 'sun spot' activity, has led to the term 'MUF' or maximum usable frequency, for such transmissions.

Besides, because of the way in which different parts of the radio frequency spectrum are afflicted differently by the possibility of ionospheric refraction, the frequency spectrum is classified as shown in Table 2.1. In this VLF and LF signals are strongly refracted by the 'D' layer, when present, MF signals by the 'Due east' and 'F' layers, and HF signals merely past the 'F' layer, or not at all.

Table 2.1. Classification of radio frequency spectrum

VLF	3–30 kHz
LF	30–300 kHz
MF	300–3000 kHz
HF	three–thirty MHz
VHF	30–300 MHz
UHF	300–3000 MHz
SHF	3–30 GHz

Additionally, the associated wavelengths of the transmissions (from 1 00 000–1000 g in the case of the VLF and LF signals, are then long that the earth's surface appears to exist smooth, and there is a substantial

reflected 'ground wave' which combines with the direct and reflected radiation to course what is known every bit a 'space wave'. This infinite wave is capable of propagation over very long distances, especially during daylight hours when the 'D' and 'E' layers are strong.

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