''ΕΡΑΣΙΤΕΧΝΙΚΟ ΔΥΚΤΙΟ ΕΛΛΑΔΑΣ ΝΟΣΤΑΛΓΙΑ''
H ΚΑΛΥΤΕΡΗ ΕΛΛΗΝΙΚΗ ΜΟΥΣΙΚΗ ΑΠΟ ΤΑ 1960 ΜΕΧΡΙ ΣΗΜΕΡΑ ΟΛΟ ΤΟ 24ΩΡΟ . Ο ΣΤΑΘΜΟΣ ΤΩΝ ΜΕΓΑΛΩΝ ΕΠΙΤΥΧΙΩΝ ΤΟΥ ΕΛΛΗΝΙΚΟΥ ΠΕΝΤΑΓΡΑΜΟΥ ΣΤΗΝ ΕΛΛΑΔΑ ΜΕ ΕΚΠΛΗΚΤΙΚΗ ΠΟΙΟΤΗΤΑ ΗΧΟΥ HD.
In this article LodeRunner explains
what an Inverted-L antenna is, how it works, and why you might strongly
consider building one for use on the lower bands. While explaining its
use with a high degree of technical information, he’s written it in a
manner that’s easy to follow and digest. The diagram data is sourced
from EZ-NEC, which a link to the software is provided in the sidebar.
The Inverted-L Antenna and NVIS
An
“Inverted-L” antenna is basically a wire antenna, typically ¼ to ½
wavelength long on the band it is designed for. The Inverted-L antenna
is a common antenna for the 160 meter and 80 meter amateur bands, where
typical ¼ wave verticals are impractically tall for most amateurs.
In
the Inverted-L configuration, the first portion of the wire rises
vertically from the feedpoint, and at some height is bent roughly 90
degrees, and then extends horizontally to the unterminated end. The
feedpoint is very close to ground level (typically not more than 3 feet
above ground), and the antenna is worked against a Ground consisting of
one or more ground-rods, and/or a counterpoise consisting of one or more
radial wires – which may be buried, laid directly on the ground, or
suspended above the ground at some low height.
Because
the input impedance of a typical Inverted-L antenna is low, and the
feedpoint is at or very close to ground level, where ground losses are
substantial, it is very important to establish a good ground to work the
antenna against.
Most
of the amateur literature regarding the Inverted-L antenna is focused
on optimizing performance of the Inverted-L antenna for “DX” operation –
that is, high efficiency in radiating its energy in a pattern that is
low in elevation (low Take-off Angle, or ToA) – typically below 30
degrees relative to the horizon – which maximizes the distance to which
communications may be achieved. Effective NVIS communications, on the
other hand, require an antenna which is optimized to produce a pattern
where the majority of the radiated energy has a high ToA pattern –
ideally between 60 and 90 degrees – to provide reliable communications
from zero to several hundred miles.
Fig. 1: DX dipole pattern on 80 Meters
First, lets take a look at the difference between a good “DX Antenna” pattern vs. a good “NVIS Antenna” pattern –
Figure
1 is a diagram of a dipole optimized for DX communications; Figure 2
represents the exact same dipole, but the height has been lowered by
approximately 1/3 wavelength to optimize the antenna pattern for NVIS
communications.
