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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING toroidal antennas. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Antennas. Theory An urgent task of antenna technology is the creation of efficient electrically small antennas. They are needed both for portable and mobile radio stations KB, VHF and microwave bands, and for stationary long-wave radio systems in conditions of limited space. The proposed article introduces readers to one of the interesting ways to solve this problem. The dimensions of an electrically small antenna are, by definition, much smaller than the wavelength λ in free space. The problem of designing such antennas is that as the size of the radiating system decreases, the radiation efficiency decreases rapidly. Difficulties arise in matching non-resonant antennas with sources (receivers). It is possible to reduce the physical dimensions of the antenna while maintaining the electrical (wave) dimensions by replacing straight conductors with spiral ones bent in the form of a helix (Fig. 1).
Such structures are called retarding. The speed of wave propagation along the axis of the spiral is less than the speed of light, so the wavelength λs in such a structure at the same frequency is less than λ. The physical length of a resonant antenna can be reduced tenfold in this way. Helical transverse (perpendicular to the axis) radiation antennas are widely used in portable and fixed radio equipment. If the linear vibrator is folded into a closed ring, we get a frame (Fig. 2, a). The distribution of electric current 1e in an electrically small frame can be considered uniform, so it will radiate uniformly in all azimuthal directions, but only with horizontal polarization (Fig. 2,6), like an elementary vertical magnetic vibrator. With uneven current distribution, the diagram will not be as symmetrical. When the length of the perimeter of the frame is a multiple of an integer number of half-waves, resonances are possible in such an antenna. So, in a "square" type antenna, two half-waves fit on its perimeter.
At medium, long and extra-long waves, due to the peculiarities of their propagation, vertical polarization is preferred. It is here that the problem of reducing the vertical dimensions of antennas is particularly acute. Let's try to imagine an amateur quarter-wave vertical vibrator in the range of 136 kHz with a height of about 550 m! However, it is not at all necessary to use electric current as a radiation source. In accordance with the principle of permutational duality, if a uniformly distributed ring electric current (Fig. 2, a) is replaced by a magnetic current IM (since there are no magnetic charges in nature, this will be a fictitious magnetic current, the density of which is proportional to the rate of change of magnetic induction), then in the field radiation vectors of the electric and magnetic components will change places. We will get a source equivalent in terms of the directivity pattern to an elementary electric vibrator, in our case a vertical one (Fig. 3).
Ring magnetic current can be obtained in a toroidal helical antenna (Toroidal Helical Antenna, THA), which is formed by folding a linear spiral into a closed ring. The shape of the spiral coil can be arbitrary (circle, rectangle, etc.). On fig. 4 shows a sketch of a toroid with a square cross-sectional shape and the size designations are indicated.
On fig. 5a shows an example of building a 7-turn toroidal antenna. Resonances are also possible in such a system, when an integer number of half-waves of the magnetic current fit along the axis of the toroid. But in a spiral, the wavelength is shorter, so the resonant TNA can be much smaller than the resonant frame from a linear wire. On fig. 5,b,c,d shows the spatial radiation patterns (RP) of HPP both in terms of individual components of the electric field Eθ, Eφ, and in the total field EΣ eddy component of the electric current of the spiral, there is always a toroidal component (along the axis of the toroid), due to which the radiation field contains not only the vertical Eθ, but also a significant horizontal component of the electric field Eφ.
To compensate for the toroidal component of the electric current, two identical windings are made, wound in different directions (left and right), and they are turned on in antiphase (Fig. 6, a).
The windings are not connected at the intersections. We received a toroidal helical antenna with counter helical windings (Contrawound Toroidal Helical Antenna, CTHA). The magnetic fields in the cavity of the toroid from both windings add up. On the diagrams of Fig. It can be seen from Fig. 6b that the fraction of the Eθ component in the radiation field has noticeably increased, the minima of the total diagram along the y axis have become less deep, but again we have not obtained the general diagram, as in Fig. 3. This is explained by the fact that the magnetic field in the cavity of the toroid is not distributed uniformly along the axis, but in accordance with the distribution of the amplitudes of the standing current wave. How to overcome this obstacle, we will show below, and now we will consider some interesting properties of the already described antennas. On fig. Figure 7 shows the calculated frequency dependences of the active (R) and reactive (X) components of the input impedance of the HP at a = 0,6 m, h = 0,8 m and N = 7. Characteristic is the alternation of even "series" and odd "parallel" resonances ( similar in nature to resonances in series and parallel oscillatory circuits).
