GP for 80 meters. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas
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When making a GP for low frequency bands, radio amateurs are usually forced to choose between antenna efficiency and antenna size. Since the effective height of the GP band 80 meters is about 13 m, it should be expected that with the optimal use of "extension" elements, an antenna of this length will be quite effective. You can tune a short antenna into resonance with a capacitive end load or / and an inductor.
A capacitive load is usually performed in the form of several conductors located perpendicular to the radiator web and located at its top. This type of matching provides the maximum efficiency of the antenna and. therefore, is a priority. For design reasons, the length of the conductors is chosen no more than 0,03 l, which limits the possibilities of this method.
The use of an inductor is less desirable, since it significantly reduces both the efficiency of the antenna as a whole and its operating frequency band. However, both methods are often used in practice to effectively shorten the antenna. The losses in the coil can be reduced if it is made in the form of one or two turns of a sufficiently large diameter. Although such inductors are more difficult to manufacture, they provide a large bandwidth (with a coil diameter of about 0.01a, it works partly as an emitter).
The advantage of this design is that the coil introduces a certain capacitance relative to the "ground", which further shortens the antenna.
The combination of these two methods is used in the antenna for the range of 80 meters (Fig. 1).
The base of the antenna is a metal pipe protruding 3 m above the ground. In the lower part, five radially divergent and 10 cm deep ground wires 25 m long are attached to the base. The ground wires are made of galvanized steel wire. In the upper part, six radially divergent counterweights 19 m long are connected to the base.
A radiator 10.5 m high is fixed (through an insulator) on the base, consisting of two pieces of metal pipes 3 m long (lower) and 7,5 m long (upper). The sections of the emitter are mechanically connected to each other through an insulating sleeve with a cross, on which the inductance coil L is located.
The design of the inductor L is shown in fig. 2.
Four bamboo sticks 1 m long are fixed in the insulating sleeve. Porcelain roller insulators are installed at the ends of the sticks, and there are two such insulators on one of the sticks. A coil made of an antenna cord with a diameter of 5 mm. fixed on these insulators and connected with its ends to the upper and lower parts of the radiator. The capacitive load at the top of the radiator is made of four sections of the antenna cord electrically connected to it, 2.5 m long and 3...5 mm in diameter. stretched along bamboo poles (fishing rods). To prevent these poles from bending, they are supported by nylon cords.
The emitter in the working position is held by two tiers of nylon stretch marks (four in each).
The antenna is fed with a 75-ohm coaxial cable 12 m long. A matching device is included between the cable and the transceiver (see article "Spiral" GP for low bands"in "Radio", 2000, No. 1, p. 64).
The antenna has shown itself well when working on ultra-long distances, providing communication with all continents.
Author: E.Osminkin (UA4ANV)
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