Practical_Antenna_Handbook_0071639586

CHAPTER 11
Vertical Arrays
Despite its longevity, the vertical antenna is either praised or cursed by its users,
depending upon their experiences with it. At HF in particular, “DXability” is
often the criterion for judging the antenna’s performance. As amateurs and
others have come to appreciate the importance of a good RF ground system to the vertical’s
performance, in recent years the vertical has come into its own on the 1.8- and
3.5-MHz bands because it provides superior low-angle radiation compared to the low
dipoles that are the only alternative for most users.
But a vertical antenna is omnidirectional in its azimuthal (compass rose) coverage;
that is, it transmits and it “hears” equally well in all directions. Thus, a single vertical
exhibits two weaknesses:
• It puts far more RF energy out in directions that are of no use (at that particular
moment, at least) than it does in the desired direction.
• It does not have any way of eliminating noise coming from all 360 degrees
around the compass when trying to receive a weak signal from a specific
heading.
With respect to the first point, there are only three things the user with a single vertical
can do to improve his/her signal at a specific faraway receiving location:
• Increase the transmitter power to the antenna.
• Improve the antenna efficiency by reducing resistive losses, especially by
making sure an adequate system of radials is installed at the base of the antenna.
• Increase the electrical length of the vertical radiator (up to a maximum of
5 l/8). This causes the low-angle field strength for a given transmitter power
to increase because high-angle radiation is being reduced as the vertical is
lengthened.
With respect to the second point, we note:
• Atmospheric band noise arrives from compass headings where the band is
“open”. That is, if it’s shortly after sunset at the site of the vertical antenna,
atmospheric noise on the MF and lower HF bands is likely coming only from
points east of the antenna, where darkness and ionospheric propagation are
prevalent. To the west, daylight absorption in the lower layers of the ionosphere
substantially reduces the noise levels at the vertical. This is true of the noise
(“QRN” in the long-established radio shorthand known as Q-signals) from
distant thunderstorms.
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