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MFJ ANTENNA-TALK #3
3-Element Bob-Tail Curtain Array
Bob-Tail Curtain Vertical arrays do not require a radial ground/earth system, nor do they need any "steering". The antenna is "broadside" array, bi-directional, and perpendicular to the run of the wire. The center ¼ wavelength wave is fed with coax using a simple "L-Network" installed at the base of the center vertical if the pole is at, or near minimum height (35 ft). A hi-voltage variable capacitor of ~ 100 -150Pf, and ~20 turns of an 8 TPI, 2 1/2" diameter Airdux coil will do. Another option is using a remotely-located, weather-protected auto-tuner (MFJ-993), which can also provide operation on other bands.
The end verticals act to "binomial-cancel" signals off the ends, and vector add to the center vertical resulting in gain (~10-15dB over a single vertical at ~2000+ miles). The real benefit comes from the deep nulls off the ends, which can effectively null-out such stations such as "JA's/Caribbean" when the interest is Europe/Oceana. The downside: noise levels are typical of verticals when compared to horizontal arrays (2-3 units higher in most cases).
Orientation is best broadsides NE/SW and/or NW/SE; however, super results are achieved with right-angle installation pairs for world coverage. Great signal reports result; while cutting through the pile-ups to DX stations of great distance. Long path results are very good, and one usually can work Europe "round the clock" in winter from higher latitudes.
The Bobtail Curtain Array is enhanced when a ½ wavelength vertical element is placed between 0.2 and 0.25 wavelengths behind the center element to afford some unidirectivity as a parasitic reflector. Similarly, a parasitic director vertical element placed in front of the center vertical at about 0.12 to 0.15 wavelengths enhances unidirectivity even more. These elements can be "loaded" to achieve their 1/2 wavelength electrical length. However, when going unidirectional, you lose the world coverage aspect of a pair of arrays in right-angle configuration.
Another configuration is adding another array aligned in parallel, spaced 1/8th wavelength, and connected by a 3/8th wavelength coaxial delay line. This provides maximum forward gain and good unidirectivity.
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