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The same strategies that worked for vertical antennas also work for horizontal antennas, although in the horizontal case we are simulating a half wavelength (180 ) balanced antenna rather than a /4 unbalanced antenna Figure 26-3 shows several different shortened, low frequency antennas that are based on the same methods as the verticals shown previously Figures 26-3A through 26-3C show discretely loaded dipole antennas Figures 26-3A and 26-3B are center-loaded versions, and Fig 26-3C is a center (of each element) -loaded version In each case, it is assumed that the radiator elements are the same physical length In the example of Fig 26-3A, the coil is tapped to provide a match to either 52- or 75- transmission line In some variants, the shield of the coaxial-cable transmission line is connected directly to the junction of the coil and one radiator element, and the coax center conductor is connected to the tap that best matches the impedance of the line The impedance-matching problem is solved a little differently in Fig 26-3B In this type of antenna, the coil is used to center-load the dipole, but the transmission line is connected to a link wound on the same form as the loading inductor The turns ratio between the loading inductor and the coupling link determines the impedance match A hairpin linear loading scheme is shown in Fig 26-3D This design is basically the same as for the vertical case, but it is balanced out of respect for the design of the dipole This type of design is used for both the driven and parasitic elements on some commercial 40-m beam antennas In addition, some add-on 40-m dipoles (designed for beam antennas that were intended for operation at frequencies in the 20-m and higher bands) use this method Such antennas are essentially rotatable dipoles The continuously loaded dipole of Fig 26-3E is constructed like the helical vertical (ie, about a half-wavelength of insulated wire is wound over the entire length of an insulated rod or pipe of some sort) The winding can be either broken at its center point to accommodate the feedline, or link-coupled to the transmitter, or receiver, as shown in Fig 26-3F Of the two feed methods, the most popular appears to be the type that breaks the winding into two pieces The form of continuously loaded dipole in Fig 26-3E has a combination of the two methods of feed The distributed loading coil is broken into two sections, as is often done on continuously loaded dipoles But in this case, a portion of the overall inductance is made by using a discrete inductor that is part of a toroidal transformer T1 The length of the wire used for the continuous loading coil is somewhat shorter (find by experimentation) than otherwise would be the case because of the inductance of the transformer secondary Transformer T1 can be wound using the same sort of toroidal ferrite or powdered iron core used for Balun transformers
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26-3 Inductance loading of a dipole: (A) center loading with impedance tapped coil; (B) center loading with transformer coupling; (C) loading at center of elements; (D) linear loading (hairpin); (E) linear or length loading; (F) transformer coupling
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Loaded tower designs
Many amateur radio stations and other services use a rotatable beam antenna on top of a tower Typical towers are from 30 to 120 ft in height In my locality, towers to 30 ft, or less than 3 ft above the roof line, whichever is taller, can be erected without a permit as long as they are attached to the house and will not fall onto a power line, or across the property line, in a catastrophic failure Higher towers must be erected under a mechanical permit, and properly inspected by the county These towers typically support a two-element (or more, three is popular) Yagi /Uda beam antenna, or a quad, or some other highly directional antenna But they can also be treated as a vertical antenna under the right circumstances If the tower is close to 66 ft high (which is a popular height for amateur radio towers), then the tower (already /4) can be used as a resonant vertical on 75/80 m The same tower can also be used on lower frequencies if proper antenna tuning unit (ATU) components are provided Figure 26-4 shows a situation in which an 80- to 110-ft tower is insulated from ground, and is considered a random-length vertically polarized Marconi antenna Like other such antennas, it is fed by a simple L-section, or reverse L-section, ATU Note the approximate values of the inductors and capacitors in each configuration, and also their relationships On the 40- and 75/80-m versions, the tower is too long, so a series capacitance and shunt inductance are used for the ATU (ie, Reverse Lsection coupler) On the other hand, the tower is too short for 160 m, so the inductor and capacitor are reversed There is only one small problem with the design of Fig 26-4: Compared with grounded towers, insulated towers are expensive to buy and install In addition, lightning protection is probably better in the grounded tower So how do we work a grounded tower See Fig 26-5 In this antenna, the tower is grounded at its base If the tower is mounted to a concrete pedestal (the usual arrangement), then a separate ground rod and ground wire adjacent to the pedestal must be provided Concrete is not a good insulator, but it is also not a good conductor A delta feed system is used with this antenna That is, a single wire from the ATU is connected to a point on the antenna where an impedance match can be achieved It is important that the ATU be spaced away from the tower base (as shown), and the wire must be run straight to the feedpoint on the antenna This system is an example of several possible shunt feeding systems
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