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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING About feeding radio receivers with free energy. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception Perhaps due to the rise in the price of rechargeable cells and batteries, or perhaps for some other reason, but recently the interest of radio listeners in the problem of powering the radio receiver with "free energy" of radiation from powerful broadcasting stations has greatly increased. In a number of periodicals, reports appeared about "loud-speaking" detector devices, as well as about receivers that work on telephones and, powered by the field of some powerful radio station, receive programs from other less powerful stations. Since the reasons for this phenomenon are to some extent shrouded in mystery, and the most incredible circuit solutions are offered in the literature, with the help of which it is supposedly possible to obtain even more incredible results. The purpose of this article is to help radio amateurs who are interested in this problem to understand it from an objective point of view and really evaluate the capabilities of radio receivers powered by the "free energy" of powerful radio stations. The issues of optimal detection and construction of the receivers themselves are supposed to be considered in one of the following articles. It is known that the EMF induced by the field of the transmitting radio station in the antenna of the radio receiver can be determined by the formula: ε = E*hд, where E is the field strength of the radio station at the receiving point, and hд is the effective height of the antenna. However, we need to maximize not the EMF at all, but the power of the received signal supplied to the detector, the input resistance of which Rin depends on its circuit, the load resistance, and to some extent on the magnitude of the EMF induced in the antenna. Since the power of the signal entering the detector P = U * I (where U is the voltage supplied to the detector, and I is the current flowing through it), and the input resistance Rvh = U/I, then you can maximize the power by changing the input impedance of the detector, choosing different schemes for matching it with the antenna, as well as increasing the voltage on the detector, decreasing the current, and vice versa. On the other hand, it is known that the source (antenna circuit) gives the maximum power to the load (detector) when its active resistance is equal to the input resistance of the load, i.e. RА = Rvh, and the reactance is compensated by the inclusion of a reactance of a different sign. These are the usual conditions for matching the source to the load. How to fulfill them in a real situation? The most powerful radio stations operate in the ranges of long and medium waves. Wet soil, fresh water, and even more so sea water, have the properties of a conductor at these frequencies, in which the conduction currents are much greater than the displacement currents. As a result, waves with horizontal polarization are significantly weakened near the earth's surface. For this reason, vertically polarized waves are used for broadcasting, emitted by vertical masts - antennas with a more or less developed horizontal part and good grounding. The issues of designing long-wave and medium-wave antennas were solved back in the thirties and covered in detail in the textbooks of the forties and fifties, this also explains the "antiquity" of the literature cited at the end of the article.
A sketch of a vertical antenna with grounding is shown in Fig. 1a. Own (resonant) wavelength emitted by such an antenna (recall that it is considered a wave at the frequency of which the resistance on the XT1 connector is active and is equal to the resistance of a quarter-wave asymmetric vibrator, i.e. ~ 37 Ohm) λ0=4*IД, and the effective height hд=2IА/π. In amateur conditions, it is almost impossible to build a quarter-wave vertical antenna, since it turns out to be too high, therefore L-shaped (Fig. 1, b) and T-shaped (Fig. 1, c) antennas are usually used, in which the parameter λ0= KIДwhere IА = h + IГ, and K is a coefficient, the value of which can be determined from the table:
One could recommend an umbrella antenna having 3-4 horizontal beams connected at one point with the vertical part, however, due to the complexity of the design, it is used extremely rarely. Only the vertical part of the antenna is involved in receiving radio waves, while the horizontal part performs the functions of a capacitive load, increasing its own wavelength and effective height. The more developed the horizontal part, the more accurately the relation hд = h and the antenna itself is more efficient. In most cases, the antenna receives signals whose wavelength is greater than the antenna's own wavelength λ >λ0, and its resistance is complex (Za) with active (RΣ) and reactive (X) components, determined by the formulas: ZА=RА -jX;
where W is the wave impedance of the antenna wire, which is approximately 450 ... 560 ohms.
To compensate for the capacitive resistance of the antenna, an inductance (extension coil) is included in its circuit, and the equivalent circuit of the antenna takes the form shown in Fig. 2. Now it is possible to calculate the power transmitted by the antenna to the load (detector), and we will not take into account the losses in its circuit for the time being. If the input resistance of the detector is equal to the active component of the antenna resistance Rvh=RΣ the power in the load is maximum and equal to Р0= (ε/2)2/RΣ. Substituting into this formula the expressions for ε and RΣ, we obtain P0= E2 hд2 λ2 / (4*1600*hд2) = E2 λ2 / 6400 The formula we have derived determines the maximum power that can be induced by the field of a radio station in an ideal lossless antenna. It is interesting to note that this power does not depend on the size and design of a particular antenna. From what has been said, the following conclusions can be drawn. - the possibility of powering the receivers with "free energy" depends only on the field strength of the radio station at the place of reception;
