Two-way UHF antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas
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Antenna modifications. For some increase in the gain (when narrowing the bandwidth), and most importantly, to increase the noise immunity of the antenna, the reader N. Turkin from St. Petersburg proposes to modify the version of the two-band antenna. To do this, he recommends making the antenna reflector active. The result is a so-called "ZL" type antenna, similar in properties to a four-element antenna (see K. Rochthammel's book "Antennas". - M Energy, 1979).
The main feature of such an antenna is the attenuation of the signal from the opposite direction up to 40 dB.
Modification of the antenna comes down to connecting the upper attachment points of the vibrator half-loops and the reflector of the two-wire overhead symmetrical criss-cross transmission line (see figure). It can be made from wires with a diameter of 3-4 mm, located at a distance of 1,5 ... 2 mm from one another. The jumper on the reflector cable is lowered down until a quarter-wave symmetrical short-circuited line is obtained. The distance between the reflector and the vibrator should be L/8, the perimeter of the vibrator is 1,04L; reflector - 5% more (1,09L).
With an active reflector, the input impedance of the antenna drops to about 30 ohms, so the connecting cable with a characteristic impedance of 75 ohms should be connected through a quarter-wave unbalanced line (resistance transformer) in the form of a piece of coaxial cable with polyethylene insulation with a characteristic impedance of 50 ohms and a length of 0,66 L / 4 .
Author: Trifonov A.; Publication: cxem.net
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