Input circuits when working with an unconfigured antenna. General information

In the unconfigured antenna circuit mode, the antenna itself is represented as an EMF source. with active and reactive resistances. If the antenna circuit is not tuned, then both active and reactive conductivity are introduced into the input circuit, which entails:

        1) Expansion of bandwidth due to insertion of active conductivity , moreover, the permissible value of the relative expansion of the band, estimated value is limited (it is usually considered that and ),

here - attenuation of the input circuit with the antenna turned off;

        -admissible change in attenuation when the antenna is connected.

        2) tuning frequency shift due to introduced reactive conductivity , and the permissible value of the relative shift settings is limited (it is usually assumed that the tuning shift should not exceed half the bandwidth, i.e. or ).

If the requirements for valid values and sufficiently rigid, the antenna parameters in the operating frequency range of the receiver vary widely, and the use of a special antenna antenna tuning unit to compensate for its reactivity is not provided, then a solution to the situation is the choice of a sufficiently weak connection with the antenna. With a weak connection from the antenna circuit, a small additional attenuation is introduced into the circuit, usually not more than 10-20% of its own, which allows preserving the selective properties of the input circuit. The transmission coefficient of the input circuit with a weak link is small.

The resonance transfer coefficient of the input circuit is determined by the expression

those. it is determined by the active losses in the input circuit at resonance and the conductivity of the antenna circuit.

The antenna circuit has its own resonant frequency, which depends on the parameters of the antenna and the coupling elements of the antenna with the input circuit. Changes frequency have the nature of the resonant curve of the antenna circuit. Depending on where the natural frequency of the antenna circuit will be in relation to the working frequency range of the receiver, the nature of the change in the resonant gain of the input circuit will also be different.

There are three cases of communication with the antenna:

        one)   - communication with the "shortened antenna";

        2)   ;

        3)   - connection with the "extended antenna".

When working with an unconfigured antenna circuit, three types of input devices are practically used: with capacitive, transformer and combined coupling with the antenna.