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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Measurement of the sensitivity of radio receivers with a magnetic antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception Magnetic antennas are widely used in radio receivers for receiving signals in the LW, MW and, less often, KB bands. To measure the sensitivity at the location of the antenna of the radio using a known technique create an electromagnetic field of known intensity. The article analyzes this technique and provides recommendations for its improvement. The sensitivity of a radio receiver is such a value of the input signal at which a certain signal-to-noise ratio is created at its output. When measuring voltage sensitivity, the input of the radio receiver is connected to the signal generator through the equivalent of the antenna - an electrical circuit that simulates the parameters of an external antenna. For radio receivers with a magnetic antenna, field sensitivity measurements are carried out, but very little attention is paid to this issue in the technical literature. Usually, everything comes down to a reference to allegedly well-known methods [1-3], the essence of which is to create a given magnetic field strength using a current-carrying loop connected to a measuring generator. By changing the generator signal, taking into account the frame conversion factor, the field strength is found at which the output signal of the radio receiver has the required parameters. Acquaintance with the sources [1-3] showed that the same technique is meant, in which a single-turn square-shaped frame with a side of 380 mm, made of a copper tube with a diameter of 3...5 mm, is used. It is connected directly to the output of the signal generator through a 80 ohm resistor. The middle of the magnetic antenna of the radio receiver is placed at a distance of 1 m from the center of the frame so that the axis of the antenna is perpendicular to the plane of the frame. In this case, the field strength (mV/m) at the location of the magnetic antenna is numerically equal to the output voltage of the signal generator (mV). The application of this technique with the use of modern RF signal generators led to depressing results - the measured sensitivity of radio receivers turned out to be about ten times worse than expected. A more detailed study of this situation showed that this technique was developed for the case of using the GSS-6 generator, in which, when the external attenuator is turned off, the output signal is ten times greater than the readings of its attenuator (the external attenuator has transmission coefficients of 10, 1 and 0,1). Consequently, the voltage on the frame is ten times higher, and the total conversion factor of the generator signal into the electromagnetic field is equal to 1 due to the fact that the conversion factor of the measuring frame is 0,1. In addition, the output impedance of the GSS-6 generator in this mode is 80 ohms, which explains the resistance of the additional resistor. But modern RF signal generators typically have an output impedance of 50 ohms. All this prompted us to adjust the well-known method for testing the sensitivity of receivers with a magnetic antenna.
Let's start with the magnetic frame itself. The so-called standard frame consists of one square-shaped coil with a side of 380 mm and is used in the frequency range of 0,15 ... 1,6 MHz. Obviously, its dimensions are much smaller than the wavelength of the radiation, and the distance from the frame to the magnetic antenna is greater than its dimensions, therefore, in the operating frequency range, it is an elementary magnetic radiator. An analysis of the field of an elementary magnetic emitter [4] shows that at distances r<λ, the magnetic field exists in all directions from the emitter. Two directions are of interest (shown in the figure). The first is perpendicular to the plane of the frame, while the axis of the magnetic antenna should be directed to the center of the frame. Theoretically, this direction in the far zone corresponds to the minimum of the radiation pattern. The second is in the plane of the frame, while the axis of the magnetic antenna is perpendicular to it. In the far zone, this direction corresponds to the maximum radiation pattern of the emitter. Using the expressions for the magnetic field strength in these directions [4] and passing from the magnetic moment of the vibrator to the frame with current [5], we obtain
where H1 H2 is the intensity of the magnetic component of the field at points 1 and 2 (see figure), respectively; S - frame area, m2; I - current in the frame, A; d is the distance between the centers of the frame and the magnetic antenna, m; A, - signal wavelength, m. Expressions (1), (2) make it possible to calculate the magnetic field strength at any distance from the frame in two directions. It can be shown that at small distances {λ/2π) they coincide with the expressions for the magnetic field of the loop with direct current. But the intensity of the electromagnetic field is usually measured by the intensity of its electrical component. In the formed electromagnetic field, there is a strict relationship between the intensity of the electric and magnetic components. To find the strength of the electric component of the field, which corresponds to the known magnetic component, it is necessary to multiply expressions (12) by the wave resistance of the medium, which is equal to 120π for air. Taking into account the fact that at small distances 2πr<<λ these expressions are transformed:
