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of a regular dipole), make A = D and B = C In all three examples, Figs 10-4A through 10-4C, you can expect to find the length needed for resonance varies somewhat from the standard 468/FMHz value, and that the feedpoint impedance is other than 73 Also, the pattern will be distorted with respect to the regular dipole Although these antennas do not work as well as a properly installed dipole, the performance is sufficient to allow successful operation
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238 Hidden and limited-space antennas
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Another limited-space wire antenna is the (so-called) half-slope shown in Fig 10-4D Although single-band versions are often seen, the example in Fig 10-4D is a multiband version Resonant traps separate the different band segments This antenna operates like the vertical, but it is not omnidirectional Also, the feedpoint impedance will be different from the regular vertical situation If the impedance varies too much for comfort, insert an antenna tuner, such as the Transmatch, in the coaxial line between the transmitter and the antenna Figure 10-5 shows another antenna that s useful for limited-space situations Although it is easily constructed from low-cost materials, the antenna is also sold by several companies under various rubrics including cliff-dweller, apartment house, townhouse, or traveler s antennas The antenna consists of a 4- to 16-ft section of aluminum or copper tubing Some of the commercial antennas use a telescoping tubing that can be carried easily in luggage As was true with the longwire, the window-sill antenna is tuned to resonance with an L-section coupler The L-section coupler must be tuned to produce the lowest possible VSWR, so either an RF power meter or a VSWR meter must be installed in the coaxial cable between the transmitter and the tuner Again, a good ground (or radial system) will
Limited-space antennas 239
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greatly improve the operation of this antenna The performance should be considerably less than that of a good longwire, but it will work DX for you even on lower frequencies Once again it is not the best antenna made, but it will get you on the air Figure 10-6 shows the use of a mobile antenna for a fixed or portable location I used this type of antenna at one QTH to good effect Mount a mobile antenna, such as the Hustler, on the window sill, or other convenient mount (at least one amateur radio operator uses one mounted on the roof of the house) Grounding is essential for this antenna, as was also true for the longwire and window-sill antenna (Fig 10-5) The operation of this antenna is improved by installing at least two radials per band as a counterpoise ground My installation worked well, even though only two radials were present A directional rotatable dipole is shown in Fig 10-7 This antenna is made from a pair of mobile antennas connected back-to-back on a horizontal length of 1 2 lumber and fed in the center with a coaxial cable The end pieces of the mobile antenna set the resonance, and you must recognize that adjusting one requires a countervailing adjustment of the other as well Two examples of helically wound antennas are shown in Fig 10-8 In this type of antenna, an insulating mast is wound with a half-wavelength of antenna wire The overall length of the antenna is considerably less than a half-wavelength, except at the highest frequencies In order to dissipate the high voltages that tend to build up at the ends of these antennas, a capacitance hat is used These hats are either disks (pie tins work well) or rods of conductor about 16 to 24 in long The version shown in Fig 10-8A is a vertical antenna, and like other verticals, it must be installed over either a good ground or a counterpoise ground The version shown in Fig 10-8B is horizontally polarized
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