The inclusion of elements, electrically lengthening the antenna in the outer parts of the dipoles is more profitable compared to their inclusion in the center as it provides a higher radiation resistance relative to the resistance losses of the system. In addition, the loss in this case turn out to be less due to the space element in the point through which Flows less current. Finally this incorporation easier and from a constructive point of view - no need to plug in the center of the dipole insulating inserts that weaken the rigidity of the structure.
In accordance with the aforementioned considerations ham D. Walsh (W8HRF) built a shortened antenna for the 20-meter Amateur band. The antenna consists of an active element with a gamma matching unit and a passive Director. Elements, lengthening the antenna, are coils and capacitors. The latter is formed with a wire circles with crosshairs fixed on the ends of the elements (Fig. 1).
Fig.1
The coils contain the active element 15 turns Director with 11 turns of aluminum wire. The diameter of the coils 75 mm., To give rigidity introduced inside the coils spacers made of plexiglass.
The coil of the active element are mounted on an insulating insert, as shown in Fig. 2. For mounting the Director of this insert may not apply.
Fig.2
Capacitive elements are made of aluminum wire. The diameter of the circles 430 mm.
Adjusting the antenna was manufactured by contraction and extension of turns of the coils and changing the length of the elements. To work in the phone part of the band Director was configured 14.8 MHz, the active element is 14.3 MHz with GAD. When measuring SWR results were: of 14.2 MHz to 1.5; Of 14.3 MHz And 1.05; 14,35 MHz to 1.3.
When working on this antenna was used, the transmitter power of 2 kW, R. E. R. (1 kW CW).
The antenna can be recommended to fans experiencing difficulty with the placement of a full-sized antennas.
Author: D. Walsh (W8HRF); Publication: N. Bolshakov, rf.atnn.ru