Dual band KB antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, adjustment, coordination
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In amateur radio practice, there is often a need for a simple temporary antenna for the main KB bands - 20 and 40 meters. In addition, the place for its installation may be limited, for example, by the size of a summer cottage or in the field by the distance between the trees that are supposed to be used for this. An interesting version of such an antenna was published in [1]. It is made from standard 450 ohm ribbon cable (Figure 1).
Rice. 1 (click to enlarge)
To reduce its size, a well-known technique was used - the ends of the dipole of the 40-meter range are turned towards the center of the antenna and are located along its web. Calculations show that the characteristics of the dipole change insignificantly in this case, if the segments subjected to such a modification are not very long compared to the working wavelength. As a result, the overall length of the antenna is reduced by almost 5 meters, which under certain conditions can be a decisive factor.
To introduce the second range into the antenna, the author used a method that is called "Skeleton Sleeve" or "Open Sleeve" in the English-language amateur radio literature. Its essence is that the emitter for the second range is placed next to the emitter of the first range, to which the feeder is connected. But the additional emitter does not have a galvanic connection with the main one. This design can significantly simplify the design of the antenna. The length of the second element determines the second operating range, and its distance to the main element determines the radiation resistance.
More details about this method can be found, for example, in [2], where there are several practical examples of its use in antennas for amateur bands.
In the described antenna for a 40-meter range emitter, mainly the lower (in Fig. 1) conductor of the two-wire line and two segments of the upper conductor are used. At the ends of the line, they are connected to the lower conductor by soldering. The 20-meter range emitter is simply formed by a piece of the upper conductor
The feeder is made of RG-58C/U coaxial cable. Near the point of its connection to the antenna there is a choke - a current BALUN ", the design of which can be taken from [3]. Its parameters are more than sufficient to suppress the common mode current through the outer braid of the cable on the ranges of 20 and 40 meters.
Fig. 2
The results of the calculation of antenna patterns. performed in the EZNEC program are shown in fig. 2 They are calculated for an antenna installation height of 9 m. Red light shows the radiation pattern for a range of 40 meters (frequency 7150 kHz). The gain at the maximum of the diagram on this range is 6,6 dBi. The radiation pattern for the range of 20 meters (frequency 14150 kHz) is given in blue. On this range, the gain at the maximum of the diagram turned out to be 8,3 dBi. This is even 1,5 dB more than that of a half-wave dipole and is due to the narrowing of the radiation pattern (by about 4 ... 5 degrees) compared to the dipole. The SWR of the antenna does not exceed 2 in the frequency bands 7000...7300 kHz and 14000...14350 kHz.
The author used for the manufacture of the antenna a two-wire line of the American company JSC WIRE & CABLE, the conductors of which are made of steel coated with copper. This ensures sufficient mechanical strength of the antenna. Here you can use, for example, the more common similar line MFJ-18H250 of the well-known American company MFJ Enterprises.
Fig. 3
The appearance of this dual-band antenna is shown in fig. 3.
The disadvantage is that it can really be used precisely as a temporary one (in the country or in the field) in spring-summer-autumn. It has a relatively large web surface (due to the use of ribbon cable) so it is unlikely to withstand the load of snow or ice adhering in winter.
Literature
- Joel R. Hallas A Folded Skeleton Sleeve Dipole for 40 and 20 Meters. - QST, 2011, May, p. 58-60.
- Martin Steyer The Construction Principles for "open-sleeve"-Elements. - mydarc.de/dk7zb/Duoband/open-sleeve.htm.
- Stepanov B. BALUN for KB antenna. - Radio, 2012, No. 2, p. 58.
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