Multi-element vertical antenna at 144 MHz. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas
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Readers are offered a description of the design of a multi-element antenna in the range of 144 ... 146 MHz. Its advantages are the original design and low weight. The antenna is placed inside the telescopic fishing rod and it is convenient to take it with you, for example, on a country trip, to the country house, etc. It takes only a few hours of free time to make the antenna. Of course, it can also be performed in a stationary version.
As can be seen from Fig. 1, the antenna consists of four half-wave vibrators 1, located vertically, fed from the end through a short-circuited quarter-wave matching line-transformer 4. Phase-shifting elements 2 provide in-phase switching of the vibrators. The antenna has a vertical polarization and a circular radiation pattern.
The design of the antenna is shown in fig. 2. The author placed it in a part of a plastic rod 4,5 m long. In the figure, for easier reading, the individual elements (knees) of the rod are shown as separate tubes. In fact, they naturally fit tightly into each other, without requiring sealing.
The transformer is made of copper wire with a diameter of 2 mm. To increase the rigidity of the structure, the wire is soldered to three spacer plates 3, made of one-sided foil-coated fiberglass. The foil in the middle of the plates is removed, leaving only at the ends, at the places of soldering to the wire. You can use spacers of a different design and from a different insulating material. A thicker wire is also suitable for the matching line. However, it should be remembered that the ratio of the distance between the conductors of the transformer to their diameter must be 8:1.
Each phase-shifting element of the antenna is a short-circuited quarter-wave segment of a two-wire line - a loop. To reduce the size, they are rolled up in the form of coils. For the manufacture of each loop, a piece of PEV-2 wire with a diameter of 1,4 and a length of 1040 mm is taken, a mandrel made of dielectric material (fluoroplast, plexiglass) with a diameter of 9 ... 10 and a length of 75 mm. The wire is folded in half and, starting from the middle of the mandrel, coil to coil is wound in different directions. The ends of the wires of the windings are passed through through holes, pre-drilled at the edges of the mandrels, cleaned and soldered.
The material for the vibrators is an antenna cord with a diameter of 2 ... 3 mm (for a stationary antenna, a copper wire of the same diameter is used).
Separate elements of the antenna are connected to each other by soldering in accordance with Fig.1.
It is advisable to choose a fishing rod as rigid as possible and with such an internal diameter that all antenna elements could fit inside. In the thinnest knee, the antenna cord is attached with a fishing line with a diameter of 0,7 mm or a suitable elastic cord (Fig. 2).
An RF connector is mounted at the base of the rod. The connector is connected to the matching transformer with a short piece of coaxial cable (50 or 75 ohms). The central conductor of the cable is soldered to the side of the transformer to which the antenna sheet is attached. The cable braid is soldered, respectively, to the opposite. The points of connection of the power cable to the transformer are selected according to the minimum of the SWR meter readings. Measurements are carried out at the maximum distance from the antenna of all foreign objects.
It is desirable to set up a stationary antenna at the place of its permanent installation. In addition, the antenna should be grounded at the base of the matching transformer jumper, and foam rubber cubes should be put on its canvas to eliminate the "rattle" effect (Fig. 2). In case of insufficient rigidity of the rod, additional stretch marks from a nylon cord should be applied, securing them in the region of the second (third) knee.
The antenna, made by the author, has a gain of about 5 dB and an SWR of no more than 1,2 at the edges of the amateur radio band.
Author: G.Bobnev (RA3WQV)
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