Calibrating antenna tuner

Discussion in 'Radio Circuits, Repair & Performance' started by KI7TGX, Apr 24, 2020.

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  1. KI7TGX

    KI7TGX Ham Member QRZ Page

    I came into a QRP antenna tuner which uses an interesting variable capacitor; these tune via a knob which "loops around" so as you go beyond max capacitance it then deceases, then increases, etc as you keep turning the knob. Unfortunately the knobs were not secured and they have no markings to indicate when the capacitance is at maximum. Is there a way I can use an antenna analyzer, for example, to determine which setting is maximum capacitance?
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Many if not most air variable capacitors have no end stops and continue round and round.

    A variable capacitor has maximum capacitance when the plates are fully meshed. Most tuners align the front panel knobs to their minimum setting (fully CCW) at maximum capacitance and advancing the capacitor tuning reduces capacitance (plates become less meshed). If you can open the tuner and see the capacitor plates then set the capacitor to fully meshed plates and align the front panel control to it's far counter clockwise (far left) position.

    In some QRP tuners the capacitor isn't a true air variable and instead some use a small broadcast band tuning capacitor in a plastic case. That can make it more difficult to see when the plates are fully meshed but it's usually still possible. If not then either a capacitance meter or some other tool (e.g. a grid dip meter and coil or a NanoVNA) can be used to determine maximum capacitance.
  3. KI7TGX

    KI7TGX Ham Member QRZ Page

    Thanks for the reply. Yes they are sealed, and very hard to get to besides a side view. I don't have those tools, but would an analyzer that gives you the complex impedance do the trick?
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yes, but ideally you'd disconnect the capacitor from the rest of the circuit and measure the capacitor in isolation. You're looking for the position on the capacitor that gives you the minimum -jX term (minimum capacitive reactance = maximum capacitance setting). You'll likely have to play with the measurement frequency a bit to find a high enough frequency to get a reliable complex impedance reading, at too low a measurement frequency the capacitive reactance will be very high and outside the measurement range of most antenna analyzers.
    KI7TGX likes this.
  5. AE1N

    AE1N Ham Member QRZ Page

    If there are any markings abound the capacitor just write down the settings.
    If there are non, make your own with a magic marker.
    Layne AE1N
  6. KI7TGX

    KI7TGX Ham Member QRZ Page

    My question is what to mark if you don’t know what setting is fully meshed. I like to tune with the capacitors as close to fully meshed as possible.

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