VHF antenna with vertical polarization. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas
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Vertical polarization of radio waves is often used in amateur radio communication on ultrashort waves, and has recently found increasing use in the transmission of television programs. The creation of directed multielement "wave channels" with vertical polarization is associated with certain difficulties. They are primarily due to the fact that the vertical metal mast supporting the antenna is in the wave polarization plane and can significantly distort the characteristics of the antenna.
The figure shows a vertically polarized four-element antenna designed for operation in the 144 MHz amateur band. Its feature is the use of the so-called "J-antenna" as an active element, which is a half-wave vibrator fed from one of the ends (in this case, from the bottom) through a quarter-wave U-knee.
In general, this combination resembles the Latin letter J, from where the name of the antenna came from. Such a radiator is easily consistent with the most common unbalanced coaxial cable, but its most important advantage is that the lower point of the U-elbow is at zero potential and can be grounded. With regard to this antenna, this means that there is no need to introduce any insulating inserts into the mast and separate the actual active element from the mast. In other words, the antenna mast can be all-metal, which is convenient not only from a constructive point of view, but also provides high mechanical strength of the antenna as a whole.
The active element is the last leg of the mast, so it is made of a fairly thick duralumin tube with a diameter of 12 mm. For the directors and the reflector, a tube with a diameter of 6 mm was used. The bearing traverse is made of a dielectric (for example, fiberglass and fiberglass rod with a diameter of 10...12 mm). To increase the mechanical strength, the traverse is pulled up to the top of the antenna with two stretch marks (shown in the figure by a dotted line) from a nylon cord. In addition, for better balancing of the antenna in the vertical plane, the reflector is removed from the active element by a distance of approximately 0,2L.
The U-elbow is formed by a piece of duralumin pipe with a diameter of 12 mm, which is attached to the mast (active element) in its lower part with a wide duralumin plate, and in the upper part - with a dielectric spacer. The antenna is fed with a coaxial cable with a wave impedance of 50 ohms. The connection points of the cable to the U-elbow are determined by the minimum standing wave ratio.
Note. The dimensions of the antenna indicated in the article correspond to an operating frequency of 147 MHz. To manufacture an antenna for other frequencies, all of its dimensions should be changed in proportion to the ratio f / 147 (f is the required operating frequency in MHz). More information about the J-antenna can be found in the book "Antennas" by K. Rothammel, MRB, vol. 637. M., "Energy", 1967.
Literature
- "QST" (USA), 1979 #11
Publication: cxem.net
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