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184 Vertically polarized HF antennas
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7-7A Antenna impedance as a function of antenna length
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The ground system for the vertical antenna 185
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Antenna length (degrees) 7-7B Radiation resistance as a function of antenna length
The ground system for the vertical antenna
The vertical antenna works well only when placed over a good ground system 28 gives details of proper ground systems for all radio antennas, verticals included, so only certain specifics are included here The reader should also examine Chap 28, however, in order to get a firmer grasp of the problem The usual way to provide a good ground for a vertical is to use a system of radials such as Fig 7-9A This case shows a view (from above) of 16 quarter-wavelength radials arranged to cover the full circle around the antenna Each radial is a quarterwavelength, so it will have a length (in fact) of 246/FMHz All of the radials are connected together at the base of the antenna, and the ground side of the transmission line is connected to this system The radials can be placed either on the surface or underground One friend of the author built an extensive radial system on the bare dirt when his house was built, so when the sod was installed he had a very high-quality underground radial system
186 Vertically polarized HF antennas
7-8 Impedance matching through broadband transformer
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If you decide to use an aboveground radial system, however, be sure to prevent people from tripping over it There might be liability implications for people who trip and injure themselves, even when the person is an intruder or trespasser Some experts prefer to place a copper wire screen at the center of the radial system The minimum size of this screen is about 2m square (6 6 ft) Connect it to the radials at the points shown using solder Other experts will drive ground stakes into the ground at these points Still another method is shown in Fig 7-9B This case shows a spider web of conductors shorting the radials at points a meter or two from the antenna Again, some authorities recommend that ground rods be driven into the earth at the indicated points The exact number of radials to use depends in part on practical matters (how many can you physically install ) Use at least two radials per band, with four per band preferred for simple, low-cost systems However, even four is considered a compromise case The general rule is: the more the better But it s also true that there is a law of diminishing returns as the number of radials is increased Figure 7-9C shows the approximate field intensity (mV/m) as a function of the number of radials Notice that the field intensity does not increase as rapidly with the number of radials above 20 or so Note that the Federal Communications Commission requires AM band (550 to 1700 kHz) stations to use 120 radials, but that number is not necessary for amateur stations A practical upper limit of 16 radials is usually accepted for amateur radio work, and four radials work quite well
Variations on the vertical antenna theme 187
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7-9A Comprehensive ground system for vertical antenna
For vertical antennas mounted above ground, there is an optimum height above ground for the base of the antenna This height is a quarter-wavelength above the actual ground plane Unfortunately, that distance might not be the height above the surface Depending upon ground conductivity and groundwater content, the height can be exactly a quarter-wavelength above the surface or slightly lower The point is found from experimentation, and will, unfortunately, vary over the year if climatic changes are usual
Variations on the vertical antenna theme
Thus far, the vertical antennas have been standard quarter- or 5 8-wavelength models This section looks at several variations on the theme Consider Fig 7-10 This antenna is the vertical half-wavelength dipole The vertical dipole is constructed in exactly the
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