.net barcode reader library High-frequency dipole and other doublet antennas in Software

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High-frequency dipole and other doublet antennas
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A MYTH AROSE IN RADIO COMMUNICATIONS CIRCLES SOME TIME AGO PEOPLE CAME TO
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believe (especially in the ham and CB communities) that large antenna arrays are absolutely necessary for effective communications, especially over long distances Overlooked, almost to the point of disdain, were effective (but simple) antennas that can be erected by inexperienced people and made to work well The simple dipole, or doublet, is a case in point This antenna is also sometimes called the Hertz, or hertzian, antenna because radio pioneer Heinrich Hertz reportedly used this form in his experiments The half-wavelength dipole is a balanced antenna consisting of two radiators (Fig 6-1) that are each a quarter-wavelength, making a total of a half-wavelength The antenna is usually installed horizontally with respect to the earth s surface, so it produces a horizontally polarized signal In its most common configuration (Fig 6-1), the dipole is supported at each end by rope and end insulators The rope supports are tied to trees, buildings, masts, or some combination of such structures The length of the antenna is a half-wavelength Keep in mind that the physical length of the antenna, and the theoretical electrical length, are often different by about 5 percent A free-space half-wavelength is found from 492 ft FMHz
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[61]
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In a perfect antenna, that is self-supported many wavelengths away from any object, Eq 61 will yield the physical length But in real antennas, the length calculated
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142 High-frequency dipole and other doublet antennas
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6-1 Simple half-wave dipole antenna
above is too long The average physical length is shortened by up to about 5 percent because of the velocity factor of the wire and capacitive effects of the end insulators A more nearly correct approximation (remember that word, it's important) of a half-wavelength antenna is L= where L is the length of a half-wavelength radiator, in feet FMHz is the operating frequency, in megahertz Example Calculate the approximate physical length for a half-wavelength dipole operating on a frequency of 725 MHz Solution: L= = or, restated another way: L = 64 ft 66 in It is unfortunate that a lot of people accept Eq 62 as a universal truth, a kind of immutable law of The Universe Perhaps abetted by books and articles on antennas that fail to reveal the full story, too many people install dipoles without regard for 468 ft FMHz 468 ft = 6455 ft 725 468 ft FMHz [62]
The dipole feedpoint 143 reality The issue is resonance An antenna is a complex RLC network At some frequency, it will appear like an inductive reactance (X = +jXL), and at others it will appear like a capacitive reactance (X = jXC ) At a specific frequency, the reactances are equal in magnitude, but opposite in sense, so they cancel each other out: XL XC = 0 At this frequency, the impedance is purely resistive, and the antenna is said to be resonant The goal in erecting a dipole is to make the antenna resonant at a frequency that is inside the band of interest, and preferably in the portion of the band most often used by the particular station Some of the implications of this goal are covered later on, but for the present, assume that the builder will have to custom-tailor the length of the antenna Depending on several local factors (among them, nearby objects, the shape of the antenna conductor, and the length/diameter ratio of the conductor) it might prove necessary to add, or trim, the length a small amount to reach resonance
The dipole feedpoint
The dipole is a half-wavelength antenna fed in the center Figure 6-2 shows the voltage (V ) and current (I ) distributions along the length of the half-wavelength radiator element The feedpoint is at a voltage minimum and a current maximum, so you can assume that the feedpoint is a current antinode At resonance, the impedance of the feedpoint is Ro = V/I There are two resistances that make up Ro The first is the ohmic losses that generate nothing but heat when the transmitter is turned on These ohmic losses come from the fact that conductors have electrical resistance and electrical connections are not perfect (even when properly soldered) Fortunately, in a well-made dipole these losses are almost negligible The second contributor is the radiation resistance Rr of the antenna This resistance is a hypothetical concept that accounts for the fact that RF power is radiated by the antenna The radiation resistance is the fictional resistance that would dissipate the amount of power that is radiated away from the antenna
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