Jordan Balmain Electromagnetic Waves Pdf
Gregory Monty
In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the directional (angular) dependence of the strength of the radio waves from the antenna or other source.[1][2][3]
Particularly in the fields of fiber optics, lasers, and integrated optics, the term radiation pattern may also be used as a synonym for the near-field pattern or Fresnel pattern.[4] This refers to the positional dependence of the electromagnetic field in the near field, or Fresnel region of the source. The near-field pattern is most commonly defined over a plane placed in front of the source, or over a cylindrical or spherical surface enclosing it.[1][4]
The far-field pattern of an antenna may be determined experimentally at an antenna range, or alternatively, the near-field pattern may be found using a near-field scanner, and the radiation pattern deduced from it by computation.[1] The far-field radiation pattern can also be calculated from the antenna shape by computer programs such as NEC. Other software, like HFSS can also compute the near field.
The far field radiation pattern may be represented graphically as a plot of one of a number of related variables, like the field strength at a constant (large) radius (an amplitude pattern or field pattern), the power per unit solid angle (power pattern) and the directive gain. Very often, only the relative amplitude is plotted, normalized either to the amplitude on the antenna boresight, or to the total radiated power. The plotted quantity may be shown on a linear scale, or in dB. The plot is typically represented as a three-dimensional graph (as at right), or as separate graphs in the vertical plane and horizontal plane. This is often known as a polar diagram.
It is a fundamental property of antennas that the receiving pattern (sensitivity as a function of direction) of an antenna when used for receiving is identical to the far-field radiation pattern of the antenna when used for transmitting. This is a consequence of the reciprocity theorem of electromagnetics and is proved below. Therefore, in discussions of radiation patterns the antenna can be viewed as either transmitting or receiving, whichever is more convenient. This applies only to the passive antenna elements; active antennas that include amplifiers or other components are no longer reciprocal devices.
Since electromagnetic radiation is dipole radiation, it is not possible to build an antenna that radiates coherently equally in all directions, although such a hypothetical isotropic antenna is used as a reference to calculate antenna gain.
The simplest antennas, monopole and dipole antennas, consist of one or two straight metal rods along a common axis. These axially symmetric antennas have radiation patterns with a similar symmetry, called omnidirectional patterns; they radiate equal power in all directions perpendicular to the antenna, with the power varying only with the angle to the axis, dropping off to zero on the antenna's axis. This illustrates the general principle that if the shape of an antenna is symmetrical, its radiation pattern will have the same symmetry.
In most antennas, the radiation from the different parts of the antenna interferes at some angles; the radiation pattern of the antenna can be considered an interference pattern. This results in minimum or zero radiation at certain angles where the radio waves from the different parts arrive out of phase, and local maxima of radiation at other angles where the radio waves arrive in phase. Therefore, the radiation plot of most antennas shows a pattern of maxima called "lobes" at various angles, separated by "nulls" at which the radiation goes to zero. The larger the antenna is compared to a wavelength, the more lobes there will be.
In a directional antenna in which the objective is to emit the radio waves in one particular direction, the antenna is designed to radiate most of its power in the lobe directed in the desired direction. Therefore, in the radiation plot this lobe appears larger than the others; it is called the "main lobe". The axis of maximum radiation, passing through the center of the main lobe, is called the "beam axis" or boresight axis". In some antennas, such as split-beam antennas, there may exist more than one major lobe. The other lobes beside the main lobe, representing unwanted radiation in other directions, are called minor lobes. The minor lobes oriented at an angle to the main lobe are called "side lobes". The minor lobe in the opposite direction (180) from the main lobe is called the "back lobe".
For a complete proof, see the reciprocity (electromagnetism) article. Here, we present a common simple proof limited to the approximation of two antennas separated by a large distance compared to the size of the antenna, in a homogeneous medium. The first antenna is the test antenna whose patterns are to be investigated; this antenna is free to point in any direction. The second antenna is a reference antenna, which points rigidly at the first antenna.
It is assumed that the two antennas are sufficiently far apart that the properties of the transmitting antenna are not affected by the load placed upon it by the receiving antenna. Consequently, the amount of power transferred from the transmitter to the receiver can be expressed as the product of two independent factors; one depending on the directional properties of the transmitting antenna, and the other depending on the directional properties of the receiving antenna.
For the transmitting antenna, by the definition of gain, G \displaystyle G , the radiation power density at a distance r \displaystyle r from the antenna (i.e. the power passing through unit area) is
Here, the angles θ \displaystyle \theta and Φ \displaystyle \Phi indicate a dependence on direction from the antenna, and P t \displaystyle P_t stands for the power the transmitter would deliver into a matched load. The gain G \displaystyle G may be broken down into three factors; the antenna gain (the directional redistribution of the power), the radiation efficiency (accounting for ohmic losses in the antenna), and lastly the loss due to mismatch between the antenna and transmitter. Strictly, to include the mismatch, it should be called the realized gain,[4] but this is not common usage.
Here W \displaystyle W is the power density of the incident radiation, and A \displaystyle A is the antenna aperture or effective area of the antenna (the area the antenna would need to occupy in order to intercept the observed captured power). The directional arguments are now relative to the receiving antenna, and again A \displaystyle A is taken to include ohmic and mismatch losses.
where G \displaystyle G and A \displaystyle A are directionally dependent properties of the transmitting and receiving antennas respectively. For transmission from the referenceantenna (2), to the test antenna (1), that is
Background: Sleep is an important process of our body and a good sleep will lead to a healthy lifestyle. In medical field, students have sleep patterns changing due to heavy academic workload. This may have ill effects on their health. Though most factors that affect sleep are modifiable and treatment for them exists still there are can be many factors that affect sleep which should be explored. The thermal model of human body is a theoretical model that accounts for thermal effects of electromagnetic waves on a given point in human body. This could be easily affect brain as it has highest electrical activity in body and may lead to sleep related disorders.
Methods: This study is conducted on medical and dental students to analyse the amount of electromagnetic field they get exposed to and any changes in sleep patterns associated with it. The findings of medical and dental students are compared to see if changes in sleep patterns are due to professional course pursued. Any other confounding factors affecting this study are screened by self-rated Pacific Sleep Questionnaire.
Results: There were significant changes seen in the time taken to fall asleep and total sleep period but the time taken to wake up from sleep remained unaffected. The sleeping habits of medical and dental students showed no significant changes.
Giri PA, Baviskar MP, Phalke DB. Study of sleep habits and sleep problems among medical students of Pravara Institute of Medical Sciences Loni, Western Maharashtra, India. Ann Medi Health Sciences Res. 2013;3(1):51-4.
Croft RJ, Hamblin DL, Spong J, Wood AW, McKenzie RJ, Stough C. The effect of mobile phone electromagnetic fields on the alpha rhythm of human electroencephalogram. Bioelectromagnet: J Bioelectromagnet Soc, Soc Phys Reg Biol Medi, Eur Bioelectromagnet Assoc. 2008 Jan;29(1):1-0.
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