.net barcode reader free 3-dB beamwidth for vertical and horizontal extents can be approximated in Software

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3-dB beamwidth for vertical and horizontal extents can be approximated
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[1932]
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19-24 Horn radiators
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404 Microwave waveguides and antennas Horizontal
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degrees
[1933]
where is the vertical beamwidth, in degrees is the horizontal beamwidth, in degrees h a, b are dimensions of the flared flange is the wavelength
A form of antenna, related to the horn, is the cavity antenna of Fig 19-25 In this type of antenna, a quarter-wavelength radiating element extends from the waveguide (or transmission line connector) into a resonant cavity The radiator element is placed a quarter-wavelength into a resonant cavity; and is spaced a quarterwavelength from the rear wall of the cavity A tuning disk is used to alter cavity dimensions in order to provide a limited tuning range for the antenna Gains to about 6 dB are possible with this arrangement
Reflector antennas
At microwave frequencies, it becomes possible to use reflector antennas because of the short wavelengths involved Reflectors are theoretically possible at lower frequencies, but because of the longer wavelengths, the antennas would be so large that they become impractical Several forms of reflector are used (Figs 19-26 and 19-27) In Fig 19-26 we see the corner reflector antenna, which is used primarily in the high-UHF and low-microwave region A dipole element is placed at the focal point of the corner reflector, so it receives (in phase) the reflected wavefronts from the surface Either solid metallic reflector surfaces or wire mesh may be used When mesh is used, however, the holes in the mesh must be 1 12-wavelength or smaller Figure 19-27 shows several other forms of reflector surface shape, most of which are used in assorted radar applications
Parabolic dish antennas
The parabolic reflector antenna is one of the most widespread of all the microwave antennas, and is the type that normally comes to mind when thinking of microwave systems This type of antenna derives its operation from physics similar to optics, and is possible because microwaves are in a transition region between ordinary radio waves and infrared/visible light The dish antenna has a paraboloid shape as defined by Fig 19-28 In this figure, the dish surface is positioned such that the center is at the origin (0,0) of an x-y coordinate system For purposes of defining the surface, we place a second vertical axis called the directrix (y ) a distance behind the surface equal to the focal length (u) The paraboloid surface follows the function y2 4ux, and has the property that a line from the focal point F to any point on the surface is the same length as a line from that same point to the directrix (in other words, MN MF) If a radiator element is placed at the focal point F, then it will illuminate the reflector surface, causing wavefronts to be propagated away from the surface in
The isotropic antenna
Antenna radiator element
Microwave window (Cavity)
Tuning disk
Waveguide or coaxial cable fitting
Tuning adjustment screw
19-25 Cavity antenna
Dipole element aR
19-26 Corner reflector
phase Similarly, wavefronts, intercepted by the reflector surface, are reflected to the focal point Gain The gain of a parabolic antenna is a function of several factors: dish diameter, feed illumination, and surface accuracy The dish diameter D should be large compared with its depth Surface accuracy refers to the degree of surface irregularities For commercial antennas, 1 8-wavelength surface accuracy is usually sufficient,
406 Microwave waveguides and antennas
A Paraboloid
B Truncated
paraboloid (surface search)
C Truncated
paraboloid (height finding)
D Orange-peel
paraboloid
E Cylindrical
paraboloid
F Corner
reflector 19-27 Reflector antennas
although on certain radar antennas the surface accuracy specification must be tighter The feed illumination refers to how evenly the feed element radiates to the reflector surface For circular parabolic dishes, a circular waveguide feed produces optimum illumination, and rectangular waveguides are less than optimum The TE11 mode is desired For best performance, the illumination should drop off evenly from the center to the edge, with the edge being 210 dB down from the center The diameter, length, and beamwidth of the radiator element (or horn) must be optimized for the specific F/d ratio of the dish The cutoff frequency is approximated from fcutoff where fcutoff is the cutoff frequency d is the inside diameter of the circular feedhorn The gain of the parabolic dish antenna is found from G where G is the gain over isotropic D is the diameter is the wavelength (same units as D) k is the reflection efficiency (04 to 07, with 055 being most common) k ( D)2
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