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192 Vertically polarized HF antennas
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7-11A Construction of the vertical coaxial antenna
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Figure 7-11B shows a construction method that has been used by amateurs with good results The radiator and shield pipe (sleeve) are joined together in an insulating piece of thick-wall PVC plumbing pipe, Lucite, or plexiglass tubing; 6 to 10 in of tubing are needed A gap of about 2 in is left between the bottom end of the radiator pipe and the top end of the shield pipe in order to keep them electrically insulated from each
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Variations on the vertical antenna theme 193
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7-11B Vertical dipole made from tubing
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other, and to allow the coaxial cable to be passed through to the outside world A hole in the insulator pipe is drilled for this purpose The aluminum tubing pieces for the radiator and the sleeve are fastened to the insulator with at least two heavy machine screws each One of the machine screws on each can be used as the electrical connection between the coaxial cable and the pipes, provided that a larger hole is cut in the insulator at that point to admit the washer that provides the electrical pathway between the screw head and the aluminum pipe
194 Vertically polarized HF antennas If you depend upon the machine screw touching the pipe at the edges of the hole cut for it, then there will probably be intermittent connection and all of the aggravation that ensues Mounting of the homebrew coaxial vertical antenna can be a pain in the neck Normally, this antenna is mounted high in the air above ground, so some form of support is needed Fortunately, you can use small-area metal supports connected to the sleeve for this purpose Figure 7-11C shows one method for mounting that is popular A pair of television antenna standoff mounting brackets are used to support the sleeve Those brackets can be bought in sizes from 6 to 24 in Note that a 2 4 piece of lumber is used between the building wall and the brackets This wood serves as an
op radiator
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Wall Bottom radiator 2 4 7-11C Mast mounting system for verticals
TV antenna stand-off brackets
Vertical antenna construction 195 insulator, so it should be varnished or painted It is held to the wall with lag bolts, wing bolts, or some other effective method of anchoring Keep in mind that the forces on the brackets increase tremendously during wind storms! It is possible for the two vertical antennas shown previously to become a shock hazard to anyone who touches them Both of these antennas are half-wavelength radiators and are of the dipole form of construction The center point is used for feeding the antenna, so it forms the low-impedance point in the antenna As a result, the ends of the antenna, one of which is close to the ground, are the highimpedance points hence the voltages at those points within reach of prying hands playing in the yard can be high It is wise to either mount the antennas so far above the ground that they cannot be reached, or build a small nonconductive fence around the bottom end of the antenna
Vertical antenna construction
There are two general cases for installing vertical antennas: ground-level-mounted and non-ground-mounted This section takes a brief look at both forms of mounting We will concentrate on installation of homebrew verticals rather than commercial, because it is assumed that the vendors of such antennas will provide their own instructions The ground-level-mounted vertical is shown in Fig 7-12 The typical vertical antenna is 8 to 40 ft high Thus, although the actual weight of the antenna is small, the forces applied to the mounting structure (especially during windstorms) can be quite high Don t be fooled by the apparent light weight of the antenna in this respect The mounting structure for the vertical antenna can be a metal or wooden fence post buried in the ground At least 2 ft of the fence post should be above ground In the case of Fig 7-12, a 4 4 wooden fence post is used as the mounting, but the principles are similar for all forms of post A fence post hole is provided that is at least 2 ft deep In some cases, it might be possible to use a 1-ft-deep gravel fill topped with back-filled dirt In other cases, especially where a steel fence post is used, a concrete plug is placed at the bottom of the hole over a 4-in layer of gravel The antenna radiator element is installed onto the fence post using standoff insulators Unfortunately, these insulators are difficult to find, so they might have to be omitted Given that varnished or painted wood is not a terribly good conductor, it is not unreasonable to bolt the radiator directly to the 4 4 fence post Use 5 16-in (or larger) bolts, and make them long enough to fit completely through both the antenna element and the 4 4 post Thus, 6-in-, 7-in-, or 8-in-long 5 16-in bolts are the candidates for this job Use at least two bolts, one at the bottom of the antenna radiator element, and one near the top of the fence post A third bolt, halfway between the other two, would not be out of order If the antenna is quarter-wavelength, then no matching is generally necessary Although the feedpoint impedance is not exactly 52 , it is close enough (37 ) to form a reasonable match for 52- coaxial cable (with VSWR = 14:1) The center conductor of the coaxial cable is connected to the radiator element, while the shield is connected to the ground system In the example shown in Fig 7-12, two ground methods are used First, an 8-ft ground rod is driven into the earth at the base of the
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