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Effects of complex impedance thru a current transformer

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W4KJG, Apr 23, 2016.

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  1. WA7DU

    WA7DU Ham Member QRZ Page


    No need for you, Ken, to aplogize. If any, the apologies should be flowing the other direction. Unless Reaston knows you, he can't possibly have the knowledge to justify some of his comments. I infer that he is assuming a lot--we all know what "assume" means when dissected.

    I know virtually nothing about Reaston, not even whether Reaston is a surname, which in polite company would call for the "mister" preceding it. And, based on his "behaviour" here, I am happier NOT knowing him.

    Reaston: If you are feeling tough and all-mighty, take a few jabs at me, just for fun.
     
  2. KA8NCR

    KA8NCR Ham Member QRZ Page

    He's an amateur radio operator that can be best described as a jack of all trades, master of none.

    He doesn't want to give his call sign, factor that into any replies given.
     
    Last edited: Apr 25, 2016
  3. WD0BCT

    WD0BCT Premium Subscriber QRZ Page

    I have to agree with two things Reaston states here. 1. It's clear....2. It's time to part company.
    I've always respected technical and social skills when demonstrated together.
    It's painful when you see neither demonstrated.
     
  4. KL7AJ

    KL7AJ Ham Member QRZ Page

    This can usually be done with a Smith chart....if you make the assumption that the transmission line is effectively 1/4 wave. The sign of the reactance will be flipped.
     
  5. AF7TS

    AF7TS Ham Member QRZ Page

    I personally believe that if I can't explain something technical to my 9 year old son, then I don't really understand it myself.

    Back to the OP: would you be willing to actually post the measurements that were confusing? I would appreciate learning more on this topic.

    73
    Jon
    AF7TS
     
  6. VK2TIL

    VK2TIL Ham Member QRZ Page

    I was one who responded off-forum to Ken; since there is a little more interest in the topic I will make my poor contribution here.

    Transformers do indeed respond poorly to reactive loads; the essence is that the reactive element in the load interacts with the transformer's reactive elements, mainly inductive but also capacitive, and the result is resonances and perhaps other consequences that upset the expected transformer action.

    This is mentioned in one reference;

    [​IMG]

    [​IMG]


    Cancelling the load reactance is mentioned but, if added components are to be used, the transformer becomes superfluous and an L or other matching method would be preferred.

    The statement that "... with reactive sources and/or reactive loads, any lossless matching circuit will be frequency-dependent ... " is profound.

    I did some experiments to help illustrate the problems; I used a small 4:1 transformer that I had lying-around, ten turns twisted on an FT50-43;

    [​IMG]

    and connected in this manner;

    [​IMG]

    Test loads were connected to the "4R" side and the impedance at the "R" side was measured. The first load was a 180R SMD resistor;

    [​IMG]

    The transformation to 180/4 was good.

    The next load was 180R in series with 0.82uH;

    [​IMG]

    The transformation of the 180R to 180/4 was moderately good although there is frequency-dependence; the transformation of the 0.82uH to 0.82/4 wasn't too bad.

    (Note that the transformer would have been running-out of winding reactance at frequencies below about 3MHz; my experience is that this kind of transformer has about 3 - 50MHz bandwidth when transforming resistive impedances).

    The next DUT was 180R in series with 33pF;

    [​IMG]

    Resonance at the low end is obvious and distorts the readings; transformation of the resistance from 180R to 180/4 begins to make sense after the resonance but the expected transformation of 33pF to 33/4 is wildly inaccurate.

    The resonance frequency, about 2MHz, is as expected from a combination of 33pF and 200uH (the approx. inductance of the "4R" winding of the transformer).

    Now, to push the poor old transformer even further, I used a 47pF capacitor alone (seen in the photo above) as the DUT;

    [​IMG]

    Again a resonance, a little below 2MHz as expected from 200uH & 47pF, but the transformation of the 47pF is wildly inaccurate over the entire frequency range; 47/4 is certainly not 198. :)

    So it seems that the kind of transformer tested is only capable of properly-transforming the real component of a complex impedance.

    Although it does reasonably well in transforming a complex impedance with an inductive component. it fails when presented with an impedance with a capacitive component.
     
  7. G3TXQ

    G3TXQ Ham Member QRZ Page

    Shouldn't we expect a transformer that steps down impedance by a factor 4:1 to transform a 47pF capacitor to an equivalent of 188pf ?

    Steve G3TXQ
     
  8. AF7TS

    AF7TS Ham Member QRZ Page

    So it sounds like the naive approximation that Zp=(Np/Ns)^2 * Zs is correct for a 'perfect' transformer (one with no leakage inductance nor capacitance). But with real transformers you will see resonances that will blow your expectations out of the water.

    I agree with G3TXQ; the impedance of a capacitor goes as 1/C, so a _lower_ capacitive impedance looks like a _higher_ capacitance.

    73
    Jon
    AF7TS
     
  9. KD6RF

    KD6RF Premium Subscriber QRZ Page

    That's great work and data Kerry, but it looks like, as said above, you forgot for a moment that Xc proportional to 1/C !!! ...it happens :confused: But the data is interesting and shows those pesky resonance effect that one has to worry about.

    (Another item brought up by KL7AJ - sign reversal in a 1/4 wave transmission line xfmr???)
     
  10. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    VK2TIL
    How is the transformer configured?
    This must be the type were one of the output terminals is also the input terminal?
    Thanks
    Pete
     

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