vertical antennas. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / HF antennas

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Vertical antennas have long attracted shortwave antennas by the ability to receive radiation at a low angle to the horizon at a low antenna installation height, as well as by an almost circular radiation pattern in the horizontal plane. In addition, such antennas, which take up relatively little space, are convenient if the radio amateur cannot find sufficiently remote and high antenna suspension points.

Vertical antennas, of which the Ground Plane is the most common, are usually designed to operate in one band, especially if tuned elements are used to match the antenna with the feeder.

Previously proposed (for example, SP3PK, UF6FB, etc.) multi-band antennas are quite difficult to manufacture and configure.

The described multiband vertical antenna is structurally simple. In the simplest version, it is designed to operate on the 7,14 and 21 MHz bands, and with a slight complication - on 28 MHz. Antenna height 10 m. 1 shows the distribution of current in the antenna and the shape of the radiation pattern in the vertical plane depending on the ranges (ie, the ratio of antenna height and wavelength).

From fig. 1 shows that in the ranges of 7 and 21 MHz, the antenna is powered by current, and at 14 and 28 MHz - by voltage, therefore, with direct power supply by a coaxial cable in the ranges of 14 and 28 MHz, the SWR will be unacceptably large.

Fig. 1

In this antenna, the task of matching with the feeder at 14 and 28 MHz is solved by using an asymmetric quarter-wave transformer (see Fig. 2).

For a matching transformer, a segment of a 600-ohm line can be used. At the same time, when operating on 7 and 21 MHz, a direct connection of the cable to the antenna base is required. It turned out that points C and D (Fig. 2) can be combined (by bending the matching segment into a circle or triangle with smooth curves). In this case, the performance of the transformer even improves. Such a combination turned out to be possible because the transformer is fed asymmetrically (with an asymmetrical load), and its lower wire serves mainly to create a constant linear capacitance in relation to the upper (in the figure) wire of the transformer.

Fig. 2

If points A and B are shorted, the transformer will be short-circuited and the cable core will be connected to the base of the antenna. In this position, the antenna operates on 7 and 21 MHz. A small capacitance between the wires of the transformer is connected in parallel with the current fed antenna and does not affect its operation.

Thus, the operation of the antenna on three bands is ensured without the use of coils or capacitors. The efficiency of the antenna is quite high, since its length is equal to or greater than a quarter of a wave on all operating bands. At 14 MHz, due to the narrowing of the diagrams in the vertical plane, the antenna gives a gain of 1,8 dB (one and a half times in power) compared to a quarter-wave vertical antenna. At 21 MHz, for certain radiation angles, the amplification reaches two times in power. Structurally, the antenna is a composite duralumin tube with a diameter of 4 cm and a length of 10 m. It is mounted on a high porcelain insulator. 12 cm. When operating at 14 MHz, a large RF voltage (hundreds of volts) develops at the base of the antenna, so the insulator must have low losses and provide a minimum capacitance of the antenna base to ground.

A simple relay with two contacts, which we still could not get rid of in this design, is located at the base of the antenna and is enclosed in a hermetic box. The wires of the transformer have a diameter of 1,6 mm, the distance between them is 12 cm. When using a different wire diameter, in order to maintain the same impedance (600 ohms), it is necessary to change the distance between the wires. Plexiglas insulators are placed between the transformer wires every 30 cm. The quarter-wave transformer is concave into an irregular triangle with smooth bends and is supported in space with two wooden or bamboo spacers 1,8-2 m long.

On the 7, 14 and 21 MHz bands, the SWR measured by the reflectometer was less than 1,6 (the reflectometer was calibrated by resistance with a 10% tolerance).

With some complication, this antenna can be turned into a four- or five-band antenna. If you use a quarter-wave transformer half the length (2,6 m), the same antenna can be used at 28 MHz. In practice, such a switch can be made when either the entire transformer or its floorboard is switched on between the cable and the antenna resistance, or it is completely short-circuited.

To do this, it is enough to take one antenna relay from the RSB-5 radio station and place it with the anchor down. With the relay de-energized, the antenna will operate at 14 MHz.

The antenna can also be tuned to 3,5 MHz by placing an extension coil between the base and ground with a slider that allows you to change the inductance of the coil. The cable should be connected to one of the turns of the coil (closer to the cold end). The connection point can be selected according to the minimum SWR.

The described antenna can be classified as Ground Plane, that is, antennas operating with artificial earth. To do this, it must be raised to a sufficient height. If you install this antenna directly on the ground, grounding will be required, consisting of a large number of radial wires with a length of more than a quarter of a wave, otherwise the efficiency of the antenna will be very low, the antenna installation height should be such that the distance between the ends of the wires - counterweights is less distance from the ends of the wires to the ground (or roof). So, for a given antenna with 6 wires, the height of the base should be 10 m, at 8-7 m, at 12-5 m, at 20-3 m. Under these conditions, the efficiency of the antenna will be close to the maximum. The wires should have a length of 10,5 m and a diameter of 2-3 mm. It is best to install such an antenna on a galvanized iron roof. In this case, the base can stand directly on the roof. The cable sheath should be securely soldered to the wires soldered in turn to several sheets of iron. The antenna on the painted iron roof works worse (due to poor contact between the sheets).

The antenna was operated at the author's radio station for a year and a half. In the 7 MHz band, RST 599 was received from all continents except Oceania (589 from VK3AZZ). Links have been established with many DXs.

On the 14 and 21 MHz bands, RST 599 has been received many times from representatives of all continents.

Author: L. Yaylenko (UT5AA), Donetsk; Publication: N. Bolshakov, rf.atnn.ru

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