Homemade KPE. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies
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In the manufacture of radio receivers, amateurs sometimes use home-made capacitors of variable capacitance - KPI (see, for example, the article by V. Guskov "VHF receiver"in "Radio", 1998, No. 4, pp. 24, 25). I would like to propose another KPI design, which is distinguished by ease of manufacture and very high reliability, since there are no rubbing contacts in it.
The design of the capacitor is traditional (Fig. 1), the differences are only in the form of plates, which can be made by etching the foil both on the main board of the device (this option is shown), and on a separate plate of foil fiberglass.
As can be seen from fig. 1a, both working plates 1 and 2 are fixed and have the shape of half rings of approximately the same area. An isolated section of the foil 3 is left to level the surface in height. The KPI outputs are connected with printed conductors 4 to other parts in accordance with the radio circuit.
The rotor, which simultaneously serves as a tuning knob, is also made of foil fiberglass (Fig. 1b). Its outer diameter is chosen from design considerations and ease of adjustment. A notch should be made along the outer circumference of the rotor. An auxiliary common lining is etched on the rotor, which, when the rotor rotates, partially or completely covers both fixed linings.
The total capacitance of the KPI is determined by the capacitances of two capacitors connected in series, each of which is formed by its fixed and common movable plates.
When assembling the KPI, an insulating gasket is placed between the stator and the rotor and the package is tightened with an MXNUMX screw with a nut, placing it under the last spring washer. After adjusting the clamping force, the nut is fixed with a locknut, paint, or simply soldered to the screw.
With a diameter of 20 mm and the use of a fluoroplastic film 50 μm thick as a gasket, the limits of capacitance change were 4–14 pF. This is quite enough to cover the VHF broadcasting range. The working angle of rotation of the rotor is about 180°.
If we change the shape of the plates, as shown schematically in Fig. 2, the working angle of rotation will be about 90°. In this option, it is also recommended to leave isolated sections of foil between the stator sectors to level the surface in height. The same design can be used for tuned capacitors.
To level the surface of the rotor fig. 1b it is useful to make a semiring similar to 1 in Fig. 1.a.
Author: V. Evstratov, Girey village, Krasnodar Territory
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