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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Device for tuning antennas Noise bridge R15. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications When carrying out maintenance work on communication equipment, a noise bridge is used as a device for measuring and testing the parameters of various antennas, communication lines, determining the elements of resonant circuits and their characteristics, measuring antenna impedances, etc. With this device, you can determine a number of necessary antenna parameters, for example, such as:
Using this device, you can determine the length of the feeder and, if necessary, select it with a multiplicity of half-wave or quarter-wave. There are no difficulties in manufacturing and its assembly is within the power of any radio amateur. The scope of the device can be significantly expanded with a fairly close acquaintance with the principle of its operation. A noise bridge, as its name suggests, is a bridge-type device. The noise source generates a signal frequency spectrum over a wide range and covers the entire amateur radio region from 1 to 30 MHz. By using high-frequency elements, this range is expanded and, if necessary, you can configure antennas in the 144-146 MHz range. The noise bridge works in conjunction with a radio receiver, which is used to detect the signal. The radio receiver determines the accuracy of the measurements. This could be a radio receiver like R-250, Kalina, etc. . In principle, any transceiver with a digital scale will do. The source of noise is a zener diode type KS156A. It should be noted here that some zener diodes are not “noisy” enough and you should choose the most suitable one. The noise signal generated by the zener diode is amplified by a broadband amplifier using transistors VT2-VT3. The signal is then supplied to transformer T1. It is wound on a 600NN toroidal ferrite ring simultaneously with 4 PELSHO wires twisted at a distance of 15 mm. Wire diameter 0.3 - 0.5 mm. The number of turns is 6. The dimensions of the ring are not critical. Particular attention should be paid to the correct winding and installation of this transformer. The adjustable arm of the bridge consists of variable resistor R14 and capacitor C12. The measured arm is capacitors C10, C11 and a connected antenna with an unknown impedance. A receiver is connected to the measuring diagonal as an indicator. When the bridge is unbalanced, a strong, uniform noise is heard in the receiver. As you adjust the noise bridge it becomes quieter and quieter. "Dead silence" indicates precise balancing, i.e. device settings. It should be noted that the measurement occurs at the receiver tuning frequency. The device is structurally made in a housing measuring 110x100x35 mm. Most of the noise bridge parts are installed on a small 50x40 mm board. On the front panel are: variable resistors R2 and R14, variable capacitors C11 and C12 and a power supply switch. On the side are connectors for connecting a radio receiver and an antenna. The device is powered by an internal battery of the "Krona" type or a battery of similar size. The current consumption is not more than 50 mA. Variable resistor R14 capacitor C12 must be provided with scales. Moreover, the larger its diameter, the more accurate the measurements will be. Tuning, balancing and calibration We connect the radio receiver with the AGC system disabled to the corresponding connector. We install capacitor C12 in the middle position. By rotating resistor R2, you should make sure that the generated noise is present at the receiver input on all ranges. Non-induction resistors of the MLT or OMLT type with known values are connected to the “Antenna” connector. You should prepare resistors for calibration, for example, 10, 25, 50, 75, 100, 130, 150, 180, 200, 240, 270,300 and 330 ohms, after measuring the resistance with a digital avometer. When connecting resistances, we achieve by rotating R14 a sharp decrease in the noise level in the receiver phones or a sharp drop in the readings of the millivoltmeter connected to the “Receiver” output. By selecting capacitor C12, we minimize the noise level and make marks on the R14 scale in accordance with the connected reference resistor. And so on, by analogy, we calibrate the device up to the 330 Ohm mark. For precise balance, you can adjust the C9 capacitance. Calibrating the C12 scale (reactive impedance meter) is a little more complicated. To do this, we alternately connect a parallel-connected 100 Ohm resistor and a capacitance (inductance) of 20-70 pF (0,2 - 1,2 µH) to the "Antenna" connector. We achieve bridge balance by adjusting R14 at the 100 Ohm mark of the scale with minimizing the noise level by rotating C12 in both directions from position "0". If there is an RC chain, we put a “-” sign on the scale, and if there is an RL chain, we put a “+” or XL sign. Instead of inductance, you can connect a capacitance of 100-7000 pF, but in series with a 100 Ohm resistor. Antenna Impedance Measurement R10 is set to the position corresponding to the cable impedance - this is 50 or 75 ohms in most cases. We install capacitor C12 in the middle position. The receiver is tuned to the expected resonant frequency of the antenna. We turn on the bridge and set a certain level of the noise signal. Using R14 we adjust to the minimum noise level and using C12 we further reduce the noise. We carry out these operations several times, since the regulators influence each other. An antenna tuned to resonance must have zero reactance, and the active one must correspond to the characteristic impedance of the cable used. In real antennas, both active and reactive resistances can differ significantly from the calculated ones. For this, certain coordination methods are used. In this case, several options for instrument readings are possible. If the active resistance is close to zero, then a short circuit in the cable is possible; if the active resistance is close to 330 Ohms, then a break in the cable is possible. If the device shows inductive resonance, then the antenna is too long, and if it is capacitive, then it is short. The length of the antenna can be adjusted. To do this, its real resonant frequency is determined. Determining the resonant frequency The receiver is tuned to the expected resonant frequency. The variable resistor R14 is set to a resistance of 75 or 50 . Capacitor C12 is set to zero, and the control receiver is tuned to obtain a minimum noise signal. If the antenna has a high quality factor, then it is easy to miss the minimum during fast frequency tuning. For a more accurate measurement, you can connect a pointer millivoltmeter to the receiver output. The receiver must be tuned up in frequency with inductive impedance and up in frequency with capacitive to obtain a minimum noise signal. By adjusting the bridge regulators, it is necessary to additionally achieve noise reduction. Determining the length of the communication line (feeder) When designing antennas, it should be taken into account that for good performance, it is necessary to make the connection line correctly. Usually, in practice, cables that are multiples of a quarter or half wave at a certain frequency are required. For this, the following method is used: • install a jumper on the measuring connector; • using the Resistance (R14) and Reactance (C12) regulators we achieve the minimum noise bridge at the required frequency, while both regulators must be in the area of \uXNUMXb\uXNUMXbthe zero positions of the scale; • remove the jumper and connect the cable under test to the measuring arm; • to determine the cable length, a multiple of a quarter wave, it is necessary to shorten the cable until a minimum signal is obtained with the open end; • to determine the length of the cable under study, a multiple of half-wave, the cable is short-circuited at the end during each measurement.
List of radio elements
Fig2. Circuit board
Publication: cxem.net
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