.net barcode reader free Helical antennas in Software

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Helical antennas
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The helical antenna (Fig 21-5) provides moderately wide bandwidth and circular polarization Because of the polarization some people find the helical antenna to be
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428 Antennas for radio astronomy particularly well suited to radio astronomy reception The antenna (of diameter D) will have a circumference C of 075 to 13 The pitch of the helix (S) is the axial length of one turn, while the overall length L NS (where N is the number of turns) The ratio S/C should be 022 to 028 At least three turns are needed to produce axial-mode main lobe maxima The diameter or edge of the groundplane G should be on the order of 08 to 11 if circular and/or square, respectively The offset between the groundplane and the first turn of the helix is 012 The approximate gain of the helical antenna is found from Gain The pitch angle 118 10 log (C2NS) and turn length D tan 1 S ( D)2 S2 The beamwidth of the helical antenna is K NS where is the beamwidth, in degrees N is the number of turns S is the pitch, in wavelengths C is the circumference, in wavelengths K is 52 for the 3-dB beamwidth and 115 for the beamwidth to the first null in the pattern The short section between the helix and the groundplane is terminated in a coaxial connector, allowing the antenna to be fed from the rear of the groundplane The feedpoint impedance is approximately 140 [215] dBi [212]
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for the helical antenna are given by [213]
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and [214]
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Multiple helical antennas
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Stacking helical antennas allows a radiation pattern that is much cleaner than the normal one-antenna radiation pattern It also provides a good way to obtain high gain with only a few turns in each helix If two helixes are stacked, then the gain will be the same as for an antenna that is twice the length of each element, while for four stacked antennas the gain is the same as for a single antenna 4 times as long Figure 21-6 shows a side view of the stacked helixes The feed system for stacked helixes is a little more complex than for a single helix Figure 21-7A shows an end view of a set of four stacked helical antennas Tapered lines (TL) are used to carry signal from each element and the coaxial connector (B) In this case, the coaxial connector is a feed-through barrel SO-239 device at the center of the groundplane (B) A side view of the tapered line system
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Multiple helical antennas 429
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21-5 Helical antenna
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21-6 Dual/quad helical antenna
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430 Antennas for radio astronomy
21-7 Frontal view of quad helical antenna
Interferometer antennas 431 is shown in Fig 21-7B The length of the tapered lines is 106 , while the center-tocenter spacing between the helical elements is 15 The length of each side of the groundplane is 25 In the case of Fig 21-7, the antenna is fed from the front of the groundplane
Interferometer antennas
The resolution of an antenna is set by its dimensions a relative to the wavelength of the received signal Better resolution can be achieved by increasing the size of the antenna, but that is not always the best solution Figure 21-8 shows a summation interferometer array Two antennas with aperture a are spaced S wavelengths apart The radiation pattern is a fringe pattern (Fig 21-9) This pattern consists of a series of maxima and nulls The resolution angle to the first null is S a [216] 573 The interferometer can be improved with additional antennas in the array Professional radio astronomers use very wide baseline antennas With modern communications it is possible to link radio telescopes on different continents to make the widest possible baseline
21-8 Interferometer array
432 Antennas for radio astronomy
21-9 Interferometer pattern
CHAPTER
Adjusting, installing, and troubleshooting antennas and transmission lines
ANTENNA AND TRANSMISSION LINE MEASUREMENTS SHOULD BE MADE WHEN THE ANTENNA
is first installed and periodically thereafter If some difference in operation is noted, the same measurements should be repeated Many antenna measurements are difficult to make with any degree of accuracy There are some things about antennas that can and should be measured, however, regardless of the difficulty For example, VSWR and the resonant frequency of the antenna are readily accessible It s also possible to measure the impedance of the antenna feedpoint You can measure the VSWR either with a special VSWR meter (often built into transmitters or antenna tuning units), or by using an RF wattmeter By frequency stepping through the band and testing the VSWR at various frequencies, one can draw a VSWR curve (Fig 22-1) that shows how the antenna performs across the band The resonant frequency is the point where the VSWR dips to a minimum (which may or may not be the much sought-after 1:1) You can use the resonant frequency to figure out whether the antenna is too long (resonant frequency lower than the hoped-for design frequency), or too short (resonant frequency above the design frequency) Figure 22-2 shows all three situations Curve A represents the desired or ideal curve that is centered on the desired frequency FD If the antenna is too long, however, the resonant frequency will be shifted downward towards F1, and curve B will be observed Similarly, if the antenna is too short, curve C will be found as the resonant frequency shifts upband to F2 But resonant frequency and VSWR curves are not the entire story because they don t tell us anything about the impedance presented by the antenna One cannot get the VSWR to be 1:1 unless the antenna impedance and transmission line impedance are the same For example, a dipole has a nominal textbook impedance of 73 ,
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