Antennas in the 27 MHz band. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / VHF antennas

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The new 27 MHz civil communication range has made it possible for many thousands of radio amateurs to go on the air. But sooner or later, the owner of such a radio station faces the question of increasing the communication range. This may be necessary to communicate with a remote object, for example, a summer house, a vacation spot, or with familiar owners of 27 MHz radio stations living at a considerable distance.

You may be interested in long distance communication on 27 MHz and collecting QSLs. Hundreds of thousands of people in the world are fond of this, and QSL cards of CB stations, in my opinion, are much more beautiful than shortwave cards.

In any case, long-distance communication with the whip antenna that is attached to the radio station is impossible. It is necessary to have an effective outdoor antenna. But the antenna must also be properly connected to the transmitter.

Most imported CB transmitters have a bayonet antenna connector, which allows you to disconnect the whip antenna and connect an external one (Fig. 1). Such a transmitter allows you to connect a 50-ohm coaxial cable loaded onto the antenna with a resistance of 30 to 100 ohms.

The antennas described in this article will just fit these parameters. CB transceivers manufactured in the CIS and simple foreign transceivers may not have such a connector. If your transceiver is to be used for long-distance communication, this connector must be installed. Further, the output of such transmitters will need a simple matching device, shown in fig. 75.

The inductor used in the matching device is frameless. It is wound with a copper wire with a diameter of 1-2 mm on a mandrel with a diameter of 2,2 cm and stretched to a length of 4 cm. The number of turns is 10. The cable is initially connected to the 2nd turn of the coil, and the transceiver antenna to the 4th.

The variable capacitor must be air. The use of a ceramic trimmer capacitor leads to a decrease in the efficiency of the device. Structurally, the device can be arranged in the form shown in Fig. 3. The box must be made of metal - copper or foil fiberglass. Joints must be carefully soldered. After tuning, the box can be closed with a lid, and the capacitor is adjusted again. You can set up a matching device using the signals of CB stations or a simple RF voltmeter. By connecting the antenna and the output of the transceiver to different turns of the coil, they achieve the maximum deviation of the voltmeter needle or the maximum signal reception.

But to set up a matching device, of course, you need an antenna. Fans of long-distance MW communications need to remember - the antenna for DX must be either high or long. Usually in summer cottages or in a private house there are no problems with installing an antenna.

This can be a simple half-wave dipole with a shoulder length of 2,7 m. For effective operation, the dipole must be raised to a height of at least 2,5 meters from the ground. The dipole has a figure-eight pattern. It can be made of copper, aluminum or iron wire with a diameter of 1-4 mm.

The central insulator is conveniently made of foil fiberglass by cutting the foil in the middle. The cable can either be directly soldered to the foil, or bent, which is better, since in this case the cable is more reliably protected from moisture getting inside.

In any case, the exposed cable must be protected from moisture with paraffin or epoxy.

End insulators can also be made of thick fiberglass, foil and non-foil, or you can simply tie a nylon cord or fishing line to the antenna canvas.

It is desirable that the cable from the dipole antenna be perpendicular to the antenna web for at least 2,5 meters. This antenna can be placed not only parallel to the ground, but also vertically and at an angle to it.

For a whip antenna (Fig. 4), you can use plastic, wood, or, even better, a special post insulator as an insulator. It is useful to stretch the upper end of the antenna with a nylon cord to increase its stability. The whip antenna has a circular radiation pattern, which is convenient in some cases. Such antennas can also be installed on the roofs of urban high-rise buildings.

If you want to contact a fishing or hunting place, it is advisable to use the Beverage antenna (Fig. 5). The antenna itself must have a web length of at least 40 meters (more possible) and can be made of wire with a diameter of 0,5-1 mm. The wire can be suspended at a small height above the ground - 1-2 meters. At the end of the antenna load and on the matching device, it is desirable to use 3-4 counterweights.

Such an antenna can also be installed on the roof of a multi-storey building. The matching device in this case is necessary both on the side of the antenna and on the side of the transceiver.

With appropriate experience, you can use any complex amateur communication antenna, having recalculated its dimensions for the MW band.

In conclusion, I want to give advice: do not try to amplify the signal of your CB transceiver. Many CIS-made CB radio stations and simple foreign portable transceivers do not allow you to get a high-quality output signal when it is amplified, because the transmitter in them is assembled according to a simple scheme, where a quartz-stabilized master oscillator operates at the radio station's transmission frequency. Due to insufficient (and sometimes completely absent) shielding of the radio station case, when its RF power is increased, the signal quality can deteriorate significantly, not to mention TV interference.

Better put the effort into installing a more efficient antenna, or buy a commercially made radio with a high output.

Author: I. Grigorov (UZ3ZK); Publication: krasnodar.online.ru/hamradio

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