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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!


Grounding and Feeding

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.



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