Calculation of the elements of a J-shaped antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas
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A particularly simple way to power a half-wave vertical antenna is to connect a closed quarter-wave line "B" to the high-resistance lower end of the radiator and search for resistance along its length, corresponding to the characteristic impedance of the power cable. Since along the length L / 4 of the line, the resistance changes from tens of thousands of ohms at the point of the lower end of the radiator (voltage maximum) to zero at the closing point of the matching line (voltage minimum), it is thus possible to match a coaxial cable and a VHF ribbon cable with any wave impedance to the antenna, as well as a two-wire power line with air insulation and a wave resistance of 600 Ohm.
The advantage of this method of feeding the antenna is that the lower closed end L/4 of the matching line "B" can be grounded and, in addition, the antenna can simultaneously serve as a good lightning rod.
The optimal matching of the power line with the antenna (size "C") is sought as follows: a neon light is connected to the emitter (near the upper point of the matching cable), and the power cable connection point is moved until the brightest glow of the light comes.
It is recommended to make the quarter-wave matching line "B" slightly longer than the calculated length and use a movable closing bridge. The antenna is then excited with a nearby auxiliary antenna connected to the transmitter. At the same time, power lines are not connected to the quarter-wave line and, by changing the location of the closing jumper, the maximum glow of the neon bulb is achieved, which is turned on at the connection point of the lower end of the emitter with the quarter-wave matching line. Thus, the emitter and the matching line are tuned to resonance with the operating frequency. Then the auxiliary antenna is disconnected from the transmitter and the vertical radiator is connected to the power line, the coordination of which is carried out in the manner described above. A radiator configured in this way is an excellent antenna for establishing long-distance communications. The antenna is omnidirectional provided that it is mounted at a high enough height.
Literature
- K. Rothammel. Antennas. M. Energy. 1979
Publication: cxem.net
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