For comparison, the table shows the calculated values of resonant frequencies (in megahertz) and resonant impedances (in kiloohms) for this antenna (TNA) and for the STNA antenna with the same parameters. The nature of the alternation of resonances in STNA is the same as in TNA, however, with the same parameters, the resonant frequencies of STNA are lower; this can be explained by the effect of capacitance between the windings. Note that both antennas do not have a strict multiplicity of resonant frequencies. The main parameters of toroidal antennas are the dimensions and the number of turns N. For calculations and modeling, we chose a cross-sectional shape in the form of a square with side h. If we neglect the influence of the medium inside and outside the toroid, then, given the frequency of the 1st resonance (MHz) and radius a (m), we can calculate the size h (m) of the above antennas using the formulas: for TNA:
for STNA:
The formulas were obtained using regression analysis based on the results of computer simulation for a wire diameter of 1,3 mm, sizes 0,6 m ≤a ≤ 4 m, 0,5m ≤h≤4m, with 0,3 ≤ h/a ≤ 1,3, and frequency range 0,7 MHz < f1 < 23 MHz. The root-mean-square error under the specified conditions is about 0,03 m. Scale recalculation is also possible for other frequencies (all dimensions change in proportion to the change in wavelength). An interesting feature of STNA is the possibility of obtaining (only for certain combinations of parameters) a radiation pattern close to isotropic (Fig. 8). This pattern was obtained, in particular, at a frequency of 70 MHz for an antenna with parameters N = 5, a = 0,2 m and h = 0,27 m in free space conditions.
On fig. 9 shows the comparative dependences of the efficiency of TNA and STNA on frequency. As a rule, the efficiency decreases rapidly with a decrease in the main dimensions of the antenna and an increase in the number of turns. The highest efficiency for TNA is in the region between the 2nd and 3rd resonances, for STNA - at the 3rd and 5th resonances, and its maximum values are lower than for TNA. Both types of antennas are characterized by deep efficiency minima at all even resonances above the second. This is explained by the unfavorable distribution of current in the windings for effective radiation.
Electrically small antennas generally have low efficiency and are therefore very sensitive to feeder antenna effects. It makes sense to use them on moving objects with a very short feeder or without it at all. The elliptic polarization of toroidal antennas is useful, for example, to ensure uninterrupted communication in mobile systems, in particular, for stable reception of VHF FM broadcasting programs. On fig. 10 shows the placement of STNA with the characteristic according to fig. 8 on the roof of the car and shows the radiation pattern, taking into account the influence of the body and the ground.
Historically, the development of toroidal antennas is associated with the desire to reduce the vertical size of the radiating system with vertical polarization and circular pattern. As noted, in a conventional STHA antenna with a single excitation source, it is not possible to obtain a uniform distribution of the magnetic current along the axis of the toroid. On fig. 11,a shows the intersection of the turns of the left and right windings on the entire outer surface of the toroid in expanded form, and in fig. 12 (curve 1) - distribution of the magnetic field strength along the axis of the toroid for an 8-turn conventional STNA at f3 = 27 MHz. As a result of the uneven distribution of the field, the radiation patterns of such an antenna are close to those shown in Fig. 6.
One way to obtain a close to uniform distribution of the magnetic current is to divide the windings into sections, in each of which the directions (left and right) of both windings change to the opposite of the neighboring ones (Fig. 11,6). In places where the windings are divided into sections, terminals are installed for connecting additional excitation sources. In this case, instead of one, you need to connect four identical common-mode sources. In this case, the distribution of the magnetic current (Fig. 12,6) is obtained without sign changes, although with small ripples. Such a solution made it possible to obtain a RP in a wide frequency band that does not differ from that shown in Fig. 3. The calculated efficiency of the sectioned STNA in this case at a frequency of 36 MHz turned out to be approximately twice that of the non-sectioned STNA (59% versus 29%). In conclusion, we note the most important advantages and disadvantages of the considered antennas and the possibility of their application. General pluses are a decrease in the vertical size of the antennas (due to an increase in the horizontal dimensions!), no requirements for counterweights and grounding. In essence, the THA is a frame made of a helical conductor, which has reduced the physical dimensions of the resonant antenna. Such an antenna is interesting already because it has an elliptical polarization, and the dependence of the RP on the shape, environment, and asymmetry of the connection allows such antennas to be widely and variedly used in communications, broadcasting, telemetry, and other portable radio equipment. The presence of a second, opposite winding in STNA, generally speaking, worsens the radiation conditions, hence the lower efficiency. However, these antennas have a better polarization ellipticity, which is important for mobile communication systems in multipath conditions. The isotropic RP of an unsectioned STNA is hardly feasible in practice due to the strong influence of the environment, but the surrounding objects (and, in particular, conductive surfaces) have little effect on the input impedance of the STNA. Non-partitioned STNA can be used in portable devices for low-level radio communication and personal radio call, in nGPS cellular communication systems. The main area of application of toroidal antennas equivalent to a vertical vibrator (with vertical polarization and uniform radiation pattern in the horizontal plane) is relatively long waves, for which the conductivity of the earth (or water) is sufficiently large. Cons STNA - a complex manufacturing technology. When sectioning antennas, there is additional trouble with connecting several power points. General disadvantages - with a decrease in size, the antenna efficiency sharply decreases, and when trying to improve it (by increasing the thickness and selection of wire material, improving the quality of dielectrics), the bandwidth decreases. Matching problems when tuning from one frequency to another make it difficult to use toroidal antennas in the frequency range. The interested reader can refer to the patent literature [1-4] and to the results of studies with the participation of the author [5, 6]. In [7], several new methods for manufacturing a vertically polarized emitter based on toroidal structures were proposed. In [8], a universal algorithm for synthesizing antennas from segments with electric and magnetic currents was proposed. Literature
Author: A. Grechikhin (UA3TZ)
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