VHF antenna with vertical polarization
Vertical polarization of radio waves is often used in Amateur radio communication on VHF, and more recently finding wide application and transmission of television programs. The creation of a multielement directional "wave channels" with vertical polarization is connected with certain difficulties. They are caused primarily by the fact that the vertical metal mast that supports the antenna is in the plane of polarization of the wave and may significantly distort the performance of the antenna.
The figure shows up antenna with vertical polarization, designed to work in the Amateur 144 MHz band. Its peculiarity is the use as the active element of the so-called "J-antenna", which is a half-wave vibrator, fed from one of the ends (in this case - from the bottom) through a quarter-wave U-knee.
Overall this combination resembles the Latin letter J, hence the name of the antenna. Such an emitter can easily adapt to the most common unbalanced coaxial cable, but its main advantage is that the bottom point of the U-knee is at zero potential and can be grounded. In relation to the antenna this means that there is no need to enter in the mast any insulating insert and separate the actual active element from the mast. In other words, the antenna mast can be all metal, and it is convenient not only from the structural point of view, but also provides high mechanical strength of the antenna as a whole.
The active element is the last knee of the mast, so it is made of fairly thick dural tube diameter of 12 mm. For Directors and reflector used tube diameter of 6 mm. Bearing cross member made of a dielectric material (e.g. fibreglass and glass fibre laminate rod with a diameter of 10...12 mm). To increase the mechanical strength of the cross member is lifted to the top of the antenna is two marks (shown by a dotted line) of a nylon cord. In addition, for better balancing of the antenna in the vertical plane reflector is removed from the active element at a distance of about 0.2 L.. It is somewhat more optimal (antenna gain) of the distance between these elements, amounting to 0.16 L..
U-knee formed by the cut dural tube diameter 12mm, which in its lower part attached to the mast (active element) with a broad dural plate and the top of using dielectric spacers. Nourish the antenna coaxial cable with a characteristic impedance of 50 Ohms. The point of connection of the cable to U-the knee is determined by the low coefficient of standing wave.
Note. Referred to in article antenna dimensions correspond to the operating frequency of 147 MHz. For the manufacture of the antenna at other frequencies all its dimensions should be changed in proportion to the ratio f/147 (f is the desired operating frequency in MHz). Read more about J-antenna can be read in C. Rothammel "Antenna", MRD, vol. 637. M., "Energy", 1967.
- "QST" (USA), 1979, NO. 11