Butternut HF9V, Инструкция по эксплуатации

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TECH NOTES—GROUND RADIAL SYSTEMS
resistance is the textbook figure of 35 ohms. The feedpoint impedance would
then be 15+0+35 = 50 ohms, and the antenna would be perfectly matched to a
50 ohm coaxial line. Since the radiation resistance is an index of the amount of
applied power that is consumed as useful radiation rather than simply lost as heat
in the earth or in the conductor, the radiation resistance must be kept as high as
possible in relation to the total feedpoint impedance for maximum efficiency.
Efficiency, expressed as a percentage, may be found by dividing the radiation
resistance by the total feedpoint impedance of a resonant antenna, so under the
conditions assumed above our vertical antenna would show an efficiency of 35/50
= 70%. As a vertical antenna is made progressively shorter than one-quarter
wavelength the radiation resistance drops rapidly and conductor losses from the
required loading inductors increase. A one-eighth wave inductively loaded vertical
would have a radiation resistance of something like 15 ohms and coil losses (or
trap losses for multiband antennas) would be in the range of 5 ohms. Assuming
the same value of ground loss resistance (15 ohms), the feedpoint impedance
would become 15 + 5 + 15 = 35 ohms and the efficiency would be
15/35=43%. From the above calculations it is clear that the shorter a vertical
antenna must be the less efficient it also must be for a given ground loss
resistance. Or to state the matter another way, more elaborate ground or radial
systems must be used with shorter verticals for reasonable efficiency. If the
ground loss of resistance of 15 ohms from the preceding example could be
reduced to zero ohms, it is easy to show that the efficiency of our one-eighth
wavelength loaded vertical would increase to 75%. Unfortunately, more than 100
radials each one-half wavelength long would be required for zero ground loss, so
lower efficiencies with shorter radials must usually be accepted for the sake of
convenience. In spite of their limitations, short vertical antennas over less than
ideal ground systems are often more effective DX performers than horizontal
dipoles which must be placed well above the earth (especially on the lower bands)
to produce any significant radiation at the lower elevation angles. Verticals, on
the other hand, are primarily low-angle radiators on all bands.
ABOVE GROUND (ELEVATED) INSTALLATIONS (rooftop, tower, mast. etc.)
The problem of ground loss resistance may be avoided to some extent by
mounting a vertical antenna some distance above the earth over an artificial
ground plane consisting of resonant (usually 1/4
8
) radial wires. Four resonant
radials are considered to provide a very low-loss ground plane system for vertical
antennas at base heights of 1/2
8
or more. This arrangement contrasts favorably
with the more than 100 radials for zero ohms loss resistance at ground level, and
since 1/2
8
is only about thirty-five feet at 20 meters, very worthwhile
improvement in vertical antenna performance can be realized, at least on the
higher bands, with moderate pole or tower heights. At base heights below 1/2
8
more than four radials will be required to provide a ground plane of significantly
greater conductivity than the lossy earth immediately below the antenna: even so,