Antenna with switchable polar pattern refers to the type of two-element linear antenna with active power and is designed to work in the range of 7 MHz. A gain of about 6 dB, the ratio of front to back 18 dB, "sideways" - 22-25 dB. Width days at the half-power level of about 60 deg.
Fig.1For 20 m range L1=L2= 20,57 m: L3 = of 8.56 m
The bimetal or ant. cord 1,6 to 3 mm.
I1 =I2= 14m cable 75 Ohm
I3= 5,64 m cable 75 Ohm
I4 =7,08 m cable 50 Ohm
I5 = arbitrary length cable 75 Ohm
K1.1 - RF relay REV-15
As can be seen from Fig.1, two active vibrator L1 and L2 are located at a distance L3 (phase shift of 72 degrees) from each other. The items supplied with opposite phase, the total phase shift is 252 degrees. K1 switches the direction of radiation by 180 degrees. I3-plume phase-shifting I4 - quarter-wave matching section. The antenna tuning is the adjustment of the size of each element one at a minimum SWR with closed-circuit through the half-wave repeater 1-1(1.2) the second element. SWR in the middle of the range does not exceed 1.2, on the edges of the range -1.4. The sizes of vibrators are for suspension height of 20 m. From a practical point of view , especially when working in competitions, well-proven system consisting of two such antennas perpendicular to each other and spaced apart. On the roof in this case is a switch that gives an instant switch days in one of four directions.
One of the options for the location of antennas among typical urban development is proposed in Fig.2.
Fig.2
This antenna is used since 1981, many times repeated on different QTH, it spent tens of thousands of QSOs with more than 300 countries in the world.
Author: D. Kolenchuk, RW1ZZ; Publication: N. Bolshakov, rf.atnn.ru