Simple and compact vertical antenna for CB band. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, adjustment, coordination

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Having worked for more than 30 years as a foreman of a radio workshop engaged in the repair of communication radio equipment with a traveling nature of work, I have repeatedly come across a situation where a compact antenna was required that can be easily transported and installed, for example, on a balcony, loggia, in a window opening or on the luggage superstructure of a car . In particular, such an antenna was needed for on-air testing of civil CBS radio stations in the 27 MHz band.

From a constructive point of view, the most optimal option for such an antenna seemed to me to be a shortened vertical radiator with an extension coil.

The vibrator of my antenna (Fig. 1), or rather, its element "a", is made of a telescopic tube from an old indoor television antenna "KTTA" (the so-called "whiskers"). This makes it possible, by changing the length of the vibrator, to quickly rebuild the antenna over the range, which is essential because of its narrow bandwidth.

Fig. 1

The extension coil L1 (element "0") is wound with a copper wire with a diameter of 1,2 mm in PVC insulation (outer diameter over the insulation - 4 mm) on a frame made of a plastic pipe with a diameter of 25 mm (photo in Fig. 2).

Fig. 2

Winding - turn to turn, for a length of 195 mm. Element "c" of the antenna is formed by the lower output of the coil.

Matching the high impedance of the antenna with the low wave impedance of the feeder (and all modern industrial communication equipment is designed to operate on a load with a wave impedance of 50 ohms) is performed by the P-loop (Fig. 3).

Fig. 3

It is mounted in a metal case from the antenna filter of the VHF radio station "Granit" (Fig. 4).

Fig. 4

Trimmer capacitors C1 and C4 with an air dielectric are also from this radio station. Capacitor C2 is made up of three capacitors KT-2 with a capacity of 30 pF + 82 pF + 82 pF, connected in parallel. Capacitor C3 is made up of three KT-2 capacitors with a capacity of 300 pF, also connected in parallel. When choosing capacitors, one should take into account the maximum power of the equipment with which to work. With a power of 50 W, their operating voltage must be at least 500 V.

Coil And matching device contains 8 turns of silver-plated wire with a diameter of 1,5 mm, wound turn to turn on a mandrel with a diameter of 9 mm and then stretched to a length of 20 mm.

To prevent the antenna effect of the feeder between the radio station and the matching device, a blocking choke is included. It consists of several turns of the same cable, wound on a ferrite annular magnetic circuit of suitable sizes with an initial magnetic permeability of 400 or more. The number of turns should be determined experimentally from changes in SWR, caused, for example, by moving the feeder, touching it with your hand or the body of the SWR meter. With good agreement and optimally selected number of turns, these changes are insignificant.

This operation should be carried out after preliminary tuning of the elements of the P-loop for a minimum SWR, for example, using an antenna analyzer MFJ-259. In this case, the section of the CB range should be selected, where at this moment the least pickups on the analyzer are noted.

The test results showed that the antenna bandwidth at SWR not more than two is approximately equal to 120 kHz. Narrow bandwidth is often considered a disadvantage of an antenna. But in this case, due to the fact that the antenna can be tuned in range, the narrow band is rather its advantage, as it reduces out-of-band interference from closely operating radio stations during reception and reduces spurious emissions during transmission.

Capacitor C1 can change the resonant frequency of the entire system within small limits without changing the geometric dimensions of the vibrator. A similar antenna can be designed for the HF band of 10 meters

Author: V. Efremov (UA6HGW), Essentuki, Stavropol Territory

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