Features of national broadband transformers. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Features of national broadband transformers. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Nodes of amateur radio equipment

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Those who often look into E Rad's books "Radio Circuit Design" and "High-Frequency Circuitry Handbook" have probably paid attention to the sections devoted to broadband transformers. In these sections, many diagrams of transformers are given and the principle of their operation is popularly explained. However, one can still hear complaints from radio amateurs about the "incomprehensible" operation of these devices on the air. For some, they heat up even at low power, for others they spoil the SWR of the antenna-feeder path, for others they work differently on different ranges.

What is the mystery here? Or did E Red call these transformers broadband in vain and guarantee their operation within six to ten octaves? This means, for example, that the transformer must retain its properties, starting, say, from 1 MHz to hundreds of megahertz.

In recent years, many of us have acquired modern imported transceivers. And someone even "revealed" their transmission paths, and got the opportunity to transmit in a continuous frequency range from 1,5 MHz to 30 MHz. The "disclosure" procedure, in general, has no practical meaning for working on the air, but it makes it possible to have a powerful and convenient measuring complex at hand. The transmitter then turns into a highly stable GSS equipped with an SWR meter. By the way, few people thought why imported transceivers are guaranteed a constant output impedance of 50 ohms and power over the entire frequency range.

Try to guess "from two times" how this is possible and in what way it is achieved. So, if you pass the signal through a homemade broadband transformer loaded on a resistor, the resistance of which is proportional to the square of the transformation ratio, then, depending on the design of the transformer, the SWR meter of the transceiver will show terrifying numbers - from 2 to 5 or more. In addition, these figures will change from frequency A with an increase in signal power, heating of the core will be observed. In a word, we will get exactly what they are talking about on the air. And, no matter how we rewind the transformer, we will not get a significant improvement in its operation.

So, what, after all, our "classic Red" was wrong? Not at all! We just didn’t read what he writes further there. And he writes that it is necessary to compensate for the parasitic inductive components of the complex resistance of the transformer windings. In principle, a good result is obtained both in series connection of trimmer capacitors with transformer windings, and in parallel.

Features of national broadband transformers

The effect resembles resonance - the SWR at the input of the "transformer-load" circuit has a clear minimum for strictly defined values of compensating capacitors. But unlike the resonant circuit, this circuit will be frequency-independent. In the transformer, losses are sharply reduced. For example, a ferrite ring with a diameter of one centimeter can easily withstand a hundred watts of power.

Using such a pre-configured transformer, you can no longer worry about this node and improve the overall SWR, for example, of a feeder system, selecting only the load resistance, removing only its reactive components.

Author: Sergey Makarkin, RX3AKT; Publication: N. Bolshakov, rf.atnn.ru

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