Parasol - umbrella antenna for 160 meters. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Parasol - umbrella antenna for 160 meters. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / HF antennas

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An article by Al Christman, K3LC (QST, Oct. 2000, pp. 43 - 49) describes a somewhat unusual 160m umbrella antenna that can easily be converted to other amateur bands. The antenna has a number of advantages: firstly, its height does not exceed half the height of a conventional quarter-wave vertical antenna, i.e., it is only λ / 8, secondly, the vertical antenna mast is grounded at the lowest point, without requiring a support insulator, thirdly , the antenna is fed at the top of the mast, allowing you to place other structures along almost its entire length, for example, high-frequency and VHF antennas. Two similar medium-wave antennas were built in Pennsylvania and are successfully used for radio broadcasting.

Parasol - 160 meters umbrella antenna

By design, the antenna repeats the Eisenberg top-fed antenna-mast known since the 30s of the last century. It requires good grounding, and better - a system of radial counterweights. On fig. 1 shows a diagram of an antenna with 120 quarter-wave balances buried to a shallow depth. All of them are connected to the base of the mast 21,33 m high. The coaxial power cable runs inside the mast, its braid is connected to the top of the mast, and the central conductor - to the "umbrella". The latter is made of three wires 14,2 m long, inclined at an angle of 30° to the horizon. The same wires are the upper part of the guys that support the mast. Stretch marks are directed towards the ends of buried counterweights.

The diameter of all wires (counterweights and umbrella) is 2 mm. Wires are copper or bronze antenna cord. The counterweight can also be made of galvanized steel. The mast itself is a two-inch (diameter 50,8 mm) duralumin pipe and rests on a half-inch galvanized steel pipe 1,5 m long, stuck into the ground.

Computer modeling has shown that such an antenna is practically not inferior to a full-size quarter-wave vertical with a similar system of counterbalances, has a gain of 1,26 dBi relative to an isotropic radiator, an angle of maximum radiation in the vertical plane of 24 ° and an input impedance (at the feed point) of about 30 ohms at a resonant frequency 1830 kHz. The dielectric constant of the earth was assumed to be 13, the conductivity was 5 mSym/m.

Without dwelling on many intermediate options described in the article, we will only talk about the following: if the length of the buried balances is reduced to 21 m, the antenna gain drops to 0,24 dBi, i.e. by 1 dB, and the length of the "umbrella" beams, which now directed at an angle of 45 ° to the horizon, it is necessary to increase to 15,4 m (to tune into resonance).

Parasol - 160 meters umbrella antenna

Since the creation of such a branched "land" is problematic, it turned out to be advisable to use raised counterweights, and in much smaller quantities. On fig. 2 shows a variant with eight radial counterweights extending from the base of the mast to a height of 3,66 m, and then horizontally, at a distance of 21,3 m from the mast. The three "umbrella" wires, inclined at an angle of 45°, are 15,66 m long. The gain of the antenna dropped another 0,7 dB to -0,43 dB with an input impedance of 23 ohms.

Short raised counterweights require adjustment. To do this, at the base of the mast, they are connected four together and connected to the mast through two coils with an inductance of 7 μH. Connecting counterweights to the mast not at the ground, but at a certain height, refusing to adjust them with coils or laying them directly on the ground gives worse results, sometimes by several decibels.

In the manufacture of the antenna, the author warns, one should pay close attention to the quality of the electrical connections - the losses in the antenna largely depend on them. At the same time, the antenna mast does not have to be aluminum - when using galvanized steel, the losses increase by less than 0,1 dB. It is also not necessary to strictly adhere to the given dimensions - the mast can be slightly higher or lower, and the antenna is tuned to resonance by changing the length of the "umbrella" beams. There may also be not three, but more. Raised counterweights are best tuned separately by changing the inductance of the coils included in their base.

Very interesting results were obtained by computer simulation of a phased antenna array of four umbrella antennas placed at the corners of a square with a side of 1/4 wavelength. Directional diagrams are obtained no worse than those of two-, three-element "wave channels" on the HF bands. The achievable gain exceeds 6 dB, and the forward/backward radiation power ratio is 23 dB. The radiation maximum is still directed at an angle of 22...23° to the horizon.

See other articles Section HF antennas.

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