To increase the gain of the antenna needs to increase its size. Maximum gain depends on its construction . But if there is no possibility to install antennas with larger sizes in the UHF band can apply another method , you can place already with the existing VHF antenna has one just like it. Thus obtained one antenna system consisting of two separate VHF antennas. In this antenna system to produce phasing power and antenna impedance transformation and selection of the distance between them. This principle of the antenna system can be used to further increase the gain. Merging the two antennas in a single antenna system stacked designated - " 5 * 5 " . Such arrangement of the two VHF antenna narrows the directivity pattern in the vertical plane and this allows an increase of gain. Gain antenna system also depends on the distance between the antennas themselves . Generally omnibus two identical VHF antennas to one antenna system leads to an increase in gain of about 3 dB. Below is the diagram of the antenna system of two antennas for VHF 144 MHz . One of the antenna gain of about 6 dB and the antenna system of the two antennas about 9 dB. This antenna system is 5 * 5 and has enough broadband directional. Figure A) shows an antenna system with conventional cutting vibrators (2) , the input impedance of each of the antennas of 37.5 ohms, is used to align coaxial cable ( two segments ) long 3L / 4 and impedance of 75 ohms. As a result, the input impedance of each antenna at the point of connection of the power line is 150 ohms (the transformation of resistance) , and with two antennas , it is reduced to 75 ohms. In Figure B) an example of folded dipole antenna for VHF system. In this case, the antenna impedance is 150 ohms as line segments used matching wiring line length L / 2 . These segments do not transform the input impedance , while merging the two antennas in the input impedance at 75 ohms.
Dimensions of antennas in the antenna system :
Reflector (1): 1028 mm . Vibrator (2): 995 mm . Director (3,4,5) : 927 , 914 , 914 mm .
Distances: Reflector - Vibrator(6-7) : 380 mm . Vibrator - Director(6-7): 508mm . The distance between the other directors ( 7-8-9 ) 508 mm .
The distance between the antennas of 1220 mm .