Single-band fast deployment antenna. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / HF antennas

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Summertime is the time for amateur radio gatherings and events. Before leaving for nature, appropriate preparation of equipment and antennas is necessary. Most often, in forest clearings, variations on the theme of "inverted V" are visible. But how effective is such an antenna? Its most important part - the one where there are voltage antinodes - these are the ends of the antenna web, and they are just in the most disadvantageous position - near the ground. And the kink at the top of the inverted V does not at all improve the radiating properties - the electromagnetic fields of both halves of the antenna partially cancel each other because of this. Another option - a split dipole with two suspension points, for example, between trees, also has its drawbacks: it is difficult to stretch the antenna well. Under the weight of the power cable, its center sags and the height of the suspension is lost.

The question of matching the resistance of the antenna with the cable is also usually far from ideal. An antenna of fixed length, once tuned to resonance, is not bound to resonate when moved to another location. As a result, due to the mismatch of the antenna impedance at the power point with the wave impedance of the cable, the SWR increases. True, if a transceiver with an antenna tuner is used, and the antenna has a power line matched to a long wave, a multiple of a half-wave, then you can not pay much attention to the impedance mismatch. The place of connection of the antenna, as it were, is transferred directly to the transceiver. But this situation does not always occur.

There is another way to "closer" the feeding point of the antenna and at the same time simplify its suspension. This is the use of a half-wave continuous vibrator with its power supply from the end in the antinode of the voltage using a matching device in the form of a quarter-wave short-circuited coaxial transformer. The cable loop additionally performs the role of a feeder. This design allows you to place the radiating and supply-matching part of the antenna on the same straight line and suspend it by pulling it at the opposite end with the help of a nylon cord previously thrown over the block.

The transceiver can be connected directly to the matching device of the antenna system at a point, at some distance from the short-circuited end of the loop. In this case, there is no need to carefully select the connection point, the antenna tuner will perform the task of optimal matching itself. If there is a need to connect a coaxial line, then in this case the proposed antenna will facilitate the matching process. The fact that the junction with the power cable is located at some distance from the radiating part of the antenna, being "in the hands" of the radio amateur, will also help here, which allows you to more carefully select the resistance at the feed point, corresponding to the characteristic impedance of the feeder used.

In urban conditions, it is sometimes possible to coordinate such an antenna without leaving the apartment in an absolutely comfortable home environment. Setting up the antenna system consists in selecting the length of the matching cable and determining the location of the cut for connecting the power supply. This work is greatly facilitated by the use of a coaxial cable with PTFE insulation. It can be easily extended by soldering the junction over the screen braid without fear of a short circuit with the central core. It is advisable to use a cable with a wave impedance of 50 ohms, since the devices necessary for tuning are mainly designed for such a load.

The antenna circuit is shown in fig. 1. The length of the radiating part is equal to half the wavelength in the middle of the selected range, taking into account a shortening factor of approximately 0,95. This length does not change during antenna tuning. The calculated length of the matching loop is equal to a quarter of the wavelength multiplied by the shortening factor of the cable used, which for many brands is approximately 0,66. Fine adjustment of the length of the loop is carried out using an RF generator, to the output of which a short-circuited cable segment is connected at the opposite end. By gradually shortening it, they achieve the maximum voltage at the output of the generator at the operating frequency.

Fig.1 Single-band antenna with a matching loop

It is desirable to make voltage measurements with the minimum possible connection between the measuring head of the voltmeter and the place of measurement. For this, an isolation capacitor with a capacity of not more than 1 pF is used. Measurements are preferably made using a voltmeter with a digital scale. After specifying the length of the matching transformer, the junction of the cable with the antenna is mounted. The end of the cable is freed from the upper insulation and braid at a distance of about 5 cm, tightly pulled together with a nylon fishing line with a Plexiglas insulator. Then, the antenna wire is soldered to the central core of the cable, which is also pulled together with a fishing line with an insulator. The junction is hydrolyzed with electrical tape impregnated with BF glue. To remove the mechanical load from the cable, the antenna is stretched in the direction of the source using a synthetic cord attached at one end to a hole in the insulator, and at the other, for example, to a peg in the ground. Such a semi-finished product of the antenna is hung out in space at a maximum distance from conductive objects, but in such a way that the matching cable is available for further manipulations with it.

The cable must be cut in the proportions indicated in the figure, cut the resulting parts, put them together and reconnect with light soldering. At the junction, through the SWR meter, it is necessary to apply a low power signal from the transceiver, after setting the P-loop or antenna tuner to a 50-ohm equivalent. By changing the ratio of the lengths of both pieces of cable within certain limits, it is necessary to achieve the minimum power of the reverse wave.

In tuning this antenna, the SWR meter of the company "DIAMOND ANTENNA" SX-100, kindly provided to the authors of the "Radio Design" collection by the "BERMOS" store, proved to be excellent.

Ultimately, during the tuning process, the compliance of the input impedance of the antenna with the characteristic impedance of a 50-ohm coaxial cable is checked. A cable bay of arbitrary length should be connected to the configured loop, and the SWR value should not change and remain close to 1. After completing the settings, you should also check the reliability of all solders and seal them.

The operating frequency band of such an antenna system on the ten-meter range was about 500 kHz, and on forty meters - 120 kHz with SWR values ​​​​not worse than 1,2. The antenna, due to its selective properties, turned out to be much better protected from powerful out-of-band pickups, and the presence of a short-circuited loop makes it immune to the accumulation of atmospheric static electricity.

Elongation of the antenna fabric in proportion to a multiple of the number of half-waves improves the efficiency of the antenna and almost does not change the size of the matching cable. Good results can also be obtained with similar matching with collinear antennas, which are mentioned in the articles by G.I. Osipov (RV3AK) in "R-D" No. 1, 2 for 1997

Calculator for calculating the approximate length of the antenna elements: