How to shorten a dipole. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, setup and matching
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If an ordinary half-wave vibrator does not fit between the suspension points, you can use a shortened antenna, the conductors of which are bent in the form of a broken line (Fig. 1, a) or a meander (Fig. 1.6).
Fig. 1
Studies have shown that with the same electrical length 2Le (for a conventional vibrator it is equal to the geometric length 2Lg, and for a "broken" one - the sum of the lengths of all its constituent segments), the "broken" antenna resonates (the input impedance becomes purely active) at a smaller size 2L,. Such an antenna with an electrical length of 2Le=0,58L and a size of l=0,0416L has an angle of half radiation power (in a plane perpendicular to the figure) of about ±41° (for a conventional half-wave dipole - ±39°).
The gain with respect to an isotropic radiator is 2 dB and the input impedance at resonance is 46 ohms with a geometric length of 2L = 0,38L. An increase in the value of 2Le to 0,67L (l= 0,0416L) leads to a slight expansion of the directivity pattern (up to ±42°) and a decrease in the gain to 1.95 dB, and the input impedance to 37 Ohm (at 2Le=0,33L).
Similar results were obtained for a vibrator bent in the form of a meander with a size l=0,0133l (2Le=0,7L). As can be seen from fig. 2. The active resistance of such an antenna at resonance (2Lg = 0,35L) is approximately 43 ohms. The directivity pattern and gain are approximately the same as those of a "broken" vibrator with an electrical length of 2Le=0,67L.
Fig. 2
The examples given show that the described method can reduce the size of the antenna by about 30% while maintaining the directional properties and reducing the input impedance to values that allow using a coaxial cable with a characteristic impedance of 50 Ohms as a feeder.
Literature
- Radio No. 6 1986, p.64
Publication: N. Bolshakov, rf.atnn.ru
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