Tri-band vertical antenna without ladders. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas
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This antenna is similar to a multi-band radiator, divided into parts by "trawl" contours. But here, instead of resonant circuits inserted between the individual parts of the radiator, quarter-wave segments of a coaxial cable are used, shorted at one of the ends. The basic idea behind this design is that a short-circuited quarter-wave line acts in the same way as conventional parallel resonant circuits. But when using it, the dimensions and mechanical design of the antenna change significantly. The cable is wound, attached to the emitter and connected so that the inner core of the cable is connected to the lower part of the emitter, and the cable screen is connected to the upper part.
The resonant frequency can be adjusted by reducing the length of the cable in steps. In this case, it is necessary to monitor the SWR - it will increase as you move from the upper to the lower end of the range.
For a 14/21/28 MHz antenna, it is advisable to choose resonant frequencies of 21000 and 28500 kHz (the best SWR is at these frequencies). Most coaxial cables (with polyethylene insulation) have a shortening factor of 0,66, so for a piece of coaxial cable between the parts of the radiator at 14 and 21 MHz, a length of 2,35 m is required, and for a segment between the parts at 21 and 28 MHz, a length of 1,75 m. The end of the cable is insulated over a length of approximately 20 mm and connected to the tube.
After adjustment, both ends of the cable must be covered with weather-resistant insulation. The cable is wound; the diameter of four and, accordingly, five turns is about 150 mm.
Three or four more 5 m radials are required for good performance. However, the simplest version of such an antenna will work with a single radial, or with a ground system of any size that the antenna installation site allows. The antenna was built and successfully tested by HA5LO.
Author: J.Heimsdorf (Y23JN), Translated by A.Belsky; Publication: N. Bolshakov, rf.atnn.ru
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