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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Noise bridge, for tuning antennas. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, setup and matching

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The noise bridge is used to measure and test the parameters of antennas, communication lines, characterization of resonant circuits and the electrical length of the feeder.

The noise bridge, as its name suggests, is a bridge type device. The noise source generates noise in the range from 1 to 30 MHz. With the use of high-frequency elements, this range is expanded, and if necessary, antennas in the 145 MHz range can be tuned. The noise bridge works in conjunction with a radio receiver, which is used to detect the signal. Any transceiver will work too.

The schematic diagram of the device is shown in Fig.1. The noise source is the Zener diode VD2. It should be noted here that some instances of zener diodes are not “noisy” enough, and the most suitable one should be chosen. The noise signal generated by the zener diode is amplified by a broadband amplifier based on transistors VT2, VT3.

Noise bridge, for tuning antennas

The number of amplifying stages can be reduced if the receiver used has sufficient sensitivity. Next, the signal is fed to the transformer T1. It is wound on a toroidal ferrite ring 600 NN with a diameter of 16 ... 20 mm simultaneously with three twisted PELSHO wires with a diameter of 0,3 ... 0,5 mm; number of turns -6.

The adjustable arm of the bridge is made up of variable resistor R14 and capacitor C12. Measured shoulder - capacitors C10, SI and connected antenna with unknown impedance. The 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 the bridge adjusts, the noise gets quieter and quieter. "Dead silence" testifies to the exact balancing. It should be noted that the measurement takes place at the tuning frequency of the receiver. The printed circuit board and the placement of parts on it are shown in Fig. 2.

The device is structurally made in a housing measuring 110x100x35 mm. 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 an accumulator. Consumption current - no more than 40 mA.

Variable resistor R14 and capacitor C12 must be provided with scales.

Tuning, balancing and calibration

We connect the radio receiver with the AGC system disabled to the corresponding connector. We set the capacitor C12 to the middle position. Rotating the resistor R2, you should make sure that the generated noise is present at the input of the receiver on all ranges. We connect non-inductive resistors of the MLT or OMLT type to the "Antenna" connector, having previously measured their ratings with a digital avometer. When connecting resistances, we achieve by rotating R14 a sharp decrease in the noise level in the receiver.

By selecting capacitor C12, we minimize the noise level and make marks on the R14 scale in accordance with the connected exemplary resistor. Thus, we calibrate the device up to the mark of 330 ohms.

Calibration of the C12 scale is somewhat more complicated. To do this, we alternately connect a 100 Ohm resistor connected in parallel and a capacitance (inductance) of 20 ... 70 pF (0,2 ... 1,2 μH) to the "Antenna" connector. We achieve the balance of the bridge by setting R14 at around 100 ohms of the scale and minimizing the noise level by rotating C 12 in both directions from the "0" position. If there is an RC chain, we put the "-" sign on the scale, and if there is an RL chain, we put the "+" sign. Instead of an inductance, you can connect a 100 ... 7000 pF capacitor, but in series with a 100 Ohm resistor.

Antenna Impedance Measurement

R14 is set to a position corresponding to the impedance of the cable - this is 50 or 75 ohms for most cases. We set the capacitor C12 to the middle position. The receiver is tuned to the expected resonant frequency of the antenna. We turn on the bridge, set a certain level of the noise signal. With the help of R14 we tune in to the minimum noise level, and with the help of C12 we further reduce the noise. We carry out these operations several times, since the regulators affect each other. An antenna tuned to resonance must have zero reactance, and the active resistance must correspond to the characteristic impedance of the cable used. In real antennas, the resistance, both active and reactive, can differ significantly from the calculated ones.

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 ohms.

Capacitor C12 is set to zero, and the control receiver is tuned in frequency until a minimum noise signal is obtained.

Author: A.Volynets (UA3YFR), Bryansk; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Antennas. Measurements, setup and matching.

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