Tri-band vertical antenna without traps
The resonant frequency can be set by reducing the step length of the cable. Thus it is necessary to monitor the SWR it will increase when moving from the upper to the lower end of the range.
For the antenna 14/21/28 MHz advisable to choose the resonant frequency 21000 and 28500 kHz (best SWR at these frequencies). Most coaxial cables with polyethylene insulation) have a velocity factor 0,66, therefore, to cut the coaxial cable between the parts of the radiator 14 and 21 MHz the required length of 2.35 m, and for the length between the parts 21 and 28 MHz - length 1.75 m. The end of the cable is sealed at the length of about 20 mm and is connected with a tube.
After you configure both ends of the cable need to close the insulation, resistant to weathering. The cable is wound; diameter and four, respectively, five turns - about 150 mm.
For good performance requires another three or four radial length of 5 m. However, the simplest version of this antenna will work with one radial, or the earthing system of any size that allows the antenna location. The antenna was built and successfully tested HA5LO.
Author: J. Heimsdorf (Y23JN), Translated by A. Belsky; Publication: N. Bolshakov, rf.atnn.ru