Bobtails and Half Squares|
These antennas offer big gun DX performance on the low bands,
yet they are surprisingly simple to build. Try one- you will like it!
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!
"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
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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