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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Work GSS with HF-bridge. 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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Standard signal generators (GSS) provide a voltage of 50 ... 1 V at a load of 2 Ohms, which is clearly not enough to work with bridge antenna resistance meters. In order to use conventional bridge resistance meters without modifying them, it is necessary to use a broadband power amplifier.

GSS operation with an RF bridge
Ris.1

A diagram of such an amplifier is shown in Fig. 1. It is based on the scheme from [1]. It has undergone some measurements that have made it convenient to work with a standard signal generator. The broadband amplifier provides at least 1 W of output power when operating in conjunction with the GSS in the frequency range from 1 to 30 MHz. If you reduce the supply voltage to 12 V and use the part ratings given in brackets, then the output power of the amplifier drops to 600 mW, which is enough to work with many types of measuring bridges. When assembling the amplifier from serviceable parts and setting the collector current indicated on the diagram, the amplifier is immediately operational and does not need to be adjusted. It is convenient to assemble the amplifier by surface mounting.

Transformer T1 is made on a ring magnetic circuit with dimensions K7x4x2 made of ferrite with a permeability of 400 ... 600. The windings contain 12 turns of PEL-2-0,35 wire, wound with a twist - one twist per centimeter. The ferrite ring can also be used in larger sizes. The amplifier can be assembled in a case made of foil fiberglass. Transistor VT1 is mounted on a radiator. High-frequency input-output jacks and amplifier power leads are output to the amplifier case.

After the amplifier is manufactured, it is desirable to take its amplitude-frequency characteristic with a specific GSS used with this amplifier, and make sure that there is no parasitic generation in any part of the operating range using an oscilloscope.

Sometimes it is inconvenient to use the GSS in conjunction with a power amplifier. These may be cases of measurements in the field; with a GSS powered by batteries, etc. In this case, you can use a bridge with a high-frequency unbalance voltage amplifier. A diagram of such a bridge is shown in Fig. 2.

GSS operation with an RF bridge
Fig.2 (click to enlarge)

Its difference from other circuits of bridge meters is that the high-frequency voltage is not detected and measured immediately, but is fed through the transformer T1 to the input of a two-stage transistor amplifier and then already detected. This makes it possible to get by with the RF voltage levels generated by the standard signal generator when tuning the antennas. The amplifier can be assembled on any high-frequency transistors such as KT315, KT312. The frequency response of the amplifier is linear up to 40 MHz. Transformer T1 contains 22 turns of PEL-0,1 wire in each winding. The windings are located symmetrically on both halves of the ring with dimensions K10x7x4 with a permeability of 400...600.

Calibration of the device consists in marking the variable resistor R2 of the load resistance on the dial. This is best done using a digital ohmmeter. The indications of the limb when balancing the bridge will correspond to the resistance of the measured antenna.

The bridge meter is assembled in a case made of foil fiberglass. Its installation should be as compact and rigid as possible. The limb of the variable resistor to improve the accuracy of measurements should have the maximum possible dimensions.

Literature

1. Stepanov B., Shulgin G. Power amplifier for all HF bands. - Radio, 1980, No. 10, S. 19-21.

Author: I. Grigorov (RK3ZK), Belgorod; Publication: N. Bolshakov, rf.atnn.ru

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