A shortened half-wave vibrator on CB. Scheme, description

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A shortened half-wave vibrator on CB. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / VHF antennas

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In some cases, due to lack of space, it is difficult to install a full-size half-wave vibrator. In this case, various methods of geometric shortening of the antenna can be used. In this case, the electrical length should remain equal to half the wavelength. If the space is limited only in the horizontal direction, then you can proceed as shown in Fig. 1.4. Part of the vibrator bends down at a right angle.

Depending on the ratio of the horizontal and vertical parts, the antenna gain and its input impedance change. These parameters change relatively little as the vertical part of the vibrators increases from 0 to 1/8 of the wavelength. When the horizontal and vertical parts are equal (as shown in Fig. 1.4), the gain is reduced by about 1 dB, and the input impedance is reduced to about 50 ohms, which makes it possible to feed such an antenna directly with a 50 ohm cable.

Shortened half-wave vibrator on CB
Fig. 1.4

A further decrease in the horizontal part and, accordingly, an increase in the vertical one leads to a sharp decrease in the gain and input impedance. Due to the presence of horizontal and vertical sections of the vibrators, the antenna receives (and radiates, of course) electromagnetic waves of both horizontal and vertical polarization. However, the vertical component is much less pronounced.

To reduce the geometric dimensions, you can use a conductor bent in the form of a meander, as shown in Fig. 1.5.

Shortened half-wave vibrator on CB
Fig. 1.5

By squeezing or stretching it, you can achieve antenna resonance at the desired frequency. The meander pitch is 15 cm, but this parameter is not critical. Instead of a meander, you can use a conductor with triangular bends or a spiral. The required length of the vibrators can be determined experimentally. As a starting point, it can be assumed that the length of the "straightened" conductor should be about 2,75 meters for each arm of the split vibrator.

When installing one mast and using a flexible wire as vibrators, the antenna design shown in Fig. 1.6 is convenient.

Shortened half-wave vibrator on CB
Fig. 1.6

In amateur literature, this antenna is called "Inverted Vee", as it resembles an inverted Latin letter V. The input impedance and gain depend on the angle between the arms of the dipole, as well as on the height of the suspension. The closer the ends of the dipole to the ground, the lower the input impedance and the gain of the antenna. With a suspension height of the central part of more than half the wavelength and an angle between the dipole arms of about 90 degrees, the resistance is approximately 50 ohms.

See other articles Section VHF antennas.

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