Once again about the antenna Five eighths of lambda. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas
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In the article by V. Polyakov (RA3AAE) "About the five-eighths lambda antenna", published in "Radio", 2000, No. 4, pp. 66, 67, the position was expressed and argued that a vertical antenna with a length of 5λ / 8 is not at all an optimal radiator on the VHF and high-frequency KB bands.
So, the widespread opinion that this antenna provides the highest gain and gives the main lobe of the directivity diagram (DN) pressed to the ground as much as possible is true only for a medium-wave antenna installed directly on a well-conducting ground (Fig. 1). Actually, this antenna was designed by S. Ballantine as a medium-wave anti-fading antenna, attenuating the ionospheric wave at such angles at which interference of surface and ionospheric waves is observed.
American radio amateur W4RNL simulated various antennas on a computer, including 58λ / 5. Here are some interesting conclusions, confirming what he said, he came to.
Vertical antennas of the 80-meter range with counterweights buried in the ground provide small radiation angles to the horizon and are especially suitable for long-distance communications. W4RNL reports that the 5λ/8 antenna has an advantage over the λ/4 antenna only in poor and very poor soils with a conductivity of less than 2 mS/m, and the gain is much less than the promised 3 dB (Fig. 1). Considering that it is not so easy to install a mast 50 m high, the game is hardly worth the candle.
In the range of 2 meters at low altitudes, which is typical for the operation of portable radio stations, the main lobe of the DN of any antenna strongly “lifts up” into the sky, and in these conditions the 5λ / 8 antenna turns out to be slightly better than a quarter-wave pin. The radiation patterns of these antennas in the vertical plane when the feed point is placed at a height of about 1,5 m above the ground are shown in Fig. 2.
The radial elements of the quarter-wave antenna were located at an angle of 45° to the horizon, and the radials of the antenna 5X/8 long were in the horizontal plane.
In the same range of 2 meters with a large suspension height (h>λ), a 5λ/8 antenna turned out to be no better than a conventional quarter-wave pin with inclined quarter-wave counterweights, and the latter even had a slight advantage! In addition, it is easier to match with the feeder. Antenna patterns at a height of the feed point above the ground of 7,5 m are shown in fig. 3.
The "jaggedness" of these multi-lobe diagrams is explained by the interference of the direct and reflected waves from the ground.
Author: W4RNL
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