For example, let's calculate the maximum power that can be induced in the antenna by the LW field of a radio station operating at a frequency of 171 kHz (λ = 1753 m) with its strength of 20 mV / m, which occurs in many areas of the Moscow region and even beyond its borders: Р0=E2*λ2/6400 =0,022 * 17532 / 6400 = 0,19 W. This power is quite sufficient for loud-speaking operation of most portable receivers, since it is equivalent to Upit = 9 V at a current of 20 mA. Unfortunately, the real situation is far from ideal. The fact is that in the antenna circuit there is a loss resistance Rп, consisting of the resistance of the antenna wire, the active resistance of the matching coil L (Fig. 2) and the ground resistance. The efficiency of such an antenna is determined by the expression η = RΣ/ (RΣ+Rп). and the power received from it - by the formula: P = P0*η = E2 λ2*η / 6400 Calculating the antenna efficiency is a completely solvable problem. The linear resistance of a copper wire with a diameter of 1 mm to direct current is 22,5 Ohm/km and increases by about 2 times at a frequency of 200 kHz [1]. For a wire with a diameter of 2 mm, the same values \u5,5b\u3bwill be XNUMX ohm / km and XNUMX times. Thus, the resistance of the antenna wire RPA 20 ... 50 m long can be estimated at 0,3 ... 3 ohms. Earth resistance Rsoftware more. M. B. Shuleikin once proposed the following empirical formula for determining losses in grounding [2]: Rsoftware = Aλ/λ0, where the coefficient A varies from 0,5 ... 2 ohms for good grounding and up to 4 ... 7 ohms - for bad. Matching coil resistance RPC depends on its constructive quality factor Q and can be calculated by the formula: RPC =X/Q. Using the data of the above example, we calculate the efficiency of an L-shaped antenna with a suspension height of 10 m and a horizontal part length of 20 m, having hд\u10d 6 m. According to the table, we determine the coefficient K \uXNUMXd XNUMX, then the natural wavelength of the antenna will be equal to: λ0\u6d 10 * (20 + 180) \uXNUMXd XNUMX m, and λ / λ0 = 10. With a wire diameter of 1 mm, the resistance RPA\u22,5d 2 * 0,03 * 1,3 \u3d 10 Ohm, satisfactory grounding can be obtained with Roe \u30d 500 * 500 \u10d 500 Ohm. With a wave impedance of the antenna wire W = 0,31 Ohm, the reactance of the antenna is X = 1600 * ctg (π / 250) = XNUMX / XNUMX = XNUMX Ohm. Given the constructive quality factor of the matching coil Q = XNUMX, we find its resistance RPC = 1600/250 = 6,45 ohms. The total loss resistance of the antenna, equal to the sum of all found, will be about 38 ohms, while the radiation resistance RΣ = 1600(hД/λ)2=1600(10/1753)2 = 0,05 ohm, which means that the efficiency η = 0,05/38 = 0,14%! Thus, the signal power given to the load by the considered antenna will be only 0,19 * 0,0014 = 0,26 mW, which is equivalent, for example, to a supply voltage of 1 V at a current of 0,26 mA. This is enough to operate the receiver on the phones, but not enough to power the loudspeaker receiver. Note that grounding contributes the bulk of the antenna loss. To make it good, it is necessary to dig the earth to the aquifer and place a metal object at this depth, perhaps a larger area, of course, then burying the hole. It can also be recommended to make a system of counterbalance wires, radially diverging from the grounding point and buried at a shallow depth. If experiments are carried out on a garden plot, then pipes of a water well, water supply can be used as grounding, and a metal fence of the site can also serve as a counterweight, if you take care of good electrical contact between its individual parts. An important question: how to ensure the necessary matching of the antenna with the detector? The introduction of extra reactive elements only worsens the efficiency due to their inherent additional losses, so it is desirable to get by with only the elements shown in Fig. 2. In this case, the recommended receiver circuit will take the form shown in fig. 3.
The variable inductor L1, together with the capacitance of the antenna, forms an oscillatory circuit tuned to the frequency of a powerful radio station. The reactive impedances of the antenna and the coil are equal and compensated. Series active resistance of the antenna circuit RА = RΣ + Rпconverted to equivalent resistance Rth = X2/RАconnected in parallel with the coil If it is too large to match the input impedance of the detector, the latter is connected to the tap of the coil in such a way that the condition n2*Rth=Rvh, where n is the ratio of the number of turns of the coil from the grounded terminal to the tap to the total number of turns. The detector circuit containing the VD1 diode, the blocking capacitor C1 and the load does not require explanation. In the example above, Rth= 16002/38 = 67,4 kOhm. If the detector has an input impedance of the order of 2 kOhm, which is true when it works on phones with a resistance of 4 kOhm, n = (2/67)0,5 \u0,17d 1, therefore, the tap must be made from approximately 6/XNUMX of the turns of the entire coil. An important problem in rural areas has always been and remains the lightning protection of antennas. It is best to permanently connect the antenna to ground. The scheme of the receiver shown in fig. 3 meets this condition. Nevertheless, even not very close lightning strikes induce pulsed EMF in large antennas, measured in many kilovolts, which is by no means safe. A gas-filled spark gap or even a simple HL1 neon bulb connected between the antenna and ground will help protect the detector diode. And yet, with a close thunderstorm, the antenna should be grounded with a special SA1 switch. Paradoxical, at first glance, the result, which consists in the independence of the power taken from the antenna, on its size in the absence of losses and in coordination with the load, can be easily explained. It is well known that a transmitting antenna, if it has no losses and if it is matched to the signal source, will radiate all the power supplied to it. Therefore, different antennas with the same radiation pattern under the above conditions create the same electromagnetic field strength at the same distance. It remains to add - regardless of the size of the antenna. Of course, as soon as it comes to real antennas with losses, this statement immediately loses practical value. When the size of the antennas decreases, their radiation resistance becomes extremely small, the reactive component of the resistance increases, which makes it difficult to match the antenna with the signal source, the losses increase, so the efficiency of the antennas drops sharply It follows from the reversibility of the antennas that for the same field strength, matching with the load and no losses, receiving antennas of different sizes will provide the same power in the load. Of course, for receiving antennas, losses and difficulties in matching with the load leave a purely theoretical value behind the result obtained. We note once again that all the calculations given in the article are valid only in the case when the dimensions of the antenna are much smaller than the wavelength. Literature
Author: V.Polyakov, Moscow
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