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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING On small-sized receiving and transmitting antennas. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Antennas. Theory Recently, many publications have appeared in amateur radio literature about small-sized receiving and transmitting antennas. They are widely used (especially in portable equipment and on mobile objects) for receiving broadcasting and television stations, radio communications, direction finding, etc. That is why a comparative analysis of such antennas, a discussion of their advantages and disadvantages, as well as a conversation about some "legends" related to electrically small antennas. Is it always, for example, that a receiving magnetic antenna is better than an electric one under the action of close interference [1]? Let's try to figure this out. Let's start with definitions. Electrically small antennas (ESA) are antennas whose dimensions are much smaller than the wavelength l or, by definition by S. Shchelkunov and G. Friis [2], when the maximum size of the antenna, measured from the input terminals, does not exceed l/8. An electrically small loop antenna is called a magnetic antenna (MA), In the near zone (at distances much less than l), transmitting MA, the magnetic component H of the electromagnetic field prevails everywhere (the ratio of the electrical component E to the magnetic - E / H - is much less than in the far zone ). The receiving MA is correspondingly more susceptible to an alternating magnetic field than to an electric one, i.e., it has component selectivity [3]. An electrical antenna (EA) - a short pin above a conducting surface or a dipole with a length much less than l - on the contrary, is more susceptible to the E component. If the perimeter of the frame is comparable to the operating wavelength, then it does not have MA properties. So, for example, a frame with a perimeter of 11 m does not have significant component selectivity in the KB range, say, in the 10-20 MHz frequency band. Similarly, a dipole comparable to l in size is not an electrical antenna in the sense indicated. The presence of a ferromagnetic core in the MA is not necessary at all, but if it is, the antenna is called ferrite. Now about the main 1. A magnetic antenna at reception under interference conditions is not always better than an electric one. MA could provide the best noise immunity among simple EMAs due to component selectivity if the sources of interference created an electromagnetic field with a predominance of the E component in the near zone of the receiving device [3]. However, this is not always done. For example, switching in power networks leads to the appearance of damped electromagnetic waves with a wide spectrum in sections of these networks. If the receiver antenna is located near the wires of such a network, then in the near field it is perceived as impulse noise. The amplitudes of the current and voltage components of the interference in a given narrow reception band are most often unevenly distributed along the wires: there are zones of current antinodes (maxima) and voltage antinodes (Fig. 1).
The electromagnetic field in the near zone is also inhomogeneous along the line. Near the antinodes of the current, the magnetic component predominates, and near the antinodes of the voltage, the electric component. In region 1 (Fig. 1), MA will give the best noise immunity, and in region 2 - EA. Experiments have shown [4] that the intensity of standing waves and the distribution of voltage and current antinodes depend on many different conditions, including the number and nature of loads connected to the network. On average, with the same probability, the receiver can be near the antinode of current or voltage. Thus, it is not always and everywhere that a magnetic antenna is less susceptible to "industrial" interference, as is sometimes reported. Moreover, this cannot be said when speaking about loop antennas in general. Why is it really always a significant improvement when moving from a short wire (pin) to a good symmetrical shielded frame, such as described in [1]? (And this fact actively supports the delusion in question). The fact is that most often a short wire as an antenna is not the only radiating (receiving) element of the antenna system; the wires of the mains, grounding, and other metal structures connected to the transmitter (receiver) housing also participate in the radiation (reception). Many are familiar with the situation when a neon lamp glows when touched by the transmitter body, heating pipes ... If such an "antenna system" is used at the reception, then all of the listed elements perceive all kinds of interference and interference in a building with many switched circuits and lines (power, telephone etc.). But to make a short symmetrical dipole is even easier than a high-quality frame. It is only necessary to eliminate the susceptibility of the feeder line to electromagnetic fields and eliminate the penetration of signals into the receiver by side paths other than the antenna. If the misconception discussed above was an overestimation of the selectivity of the receiving MA, then another, also very common misconception is that supposedly transmitting MAs are much worse than EA. In a number of publications, it is argued that when working on transmission, small frames are much less effective than electric antennas of comparable sizes, due to the much lower radiation resistance. Indeed, for a dipole of length lSD=20p2(l/l)2, while a round frame with a perimeter lSP=20p2(l/l)4. With the same l=1 m and l=80 m, RSP/RSD=1/6400. Radiated power is: PS=Ia2RS, where Ia is the effective value of the antenna current at the connection points. From the last expression it follows that we can expect the equality of the powers radiated by our antennas if the current in the loop is 80 times the input current of the dipole. Is it real? It turns out quite. 2. Taking into account the losses in the matching circuits, the electrically small dipole and the loop are approximately equivalent in terms of efficiency when working on transmission. The efficiency E of the antenna, which is equal to the ratio of the radiated power to the power taken from the generator, depends not only on the antenna's own loss resistance (Ra), but also on the loss resistance in the required matching element (reactance compensation) Rc: E \uXNUMXd RS/ (RS+RA+Rc), see fig. 2.
