Dual band UHF antenna
Modifications of the antenna. For some increase in gain (narrowing of the passband), and most importantly, to improve the noise immunity of the reader antenna N. Turkin from St. Petersburg offers the option to modify a dual band antenna. For this purpose, the reflector antenna he recommends to make active. The result is a so-called antenna type "ZL", similar properties up to the antenna (see the book of K. Rothammel "Antenna". - M Energy, 1979).
The main feature of this antenna the attenuation of the signal from the reverse direction to 40 dB.
Finalization of the antenna is limited to connection of the upper attachment points of Polymetal of the vibrator and the reflector two-wire aerial symmetric coupled transmission line (see figure). It can be run from wires with a diameter of 3 to 4 mm, raspolojennyh at a distance of 1.5...2 mm from one another. The jumper on the trail of the reflector is lowered down to obtain a quarter-wave symmetrical short-circuited line. The distance between the reflector and the vibrator must be equal to L/8, the perimeter of the vibrator-1,04 L; reflector is 5% more (1,09 L).
With the active reflector antenna impedance is reduced to about 30 Ohms, so connecting cable with wave resistance of 75 Ohm should be connected through a quarter-wave unbalanced line (transformer resistances) in the form of a segment of coaxial cable with polyethylene insulation with a characteristic impedance of 50 Ohms length 0,66 L/4.
Author: A. Trifonov; Publication: www.cxem.net