W6BCX, the inventor of the bobtail antenna, writes
in Ham Radio Magazine, February/March 1983,
regarding voltage feeding bobtails (and half squares):
"a small ground screen laid on the ground or a flat
roof, or suspended...
near the... feed point, (such as a)
3 by 5 foot piece of (wire) cloth... makes a highly effective
RF ground, much better than something buried in or
driven into the soil, for a ground-independent antenna..."
Note that he is talking here about a floating ground screen
or, in other words, a counterpoise.
"If the impedance between (the) antenna feedpoint and ground
is over about 1000 ohms there will be very little current
flowing to ground... or ground substitute. The bobtail falls
in this category."
Remember, the high current points are
a quarter wave up in the air, and the horizontal phasing
line is, in effect, the 'radials'- these are inverted ground
plane (IGP) antennas!
He adds,
"resonant radials above ground...
are not required for efficient operation... and may actually
upset the pattern under some conditions. An earth
ground is useful primarily for lightning protection,
and even if one is employed near the feedpoint for
this purpose, a small ground screen in addition is
recommended."
Local earth quality is important, however.
ON4UN, writing in "Low-Band DXing" advises
"Do not be misled into thinking that the bobtail array
does not require a good ground system because it is a
voltage-fed antenna. As for all vertically polarized
antennas, it is the electrical quality of the reflecting
ground that will determine the efficiency and the low-angle
radiation of the array."
Some hams have reported that performance
was improved by adding a large area ground
screen or lots of non-resonant radials on or in the earth
under their antennas. Presumably, these were not
directly tied to the coax/tuning network ground return, but were
added rather to enhance the local ground conductivity.
The cold, hard reality is, however, that the surrounding
ground quality
determines the "pseudo-Brewster angle" below which
a vertical antennas radiation drops off sharply. On the low
bands, the areas around the antenna where ground reflections
may contribute to lower transmission angles includes
distances from tens to thousands of feet,
or even a mile away. With IGPs, improving local ground
conductivity might slightly improve efficiency and local
field strength measurements, but realistically, there
is nothing that can be done to change your minimum possible
transmission angle, other than moving to a new location.
As always, verticals work superbly well over salt water, good
over average earth, and poorly over dry, arid earth.
Editors note, June 2015:
W6BCX, writing as the editor of Antenna Manual (1948) mentions use
of a ground screen at the antenna feedpoint, but also says that
a short ground rod alone may suffice if it is nearby. Notice that
by the time he wrote his two part article in Ham Radio Magazine, Feb/Mar
1983, some 35 years later, his emphasis had shifted sharply.
He advised the use of a ground screen, with the ground rod valued mainly
as a lightning conductor only. The relevant passage from the 1948 Antenna Manual
is quoted below:
"The ground connection need not have very low resistance, as the current
flowing in the ground connection is comparatively small. A stake or pipe
driven a few feet in the ground will suffice. However, the ground lead
should be of heavy wire and preferably the length should not exceed about
10 feet at 7 Mc. or about 20 feet at 4Mc. in order to minimize reactive
effects due to its inductance. If it is impossible to obtain this short a
ground lead, a piece of screen or sheet metal about four feet square may be
placed parallel to the earth in a convenient location and used as an
artificial ground. "
(Editor: Read 'artificial ground' as 'counterpoise')
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