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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Wave channel of the 1296 MHz range. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas Once upon a time, the radio amateur DL6WU proposed a successful design of a 49-element "wave channel" antenna in the 23 cm band. This was before the widespread use of computer programs for antenna modeling. I put the data of this design into the MMANA program, slightly corrected them and got very good results, summarized in Table. 1.
The calculations were made for a frequency of 1296 MHz. As the simulation results showed, it turned out to be possible to make the antenna in three versions: 21, 37 and 49 elements, without changing the length of the elements and the distance between them. Only the length of the traverses will be different: 4,1 m (for 49 elements), 3 m (for 37 elements) and 1,5 m (for 21 elements). And what is most important and pleasant - the active input impedance of the antenna in all three cases is close to 50 ohms, and the reactive is negligible. This makes it possible to directly connect a coaxial cable with a characteristic impedance of 50 ohms to an active vibrator. A simple split vibrator made of copper wire with a diameter of 3,2 mm (its length is 112 mm) was used as an active one. The good quality of the cable and the minimum length of its leads when soldering to the vibrator is the only and quite important requirement here. As an active vibrator, one could use a classic loop vibrator with a balancing U-elbow, but it is very doubtful that the gain from balancing will exceed the losses created by the presence of additional connections. For example, I’ll say that during measurements, even a simple “pull-up” of the SR-50-164 RF connector leads to a change in SWR at this frequency, and what can we say about additional solder joints .... The antenna gain can be increased by another 0,5 ... 0,6 dB, but this requires a more radical processing of the antenna and is associated with a decrease in input resistance to 20 ... 30 Ohms, which again necessitates additional matching and, as a result, additional connections. Antenna design. A schematic representation of the antenna (without observing geometric proportions) is given in the figure. The letters indicate: R - reflector, B - active vibrator, D1 ... D47 - directors.
A duralumin (D16T) tube with a diameter of 14 mm was used as a bearing traverse (and in the version with 49 elements it was added with another tube with a diameter of 12 mm for the last directors). All elements (except for the active vibrator) are made of duralumin wire with a diameter of 3,2 mm. The elements are passed through the traverse through insulators, which were pieces of the outer insulating tube (usually black) of a coaxial cable with a diameter of about 4 mm. They do it like this: 1. The diameter (type) of the coaxial cable is selected so that a piece of insulation removed from it 2,5 ... 3 cm long is put on the element with friction. 2. In the test (dummy) section of the tube (the same one that will be used for the traverse), a through hole is drilled, the diameter of which is selected so that the insulating tube removed from the cable passes into the hole with friction and protrudes slightly on both sides of it. 3. Having fixed the tube with the insulation threaded through the hole in a vice, rotate the element as far as possible into the insulation, and then lightly hammer the element through the insulation and the traverse, trying not to move the insulation. Sometimes the insulating tube is pushed out of the hole in the traverse - then the operation must be repeated. To facilitate this process and reduce friction, the edges of the element are first slightly rounded with a file and the element is moistened with water. Finally, the element is centered in the traverse by tapping it with a hammer from one side or the other. The element fixed in this way must be firmly fixed in the traverse so that it would be rather difficult to move it manually. An active vibrator is fixed on a small fiberglass plate. The described technology for installing elements, used by many ultrashortwaves, is also suitable, by the way, for YAGI antennas in the 144 and 432 MHz bands. Its main advantage is the absence of galvanic contact of the elements with the traverse, which often turns out to be the cause of electrochemical corrosion, which worsens the parameters of the antenna. However, an element mounted in a traverse in this way requires some correction in its length compared to its length in free space, usually used in computer simulation calculations. Therefore, in Table. 2 shows two lengths of the elements: with (total length of the element through the traverse) and without (length of the element in free space) boom correction (+ 4,8 mm for a traverse with a diameter of 14 mm and + 4,2 mm for a traverse with a diameter of 12 mm).
The cutting of elements must be done as accurately as possible, otherwise the characteristics of the antenna will be worse than those given. All design data of the antenna are given in Table. 2. If a wire (tube) of a different diameter is used for the elements or the diameter of the traverse tube differs from the specified one, then the lengths of the elements must be adjusted. In the 49-element version, the assembled antenna sags a little, so it is advisable to use braces that support the traverse, for example, from fishing line. An antenna of 49 elements in the "Field Day" competition was able to communicate at a distance of up to 650 km. Author: Nikolai Myasnikov (UA3DJG), Ramenskoye, Moscow Region.
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