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  • Helical resonator bandpass filter

    Features of spiral resonators filter

    On the left photo you can view the spiral resonator that has been calculated. The design of the spiral tank have very high requirements. The quality of materials, accurate skill largely affect the value of the quality factor. With a very narrow bandwidth begin to be appreciable the impact of thermal instability of the resonator. These and other effects must be taken into account.
    Here are the basic requirements for the design:

    • The side faces of the tank in any case don't to tin. It's best just to buff to a shine.
    • Wire of the helix also not to tin.
    • Silver plated tank and helix provide encrease Q of about 3%. However, quality of the homemade silvering, with a lack of experience, often leaves much to be desired and may even lead to a reduction in the quality factor if the silver layer has the microscopic irregularities.
    • The body of tank should be made of durable, highly conductive material such as brass of 2 mm or more, or use a molded case.
    • Inside the cavity must be as small as possible projections and other parts (such as screws and patches).
    • The shield can't have the soldered seams that arranged parallel to the coil axis, and if any exist, to provide the low resistance of contacts need to well caulk by soldering.
    • Soldering is highly desirable with using a silver-solder. Soldering should have the high quality.
    • The grounded end of the helix should be positioned to the side wall of the shield as straight as possible and soldered to it.
    • As a tuning element is reasonable to use a brass core with diameter from 3 to 8 mm. When mounting should ensure that the core entered no deeper than 5-10% of the length of the helix. Good results are obtained with a core diameter at the 60-80% of the diameter of the helix. Contact resistance trimmer with the shield should be as small as possible, after tuning it must be well fixed.
    • It is necessary to warm up the tank is evenly by volume. Attachment of the filter to the device must be by the single side. It is desirable that the filter case has only contact with the chassis or with only one unit of device.

    It should be remembered that if the helix resonator is included in the output of FM transmitter with an output power of 10 W, at the hot end of the helix the voltage amplitude reaches 60-80 kV! There are design of spiral resonators in a round case, even in a jar of coffee. Such construction can use as a notch filter to put down unwanted interference. [ Example ]


    At frequencies below 30 MHz resonator dimensions are rather impressive, to say the least. To reduce them we should to increase the capacitance of the resonator. In this case, such a construction is more like to a conventional LC-circuit. Indeed, there is no fundamental difference between the LC-circuits and spiral resonator. The coil in both cases, as shown in the article about its self-capacitance , correctly described only as a helix transmission line. There is only difference in external capacitor. To maximize the Q of LC-circuit, we should approximate its parameters to the spiral resonator parameters, namely:

    • The sizes of the coil and of the tank we should to make large as possible.
    • Accordingly, additional capacitance should be a minimum.
    • In ideal, if the external capacitor is an air condenser with a small capacitance, its quality must be high, preferably with copper plates and without a current collector, is even better - a vacuum condenser.
    • Almost all the requirements for the design of the spiral resonators are valid for such LC-filters.