where E1,E2 are the strength of the electromagnetic field at points 1 and 2 (see figure), respectively. The obtained expressions show that the strength of the electromagnetic field near the loop with current depends on its area, the value of the current, is inversely proportional to the cube of the distance and does not depend on the wavelength. In this case, the field strength in the first direction is two times greater than in the second. This, in particular, explains the fact that in metal detectors, in most cases, the position of the coil is used, which is parallel to the surface being examined. Using expressions (3), (4), one can calculate the field strength for a frame of any acceptable size with a known current and distance. However, it is more convenient to relate the field strength to the output signal of the signal generator to which the loop is connected. To set the current, an additional resistor is connected in series with it. Usually, the inductive reactance of the loop is negligible and can be ignored. In this case, the current in the loop without taking into account its inductive resistance is equal to
where U is the output voltage (according to the readings of its attenuator) of the generator, V; Rr - generator output resistance, Ohm; Rd is the resistance of the additional resistor, Ohm. As a result, the expressions
where K1 K2 is the conversion factor of the generator signal voltage into the electromagnetic field strength at the position of the receiving antenna at points 1 and 2 (see figure), respectively. Expressions (5), (6) make it possible to calculate the coefficient of conversion of the output signal of the generator into the value of the electromagnetic field strength, or to determine the area of the frame or the distance to it for a given value of the conversion coefficient. In accordance with them, in a well-known technique, the conversion factor for a square frame with a side of 380 mm, a generator with an output resistance of 80 Ohms and an additional resistor with the same resistance gives a value of 0,108 at a distance of 1 m. Obviously, in this technique, the frame was calculated for the conversion factor 0,1. A small error, most likely, is caused by rounding the frame sizes upwards and is not significant for measuring sensitivity. For modern signal generators with an output impedance of 50 ohms with such a frame with an additional resistor of 80 ohms, the conversion coefficient K1 = 0,133, and with an additional resistor of 51 ohms K1 = 0,172, which is inconvenient for practical use. The dimensions of the frame (its area) with a conversion factor K, = 1 can be determined from expression (5). For r \u1d 50 m, Rr \u51d 0,84 Ohm, Rd \u2d 0,917 Ohm, the area should be 1,035 m4. This corresponds to a square frame with a side of about 4,5 m or a round frame with a diameter of 1 m. But its inductance, depending on the wire diameter used, will be XNUMX ... XNUMX mH, which will lead to a noticeable dependence of the current in the frame on the signal frequency at frequencies above XNUMX MHz. In addition, such dimensions become commensurate with the distance to the antenna, due to which the formulas obtained for an elementary magnetic radiator become inapplicable. It is more convenient to use the conversion factor K1 = 0,1, which will allow using a relatively small frame with an area of 0,085 m2 - this corresponds to a square frame with a side of 291 mm or a round frame with a diameter of 328 mm. With a conductor diameter of 3 mm, its inductance is about 1 mH. For such loops, with an additional resistor of 51 ohms, the output signal of the generator, equal to 15 mV, will correspond to a field strength of 1,5 mV / m at a distance of 1 m. Taking into account the influence of the loop inductance shows that it can be used to measure the sensitivity of radio receivers with a magnetic antenna up to a frequency of 8 MHz, at which the field strength will decrease by about 9%. At higher frequencies, you can use a frame with an area of 84,17 cm2 (which corresponds to a square with a side of 92 mm or a circle with a diameter of 104 mm), made of a copper tube or wire with a diameter of 3 mm. With such a frame and an additional 51 Ohm resistor, the conversion coefficient will be K, = 0,01, so generating a 1,5 mV/m field at a distance of 1 m would require a generator output of 150 mV. Sensitivity measurements can be made up to a frequency of 30 MHz, at which the field strength will decrease by about 8%. The same frame will provide a conversion factor K, = 0,1 at a distance of 465 mm, however, in this case, a high accuracy in setting the distance between the frame and the antenna will be required. The accuracy of setting this distance affects the measurement error. So, at a distance of 1 m, an error of ±3,33 cm leads to a measurement error of ±10%. At a distance of 465 mm, the same measurement error will be with an installation accuracy of ± 1,55 cm. Round and square frames are equivalent, you can also use frames of a different shape, such as a triangular one, it is important that their area is exactly equal to the required one. Therefore, from a constructive point of view, it is more convenient to use a square frame, since in this case it is easier to obtain a given area. All the above examples are valid for the case when the axis of the magnetic antenna is located on a perpendicular to the plane of the frame, drawn through its center (position 1, see figure). But another direction can be used to measure the sensitivity (position 2). In accordance with expression (6), in this position, the conversion coefficient will decrease exactly by a factor of two. Therefore, to create the required field strength, ceteris paribus, it is necessary to double the generator signal or reduce the distance to the center of the frame in However, the second position can be convenient from the point of view of the compactness of the workplace, since the frame can be placed, for example, above the desktop. But in all cases, it is important that there are no large metal objects in the measurement zone that can noticeably distort the field. Literature
Author: D. Alkhimov, Smolensk; Publication: radioradar.net
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times. But a distance of less than 0,5 m is not recommended, since the cubic dependence greatly increases the measurement error from the inaccuracy of setting the distance to the antenna. In addition, when the distance to the frame becomes commensurate with its dimensions, the above expressions give an overestimated value of the electromagnetic field strength, since the emitter can no longer be considered as a point.
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