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Impedance matching transformer

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W6OGC, Apr 3, 2015.

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  1. W5DXP

    W5DXP Ham Member QRZ Page

    What impedance did the analyzer yield when only the 100 ohm resistor was attached? How about a picture of your test setup? Could you have hooked up the transformer backwards?
     
  2. W6OGC

    W6OGC Ham Member QRZ Page


    Only a 100 ohm resistor was attached, with the analyzer on the other side. I tried it each way, actually, with no difference. I probably need to come up with a better test set up, as this makes no sense whatsoever.

    I assume both primary and secondary gave to be wound as depicted. Otherwise, there is not enough room on the toroid.
     
  3. W5DXP

    W5DXP Ham Member QRZ Page

    Again, what is the measured impedance of the 100 ohm resistor?
     
  4. W6OGC

    W6OGC Ham Member QRZ Page

    The analyzer showed a very high impedance for the transformer, >350 ohms, and SWR. I should check if it showed the same whether the resistor was connected or not.
     
  5. W5DXP

    W5DXP Ham Member QRZ Page

    I have a 12.5 ohm resistor that at 14 MHz, measures 12.5 + j250 ohms. If I hooked that 12.5 ohm resistor to a 1:4 balun for testing, an ideal balun would show an SWR of 400:1 even though the transformed resistance is 50 ohms. Of course, I chose a wire-wound resistor to illustrate my point.
     
  6. W6OGC

    W6OGC Ham Member QRZ Page

    I have seen the idea of using an ordinary resistor of appropriate value to "tune" a matching device in several articles.

    Is there more to it than that?

    Is there a better way with simple methods/equipment?
     
  7. W5DXP

    W5DXP Ham Member QRZ Page

    My wire wound 12.5 ohm resistor is an "ordinary" resistor that is completely useless as an HF dummy load. That's why I asked you to directly measure the impedance of your 100 ohm load resistor at the same frequency at which the transformer testing was done in order to eliminate the resistor as the problem. It's an Occam's razor approach to the problem. First, eliminate the simple things that could be wrong. The resistor may be bad, mismarked, misread, or unsuitable for HF use.
     
    Last edited: May 11, 2015
  8. W6OGC

    W6OGC Ham Member QRZ Page

    This is an ordinary composition resister, carbon, I believe, that came from a kit I recently bought, and verified as 100 ohms by color code and ohm meter. I can see where a wire wound might be a potential problem, but are these ordinary carbon whatever they are's?
     
  9. W1VT

    W1VT Ham Member QRZ Page

    Hook up the resistor directly to the analyzer to verify that you are doing the measurement properly--you should see a 2:1 SWR. Garden variety 1/4W and 1/2W resistors are reasonably non-inductive for these sorts of measurements at HF.

    Simple transformers with different numbers of windings on the input and output tend to be more difficult to make in practice than transmission line transformers, particularly if you are using low permeability iron powder material.

    Zack W1VT
     
    Last edited: May 11, 2015
  10. W6OGC

    W6OGC Ham Member QRZ Page

    Well, nobody told me that. I figured this would be a good opportunity to try this toroid thing, since I never have done that. Of course, not knowing what I am doing has seldom stopped me from something I wanted to do, with mixed results.
     

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