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The BobTail Curtain

as plagiarized from the ARRL Antenna Book

click on the image for a larger one


 
 

The antenna system of Fig 62 uses the principles of cophased verticals to produce a broadside, bi-directional pattern providing approximately 5.8 dB of gain over a single element.  The antenna performs as three in-phase top-fed vertical radiators approximately 1/4 wavelength in height and spaced approximately 1/2 wavelength.  It is most effective for low angel signals and makes an excellent long-distance antenna for either 3.5 or 7 MHz.

The three vertical sections are the actual radiating components, but only the center element is fed directly.  The two horizontal parts A, act as phasing lines and contribute very little to the radiation pattern.  Because the current in the center element must be divided between the end sections, the current distribution   approaches a binomial 1:2:1 ration. 

The vertical elements should be as vertical as possible.  The height for the horizontal portion should be slightly greater than B, as shown in Fig 62.  The tuning network is resonant at the operating frequency.  The L/C ration should be fairly low to provide good loading characteristics.  As a starting point, a maximum capacitor value of 75 to 150 pF is recommended, and the inductor value is determined by C and the operating frequency.  The network is first turned to resonance and then the tap point is adjusted for the best match.  A  slight readjustment of C may be necessary.  A link coil consisting of a few turns can also be used to feed the antenna.