J-antenna options. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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J-antenna options from DH1NAW. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / HF antennas

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The article presents several designs of the popular j-antenna. As you know, this is a half-wave emitter, fed from the end using a quarter-wave short-circuited line.

Figure 1 shows a 2 m antenna, where a 6 mm radiator 1 m long is connected from below to a matching line segment 50 cm long and 8 mm in diameter. At a distance of 5 mm from this segment, the second segment of the matching line is located, 50 cm long and 2 mm in diameter. The 50 ohm power cable is connected at a distance of 2,5 cm from the bottom, as shown in Figure 1.

J-antenna options from DH1NAW
Ris.1

The same 70 cm antenna consists of a radiator with a diameter of 4 mm and a length of 30,8 cm, a matching line 15,3 cm long (its left conductor is made of the same rod as the radiator, and the right conductor is made of a rod with a diameter of 2 mm at a distance of 8 mm), feed points 1,3 cm from the bottom. The matching line can also be made from a coaxial cable.

J-antenna options from DH1NAW
Ris.2

Figure 2 shows this option for 2 m or 70 cm, the total length of the line is a quarter wave, taking into account the velocity factor of 0,68 for the RG213 cable, and the power cable is connected at a distance of 5% of the entire line length, counting from the lower end, where the cable braid connects to the central vein.

J-antenna options from DH1NAW
Ris.3

The same line can be applied to short-wavelength dipoles (Fig. 3), where the dimensions C and B are indicated in Table 1.

Table 1

Range, MHz Cm V, m
28,5 1,501 0,236
27,25 1,569 0,247
24,96 1,713 0,27
21,225 2,015 0,317
18,14 2,358 0,371
14,175 3,017 0,475
10,125 4,224 0,665
7,05 6,066 0,955
3,65 11,717 1,844
1,834 23,32 3,671

A sketch of such an antenna for 10 m is shown in Fig. 4, and the dependence of SWR on frequency is shown in Fig. 5 (the upper curve is calculated, the lower one is the practical result).

Fig.4,5. An interesting antenna option is a dipole made of coaxial cable type RG213 or RG58U, depending on the operating frequency (Fig. 6) (click to enlarge).

Fig.6 (click to enlarge)

This dipole does not require additional balancing or matching. Due to design features, the antenna has an impedance of about 50 ohms. Tuning to the desired resonant frequency is carried out exclusively by lengthening or shortening the arms La. As an example, the calculation for 70 cm from the RG213 cable is given: L1=140,8/f[MHz], L2=99,4/f[MHz], here the shortening factor has already been taken into account. For 423 MHz, L1 = 0,326 m and L2 = 0,23 m, and La = 0,048 m. From the middle of a cable segment 32,6 cm long, 5 mm of external insulation are removed on the left and right, and the screen in the middle is removed at a length of 8 mm, leaving 1 mm each, which are carefully tinned, they are the power connection points (Fig. 7).

J-antenna options from DH1NAW
Fig.7

From the outer ends towards the middle, the outer insulation is removed for a length of 48 mm, and the screen braid is removed so that 3-4 mm is still visible. The core insulation is also removed at a length of 48 mm. Then the remaining 3-4 mm of the screen are bent onto the central core and soldered. As can be seen from Fig. 8, a coaxial dipole can also be used as a Yagi emitter without any problems.

J-antenna options from DH1NAW
Fig.8.

Author: Günther Grünbeck, DH1NAW; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section HF antennas.

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