6 meter plate choke

Discussion in 'Amateur Radio Amplifiers' started by K9FV, May 12, 2011.

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  1. K9FV

    K9FV Ham Member QRZ Page

    Hello all, Most of the schematics I've seen call for a 36 to 40 turn plate choke on a 1/2" to 1" form giving from 5 uH to 13 uH inductance. How critical is this value?

    I have a plate choke with about 130 turns on a 7/8" Teflon (nylon?) rod with 86uH inductance and shows a resonant freq of 2.7 and 19.7mhz - both values well away from the desired 50 mhz.

    My concern was voltage drop, but at 1 amp current, this is only 1 or 2 volts drop across 30 ft of #22 AWG wire Not enough to measure on a 3,000 volt line.

    Will this excessive inductance cause issues other than voltage drop? It will be easy to remove wire to make only 40 turns if needed.

    Thanks to all for the help that's been provided on this quest - I've sure learned a LOTS.

    73 de Ken H>
  2. W8JI

    W8JI Ham Member QRZ Page


    A monoband choke is easy. The important parameters are impedance at the operating frequency range, current rating (for the combined DC and RF currents), and voltage breakdown.

    I think I started some stuff on chokes here:


    What RF power, plate voltage, and plate current are you planning on????

    73 Tom
  3. K9FV

    K9FV Ham Member QRZ Page

    The choke is for this amp & schematic I've been working on - 6 meters, KW power (hoping:), 3200 vdc no-load, maybe 3,000 vdc loaded?, and around 700 to 800mA.... maybe?

    I wound 40 turns on a 5/8" rod close spaced (worse type) and have a Zmag over 10,000 ohms with first resonant point at 59 mhz per the AIM4170 analyzer I have. Theta goes from positive to net at this point for first time on a scan from 30 mhz to 100 mhz.

    My next plan is to test per your setup on the webpage.

    Ken H>
    Last edited: May 12, 2011
  4. W8JI

    W8JI Ham Member QRZ Page

    Let's assume you have 2500 volts of RF across the choke. That would be a good guess.

    With a choke impedance of 5000 ohms the choke would have 1/2 amp RF current. Stray C is the killer.

    If I was doing a six meter choke, I'd probably just measure current out the bottom of the choke and apply 50 MHz to the top and adjust the turns for minimum current out the bottom. With a 5/8th inch form, it might take more turns than expected. You might be about right. You really do what the choke right at the point where impedance is highest and reactance sign changes from positive to negative.

    The problem is the choke will probably change when placed in the actual sheetmetal.
  5. K9FV

    K9FV Ham Member QRZ Page

    Thanks Tom - looks like I'll have to chase a 12 light bulb down tomorrow. I would have bet I had a couple or more lying around..... and may have, just can't find it.

    tray C? Is that the pF number I see on the AIM4170 test results? Under the Equivalent Circuit it has series and parallel Rs & Xs and Rp & Xp values, in this same back it has Cs and Cp
    At 51 mhz:
    Rx = 1000
    Xs = 11817
    Q = 11.8
    Ls = 36.7 uH

    This "stray C" you refer to, is that the 0.3 pF reading as Theta goes neg?

    Thanks again for all the input and help.

    73 de Ken H>
  6. W8JI

    W8JI Ham Member QRZ Page


    When stray capacitance from end-to-end on the choke resonates with the choke, the net reactance becomes zero and the choke appears as a very high resistance. That's parallel resonance and OK.

    With too many turns the choke becomes resonant from the ends to the middle of the winding. Then the choke becomes resistive with a very low impedance. That's called series resonance. That's why you don't want too many turns.

    73 Tom
  7. K9FV

    K9FV Ham Member QRZ Page

    Very clear explanation Tom - I thank you.

    73 de Ken H>
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