本帖最后由 choi 于 5-4-2019 10:26 编辑
(3) To understand physics underlying slotted waveguide, one starts with three areas of physics -- (a) to (c), which will be united at (4).
(a) double-slit experiment
https://en.wikipedia.org/wiki/Double-slit_experiment
(was first performed with light by [Englishman] Thomas Young in 1801; section 1 Overview: " * * * when this 'single-slit experiment' is actually performed, the pattern on the screen is a diffraction [different from refraction] pattern in which the light is spread out. The smaller the slit [ie, the shorter], the greater the angle of spread. The top portion of the image shows the central portion of the pattern formed when a red laser illuminates a slit and, if one looks carefully, two faint side bands [end on end, not side by side]")
(i) Below the photo of the single slit experiment is the double-slit one. Please understand that in the latter the central -- and the brightest -- (horizontal) line is flanked by the fainter ones both above and below. Indeed the one line right above and the line right below are created by the light passing THROUGH the two slits (without diffraction). The brightest line is positioned in the MIDDLE (which should not be there if photon is nothing but a particle, not wave also) is of these two afore-mentioned line.
Because a slit has four edges, each in theory should diffract. So there should be faint line on both SIDES also.
(ii) From double-slit experiment, you intuitively know immediately that in slotted waveguide, the radiation pattern (or electric field; with which a radar both emits and receives electromagnetic wave) is decided by orientation of slots, rather than of the waveguide (one which the slots lie).
(iii) Slits - Diffraction Effects. Undated.
https://www.cyberphysics.co.uk/topics/light/A_level/slits.htm
(iv) A slotted waveguide with a series (or an array) of transverse slots are akin to an expanded version (ie, with more than two slits) of double-slit experiment -- except that a waveguide is not exactly the same as Thomas Young's setup, because the source of electromagnetic radiation is vastly different. See (5) below.
(b)
(i) dipole
https://en.wikipedia.org/wiki/Dipole
(In electromagnetism, there are two kinds of dipoles: an electric dipole and magnetic dipole)
(A) The third figure shows "electric field lines" of an electric dipole.
(B) The online Merriam-Webster dictionary says for dipole, its first known use was in 1912 as International Scientific Vocabulary.
(ii) dipole antenna
https://en.wikipedia.org/wiki/Dipole_antenna
"is the simplest" antenna which can elucidate more complicated antenna.
(A) Please view the animation depicting "[a] half-wave dipole antenna." Take notice that the electric field (E) is perpendicular to the dipole.
(B) In section 3 Dipole characteristics, section 3.2 Radiation pattern and gain, one can see the radiation pattern (or electric field) of a "vertical" (or upright) dipole. The "radiation pattern" is just extension of the preceding (A).
(iii)
(A) Radiation pattern of a radar means its electric field E.
Antenna Characteristics.
http://www.waves.utoronto.ca/pro ... /06-antennachar.pdf
(first 2 paragraphs: "The radiation pattern of an antenna is a graphical representation of the radiation properties of the antenna. Graphically, we surround the antenna by a sphere and evaluate the electric / magnetic fields (far field radiation fields) at a distance equal to the radius of the sphere. Usually we will focus on one field component (Eff or Hff ) radiated by the antenna. Usually we plot the dominant component of the E-field (eg Eθ for a dipole). This can be done by plotting the field component over all angles (θ, φ), yielding a 3D plot. For a dipole, this leads to the doughnut pattern in 3D because of the dependence of Eθ on sin θ")
THERE IS NO WAY TO SUBSSCRIPT IN THIS WEBSITE. YOU HAVE TO READ ORIGINAL.
Antenna Characteristics. In course "ECE422 Radio and Microwave Wireless Systems" by Professor Sean Victor HUM, University of Toronto, undated
http://www.waves.utoronto.ca/prof/svhum/ece422.html
(B) In a moment, magnetic field of a electromagnetic wave (of a radar) will be mentioned briefly. Why is it represented by letter H, not M? It started with James Clerk Maxwell, whose used vectors A to H.
Lindsay Guilmette, The History of Maxwell's Equations. Fairfield, Connecticut: Sacred Heart University, 2012
https://digitalcommons.sacredhea ... p;context=wac_prize
("Maxwell translated Faraday's ideas into mathematics. Maxwell created vectors to describe the main players of electromagnetism: 'E, the electric field intensity, H, the magnetic field intensity, B, the magnetic flux density, and I the electric current density. E and H are forces and B and I are fluxes (lines of force) produced by the forces' (Peters, 2000, p. 9). A way to picture flux is to imagine having a square loop of wire in a flowing river. The flux of the velocity of the water would be like considering how much water will flow through the loop. The flux of an electric field is proportional to the number of electric field lines that go through such a loop (Sciolla, 2004)" )
(c)
(i) Babinet's principle
https://en.wikipedia.org/wiki/Babinet%27s_principle
(was formulated in the 1800s by French physicist Jacques Babinet; section 1 Explanation: " * * * Diffraction patterns from apertures or bodies of known size and shape are compared with the pattern from the object to be measured. For instance, the size of red blood cells can be found by comparing their diffraction pattern with an array of small holes")
(A) There is no need to read the rest. This Wiki page is not well written, so much so that I did not understand it after reading it many times. See (ii) for enlightenment.
(B) The sentence in section 1 Explanation is not an explanation, but points t clinical use, to decide size of red blood cells.
(ii) Anshul Kogar, Diffraction, Babinet and Optical Transforms. This Condensed Life (name of Jogar;s blog), May 21, 2016
https://thiscondensedlife.wordpr ... optical-transforms/
("To get an idea of what this means, let's look at an example")
As you can see, the center of slotted waveguide -- whether slot array is linear (but definitely not in a straight line) or symmetrical (arranged as a circle, see, eg,
Slotted Array Antenna. Sylatech, undated
https://sylatech.com/slotted-array-antenna/
(waveguide)
or a square or a rectangle) -- is brightest (and therefore the longest reach of electric field/ radiation pattern).
(iii) Babinet's principle. But why?
Jessie Segal and Alyssa Cedarman (two undergraduates under then associate professor Dan MacIsaac), Diffraction with Hair or Wire. Buffalo State College (part of SUNY), undated
physicsed.buffalostate.edu/pubs/StudentIndepStudy/EURP09/Young/Young.html
("This is a variation of Young's Double Slit, in which light goes through two thin, parallel slits. When light goes through, the light will diffract, or 'bend.' When light touches the edges of the slit, it is treated as a new source of light, as described by Huygen's Principle")
Take notice that, depending on the distance of the screen from the "opaque body,' (quoting Babinet's principle), the area directly behind the opaque body may be the brightest.
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发表于 5-1-2019 16:22:40
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