Combined antenna for 80 and 20 meters. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / HF antennas

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This antenna (Fig. 1) does not differ in the original radio engineering solution, but the design itself is interesting in that it can be installed on the very edge of the roof of a residential building. It consists of two electrically independent antennas: a loop (DELTA LOOP) for a range of 20 meters and a shortened pin (GP) for a range of 80 meters.

The mast 1, about 3 m long, supporting the DELTA LOOP and part of the GP, is securely fixed to the railing 2 at the edge of the roof. In the upper part, to the mast, through insulating gaskets 4, an emitter of a range of 80 meters, 5,3 m long, is attached, made up of three segments of thin-walled duralumin pipes, which are inserted one into the other. A conductor 3 5 m long is attached to the lower part of the emitter 4,3, supported by a dielectric stretch 6. It goes to the matching block 7. In the lower part of the mast 1, just above the fence 2, a dielectric pipe 8 is attached. It consists of two rods 3.5 m long This tube supports the lower part of the frame 9. In the upper part, the frame is attached to the 80-meter range transducer through a dielectric insert (not shown in Fig. 1).

A matching transformer 11 is also located on the dielectric plate 13, which fixes the rods. Two extensions 12 additionally fix the mast. The antennas are fed through separate coaxial cables 10 and 14.

Schematically, the antennas are shown in fig. 2.

Frame 1, which has an input impedance of about 120 ohms, is fed through a balancing transformer 2 and a transforming quarter-wave line 3 with a wave impedance of 75 ohms. Power cable 4 is used with a characteristic impedance of 50 ohms.

The design of the upper frame insulator is shown in fig. 3.

A dielectric rod 1 is inserted into the upper section of the pipe of the emitter 80 of the range of 2 meters. Screw with nut 3 prevents the rod from falling into the pipe. In the upper part of the rod there is a through hole 4. through which the wire of the loop 5 passes.

The matching device for the range of 80 meters (Fig. 4) consists of an extension coil 8 and a choke 5. The coil is wound on a frame with a diameter of 6 cm and a length of 25 cm. It has 50 turns of bare copper wire with a diameter of 1,6 mm. Winding pitch - 1,6 mm. The braid of the supply cable 4 is connected to the end of the coil, and the point of their connection with conductor 6 to the "ground" is a metal fence. The central conductor of the cable is connected to the tap of 1 coil (approximately 1,5 turns, counting from the "cold" end of the coil).

The emitter is also connected to the coil outlet (from about the 16th turn). On fig. 4 shows two taps - 2 and 3. The point is. that the operating frequency band of this emitter is relatively narrow (due to a noticeable shortening) and for operation at different ends of the range it is necessary to change the point of its connection to the matching coil. You can use a relay to switch.

Since the fence is not the best "ground", a choke 5 is introduced to eliminate currents through the braid of the supply coaxial cable. It is wound with two wires in isolation on the rod from the magnetic antenna of the broadcasting receiver. The number of turns of this inductor is about 20 (non-critical).

The number of turns of the extension coil was chosen by the author with a margin for experimentation, so you can try to tune it on a range of 160 meters.

Note that the GP length is close to a quarter wavelength for the 40m band. It requires only a small extension coil to operate, and the antenna should be relatively efficient, especially if additional counterweights are added on this band.

Author: JJ7XTV

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