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Multi-band version of the loop antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas Long-term experiments of the authors have shown that in the range of 160 m, loop antennas with a perimeter close to the wavelength have quite acceptable efficiency. Despite the relatively low suspension height for this range, which usually does not exceed 20 ... 25 m above the ground, such antennas work well both for intra-union communications and for DX QSOs. This design, although simple to manufacture, takes up a large area. Therefore, it is natural to want to ensure its operation on other bands, even if only as an auxiliary one. In this case, it should be expected that the efficiency of the antenna will increase with increasing frequency. However, the direct use of a loop antenna made for the 160-meter band is not possible due to the frequency multiplicity in amateur bands. The table shows the SWR values (option 1) of the antenna in the form of an equilateral triangle from a copper wire with a diameter of 2,2 mm. The suspension height is about 20 m. The plane of the frame is strictly horizontal and parallel to the ground. Power is supplied to one of the corners of the triangle. In all cases, hereinafter, the minimum SWR values are indicated. To optimize the antenna matching in several bands, we used the method (it was proposed by DL7AB), which makes it possible to tune the "long wire" antenna to resonance on all KB bands and is described, for example, in [1]. The idea is as follows. Inductance coils are included in the frame wire break to the right and left of the power point, the lengthening effect of which is most pronounced when they are in the antinode of the current, and gradually decreases as the current nodes approach the coils. Thus, there are two variables that most strongly affect the operation of the antenna in all ranges in real conditions: the first is the inductance of the coils, the second is the place where they are turned on. Minimum SWR value of two antenna options Table 1
The same table (option II) shows the SWR values by bands for an antenna made in a similar way. It was an equilateral triangle with a total length of 158 m, also located horizontally relative to the ground. Power was supplied via a coaxial cable with a characteristic impedance of 75 ohms. In the place of its connection to the frame, balancing was performed (by any of the known methods) of the power circuit. The authors tested two variants of balancing, which gave practically the same results. In the first case, 120 turns (distributing them evenly around the circumference) of the feeder were wound on a ring (size K80X20X50) made of 2VCh-10 ferrite. In the second case, on a similar ring, 15 turns of two pieces of the MGShV installation wire twisted together with a cross section of 1 mm were wound on a similar ring along the lakoshelka layer. A frame is connected to one end of the coil, a feeder is connected to the other. In both cases, the coils were carefully protected from climatic influences. The extension coils are included in the generators of the corner from which the antenna is fed, at a distance of 12 m from its top. Coils - frameless, wound on a mandrel with a diameter of 45 mm and contain 4 turns (pitch 8 ... 10 mm) of a copper tube with an outer diameter of 3,5 mm. Setting up the antenna begins with tuning the resonance of the entire system on the 160-meter range. For this purpose, the perimeter of the antenna was originally somewhat smaller (about 156 m) than indicated above, the excess in the form of loops was left at the feed point. By changing their length, they achieve a minimum SWR in the middle of the range. After checking the value of this parameter in the remaining ranges, if necessary, change the inductance of the coils within a small range, shifting or pushing their turns. In the event that it is not possible to achieve a satisfactory SWR value on some of the HF bands, it may be necessary to change the place of switching on the coils within a small range, which is done in a very simple way, described in [L] in the "double square" section. To do this, behind the coils, the authors made loops for tuning, with a change in the length of which the coils seemed to move along the antenna canvas. The loops were chains of insulators about 0,75 m long, bridged by a conductor. By varying its length, thereby changing the length of the frame behind the coils. After this operation, you need to change the length of the loops at the feed point in the opposite direction in order to maintain antenna resonance on the 160 m band. However, as a rule, such an exact fit is not required, as evidenced by the repeated repetition of the design in different conditions and from different materials. Almost after tuning the antenna to resonance on the 160-meter band, the SWR on all bands was quite acceptable. Moreover, as experiments have shown, the configuration of the frame does not play a significant role, i.e. it can be a triangle, square or polygon. It is only important that when setting up all operations are performed symmetrically, i.e. if the length, inductance or place of switching on the coil is changed, then this must be done in both "branches". The authors compared the described antenna with some others. On a range of 160 m with a distance to correspondents of no more than 1000 km, it gave a gain in signal level of at least one point compared to half-wave and wave dipoles, as well as a beam 106 m long. On longer paths, the difference reached two points compared to a half-wave dipole and a beam, and one point in relation to a wave dipole with an average suspension height of about 27 m. On the bands of 80 and 40 m, loop antennas of the corresponding bands, a dual-band "INVERTED V", W3DZZ and a 7 MHz whip were tested. The advantage over the first three antennas was undoubted on all routes, especially significant (up to 12 dB) it was expressed in relation to "INVERTED V" and W3DZZ. According to correspondents, only at a distance of more than 2000 km, when switching to a whip antenna, an increase in the signal by one point of the scale 5 was observed. Of greatest interest is the operation of such a frame on the HF bands 14-28 MHz. Almost always, when switching from a dipole or its modification to this antenna on any of the ranges, the signal increased by a maximum of two points. When conducting DX QSOs under certain conditions of passage, not always, but there was a slight increase in signal compared to when a quarter-wave pin was used. Using the "electrical extension" method, it is possible to construct a loop antenna with a resonance at a frequency within 3,7 ... 3,8 MHz, which works well on all multiple higher frequency ranges. Literature
Authors: G. Bolotov (UA3QA), S. Zemaitis (UW3QR); Publication: N. Bolshakov, rf.atnn.ru
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