Absorption frequency meter

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Circuit diagram of an absorption frequency meter
Wave meter for measurements in the Ku band . The waveguide connections can be seen on the left and right.

The absorption frequency meter (also wave meter or wave meter ) is a mechanically adjustable oscillating circuit , conduction circuit or cavity resonator for determining the wavelength or the frequency of radio waves and microwaves. An adjustment device is provided with a scale on which the wavelength or the frequency can be read directly.

It is used to measure the frequency of an active resonant circuit , e.g. B. from oscillators , vibrating audions or transmission amplifiers and is now mostly replaced by a frequency counter.

Low frequencies

An absorption frequency meter for frequencies below 500 MHz consists of a variable capacitor and a - mostly exchangeable - coil. This tunable resonant circuit is normally weakly coupled to the magnetic field of the measuring object. In the case of resonance, it absorbs so much energy that the resulting voltage on the resonant circuit can be rectified and displayed. To measure the frequency, change the variable capacitor until the voltage on the absorption circuit is at its maximum. This resonant energy extraction from the measurement object can be sufficient to stop the vibrations in the case of a weak oscillator. Since the absorption frequency meter has a very simple structure and still has sufficient accuracy for many practical purposes, it is particularly popular with radio amateurs . Usually, dipmeters also work in “passive mode” as absorption frequency meters.

High frequencies

Open wave knife for microwaves (max. Wave length approx. 10 cm). The micrometer (right) moves the short-circuit slide (arrow)

Wavemeters for wavelengths below about 100 cm work with Lecher lines or a waveguide section with a short-circuit slide. The required length is λ / 4 (see line theory # λ / 4 line ).

Standing waves occur on the lines or in the cavity resonator due to the reflection at the short-circuit slide, the amplitude of which is particularly large if their natural frequency corresponds to the coupled wave. They then extract a particularly large amount of energy from the source via the coupling element (e.g. a slot between the feeding waveguide and the wave cutter). A voltage is coupled out through a coupling loop and rectified with a diode.

When moving the short-circuit slide, resonance maxima occur at a distance of half the wavelength. The distance between them can be read off directly on a scale, which is usually metric for this purpose, on the short-circuit slide, which is moved by a spindle drive.

literature

  • Author collective: amateur radio . German military publisher, Berlin 1963.

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