The active resistance (in ohms) of the antennas, taking into account the skin effect for a frame with a perimeter l, is equal to
where d is the conductor diameter (mm), mg is the relative permeability of the antenna material, s and sм - specific resistances of the antenna material and copper, respectively, of the dipole of length l: Rhell=RaP/3. Active losses in the matching elements depend on their parameters and quality factors: Rc=¦Xa¦/Qc, where Xa is the reactive component of the antenna input impedance, which is capacitive for l and inductive for the frame, and for EMA ¦XaP¦<¦XaD¦ The matching element provides a series resonance in the antenna circuit (Xa + Xc = 0). Real quality factors for the dipole Qsd=200...400, for the frame Qsr=1000...2000. Reactances (in ohms) can be calculated using the formulas:
They are obtained, like the previous ones, on the basis of known relations (see, for example, [5–7]). The results of calculations of the dipole and single-turn loop antennas made of copper (d=10 mm), for l=80 m, Qsd=200, Qcp=1000, are shown in the tables. Table 1. Calculated data for a dipole of length l
Table 2. Calculated data for a frame with a perimeter l
Table 3. Calculated data for a frame with a diameter l
They show that in terms of efficiency, a small loop can be even better than a dipole of comparable size. Although, of course, the efficiency itself is very small and drops sharply with decreasing relative sizes. Similar calculations for aluminum gave a deterioration in efficiency of no more than 12% for the frame and 0,2% for the la. For l=160 m, with the same other parameters, the efficiency turned out to be worse by an average of 20%. The results presented are in good agreement with the data of [8] obtained for a pin above a perfectly conducting surface. So, if the efficiency of the frame drops rapidly due to the decrease in RSP, then the efficiency of the dipole decreases just as quickly due to the growth of losses in the matching element. 3. What is better, a small frame or a small dipole, if they are approximately equivalent in terms of efficiency? The most important advantage of working in a lossy dielectric environment (operator's body, building materials, etc.) is that the influence of the environment on the resonant frequency (detuning) and on the efficiency (insertion loss) of the loop is much weaker than the effect on the dipole. The author tested transmitters with generators of the same power and antennas: frame diameter 42 cm and dipole length 120 cm; wavelength 82 m. The efficiency of both antennas located in free space (estimated from the far field) turned out to be approximately the same. The tree trunk, the operator’s body and hands next to the dipole changed the field strength dozens of times, and the frame could be put in a backpack on the operator’s back, put on the neck or completely buried in the snow, and this did not lead to a noticeable deterioration in field parameters. Electrical contact with a metal object, of course, can greatly affect the frame, but there is a simple remedy for this - isolation. Other advantages of small frames: they do not require a counterweight (like, for example, a short pin), are less demanding on the quality of insulation, have less effect on the tissues of living organisms when transmitting (losses in the electric near field of a small dipole are much greater), and are mechanically stronger. Directionality with vertical polarization may be useful in some cases, but not in others. The bandwidth of a magnetic antenna is somewhat narrower than that of an electric one. However, as can be seen from the tables, it is a mistake to think that the smaller the antenna, the narrower the bandwidth. An increase in the quality factor Qef of the dipole circuit is prevented by an increase in losses in the matching coil, and an increase in the quality factor of the MA circuit with a decrease in size is prevented by a decrease in its own inductance. Difficulties in the manufacture and operation of MA are to ensure minimal active losses in the connections. The loop current is tens of times greater than the dipole current, so the energy loss on bad contacts is hundreds and thousands of times greater. In practice, this means the unsuitability of threaded connections (only soldering or welding) and the need for non-contact adjustment elements. Thus, the advantages of a magnetic antenna are greater, especially when operating in non-ferromagnetic environments. 4. Does a multi-turn small frame have an advantage over a single-turn frame of the same diameter? This is also one of the questions, the answer to which is not quite obvious. From Table. 2 and 3, it can be seen that for a single-turn frame RE1<S1/2RA1. Since the radiation resistance and loss resistance in the matching element are proportional to the square of the number of turns (N2), and the intrinsic loss resistance is proportional to the number of turns (N), the efficiency of the N-turn frame is approximately estimated by the formula: EN=RS1N/(1+N)RA1. Accurate calculations at l/l=0,0125 (according to Table 2) showed that at N=2, the efficiency with the same diameter (l is the perimeter of the coil) increased by 29%, at N=4 - by 54%, at N \u10d 75 - by 2%. Consequently, the efficiency of a small N-turn loop will be somewhat higher than that of a single-turn loop, but no more than XNUMX times. In conclusion, we emphasize that all conclusions about the efficiency made for transmitting antennas are valid for these antennas and in the receive mode. It is wrong to assume that only the effective height will determine the effectiveness. The efficiency of a small loop at reception is no worse than that of a dipole of the same size, despite the fact that the effective height of the dipole is ten times greater. Also, the efficiency of an N-turn frame at the reception will not be N times greater than the efficiency of a single-turn frame, despite the fact that the effective height is proportional to N. Everyone who has dealt with the manufacture and testing of sports direction finders has been convinced of what has been said many times. Literature 1. Andrianov V. Broadband loop antenna. - Radio, 1991, No. 1, p. 54-56.
Publication: N. Bolshakov, rf.atnn.ru
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