Multiband vertical antenna. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / HF antennas

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The "Ground Plane" vertical antennas, which have become widespread among shortwavers, do not have sufficient broadband and, without additional adjustment, can only be used to operate in a narrow frequency band. The so-called "thick" vertical antennas, whose radiating surface has various shapes, are free from this disadvantage and work satisfactorily in the frequency range with an overlap factor of up to 3.

The most widespread are conical (Fig. 1, a) and exponential (Fig. 1, b) antennas. The wave impedance of a conical antenna is constant along its length and depends on the angle a at the top of the cone. The broadband properties of the antenna increase with increasing a and reach an optimum at a=60-70°; in this case, the wave impedance of the antenna is approximately 70-80 ohms.

Ris.1

An exponential antenna, whose wave impedance increases along its length approximately according to an exponential law, has the same broadband properties as a conical one. At the same time, the exponential antenna has a great advantage - its maximum diameter is 3 times smaller than that of the conical one.

For the short-wave range, it is practically not possible to implement an antenna with a continuous radiating surface in the form of figures shown in Fig. 1. Such antennas are made of tubes or wires. For exponential antennas, in addition, the smooth envelope is replaced by a broken line.

The UW4HW radio station uses an exponential antenna for the 14, 21 and 28 MHz bands, the design of which is shown in fig. 2. The radiating system of the antenna is formed by six wires located in vertical planes at an angle of 60 ° to one another.

Ris.2

At the base and at the top of the antenna, the wires are electrically connected together and fixed to the carrier mast with the help of insulators. The latter is made of three pipe sections of the same length, connected by insulating inserts. A wooden pole can also be used as a carrier mast. The shape of the antenna is provided by braces fixed at one third of the total height of the antenna. Each spacer ends with an insulator through which the antenna wire passes. If necessary, you can refuse to install spacers and provide the shape of the antenna with the help of guy wires attached to the wires at the inflection points using insulators. In this case, if the mast has sufficient rigidity, additional stretch marks can be dispensed with.

The antenna is powered by a coaxial cable with a characteristic impedance of 75 ohms. The center conductor is connected to the lowest point of the antenna, and the braided shield to a good earth ground if the antenna is installed directly on the ground, or to artificial earth if the antenna is installed on the roof of a house. The artificial ground can be a metal roof or six horizontal wires radiating from the base of the antenna. Artificial earth wires are located in the same vertical planes with the corresponding antenna radiating wires and have a length equal to the length of the radiating wires.

The antenna and artificial earth are made of copper wire with a diameter of 1,5 mm. Practically measured SWR values ​​in the frequency range from 14,0 to 29,7 MHz are in the range of 1,2-1,9. It is easy to calculate the dimensions of the antenna for other frequency ranges by setting the length of the antenna wires within (0,24-0,28) l min and the angle a at the base of the antenna within 60-70 °.

The experience of using the described antenna shows that in terms of its characteristics it is superior to the "Ground Plane" type antenna and, due to the simplicity of execution, can be successfully used in amateur radio practice.

Author: Yu. Matiychenko (UW4HW); Publication: N. Bolshakov, rf.atnn.ru

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