.net barcode reader library Other dipoles in Software

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Other dipoles
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Thus far, the dipoles covered in this chapter have been the classic form, in which a half-wavelength single-conductor radiator element is connected to a coaxial transmission line This antenna is typically installed horizontally at a half-wavelength above the earth s surface (or wherever convenient if that is impossible) This section looks at other forms of the dipole Some of these dipoles are in every way the equal of the horizontal dipole, and others are basically compensation antennas in that they are used when a proper dipole is not practical
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Inverted-vee dipole
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The inverted-vee dipole is a half-wavelength antenna fed in the center like a dipole By the rigorous definition, the inverted-vee is merely a variation on the dipole theme But in this form of antenna (Fig 6-7), the center is elevated as high as possible from the earth s surface, but the ends droop to very close to the surface Angle a can be almost anything convenient, provided that a > 90 degrees; typically, most inverted-vee antennas use an angle of about 120 degrees Although essentially a compensation antenna for use when the dipole is not practical, many operators believe that it is essentially a better performer on 40 and 80 m in cases where the dipole cannot be mounted at a half-wavelength (64 ft or so) By sloping the antenna elements down from the horizontal to an angle (as shown in Fig 6-7), the resonant frequency is effectively lowered Thus, the antenna will
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Other dipoles 153
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a L Coaxial cable to transmitter L
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L R S I I S R Insulated support
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220 Feet FMHz R I S
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6-7 Inverted-vee dipole
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need to be shorter for any given frequency than a dipole There is no absolutely rigorous equation for calculation of the overall length of the antenna elements Although the concept of absolute length does not hold for regular dipoles, it is even less viable for the inverted-vee There is, however, a rule of thumb that can be followed for a starting point: Make the antenna about 6 percent shorter than a dipole for the same frequency The initial cut of the antenna element lengths (each quarter wavelength) is 220 L= ft [616] FMHz After this length is determined, the actual length is found from the same cutand-try method used to tune the dipole in the previous section Bending the elements downward also changes the feedpoint impedance of the antenna and narrows its bandwidth Thus, some adjustment in these departments is in order You might want to use an impedance matching scheme at the feedpoint, or an antenna tuner at the transmitter
Sloping dipole ( sloper or slipole )
The sloping dipole (Fig 6-8) is popular with those operators who need a low angle of radiation, and are not overburdened with a large amount of land to install the antenna This antenna is also called the sloper and the slipole in various texts The author prefers the term slipole, in order to distinguish this antenna from a sloping vertical of the same name Whatever it is called, however, it is a half-wavelength dipole that is built with one end at the top of a support, and the other end close to the
154 High-frequency dipole and other doublet antennas
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6-8 Slipole or sloper
ground, and being fed in the center by coaxial cable Some of the same comments as obtained for the inverted-vee antenna also apply to the sloping dipole, so please see that section also Some operators like to arrange four sloping dipoles from the same mast such that they point in different directions around the compass (Fig 6-9) A single fourposition coaxial cable switch will allow switching a directional beam around the compass to favor various places in the world
Broadbanded dipoles
One of the rarely discussed aspects of antenna construction is that the length/diameter ratio of the conductor used for the antenna element is a factor in determining the bandwidth of the antenna In general, the rule of thumb states that large cross-sectional area makes the antenna more broadbanded In some cases, this rule suggests the use of aluminum tubing instead of copper wire for the antenna radiator On the higher-frequency bands that is a viable solution Aluminum tubing can be purchased for relatively small amounts of money, and is both lightweight and easily worked with ordinary tools But, as the frequency decreases, the weight becomes greater because the tubing is both longer and (for structural strength) must be of greater diameter On 80 m, aluminum tubing is impractical, and at 40 m it is nearly so Yet, 80 m is a significant problem, especially for older transmitters, because the band is 500 kHz wide, and the transmitters often lack the tuning range for the entire band Some other solution is needed Here are three basic solutions to the problem of wide-bandwidth dipole antennas: folded dipole, bowtie dipole, and cage dipole
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