Debate: Does It Matter If The Secondary Windings Of A 49:1 Impedance Transformer (EFHW Antenna Unun) Change Direction Halfway Around? Does Bare Copper Wire Touching The Toroid Mess Things Up? Is “The Crossover” A Scam?

QUERY:

Here are the results of a quick 49:1 [14:2 turn ratio] toroid transformer test.
1) Does it matter to transformer performance if the windings of the secondary change directions halfway through? [Answer: Yes]
2) Does it matter to transformer performance if bare copper wire shorts to a ferrite toroid? [Answer: No]
3) Does it matter to transformer performance if your winding technique crosses over the toroid? [Answer: No]

TEST:

– 7 MHz, 1Vpp sine wave from a function generator input and ground to the primary windings (2 turns)
– Oscilloscope channel 2 passive 1x probe to both ends of the primary winding (polarity matching function generator placement)
– Oscilloscope channel 1 passive 1x probe to “antenna” output of secondary winding (with ground lead to the other end of the secondary winding)
– Oscilloscope display standardized throughout test to 100 ns and 200 mV per grid square on each channel.

RESULTS:

Voltage measured directly from generator output:

Transformer built by secondary crossing through toroid at turn 8. Output voltage is larger than input voltage. Output voltage is approximately 180° out of phase with input voltage.

Transformer built by secondary crossing over toroid before turn 8. Output voltage is smaller than input voltage. Output voltage is approximately in phase with input voltage.

Next, a portion of the primary winding insulation is removed and the bare copper wire is shorted to the toroid. There is no appreciable difference between these oscilloscope traces and those seen for the “crossing on top of toroid” situation.

And finally, what about the so-called crossover? Is it just a scam of internet hucksters (like yours truly apparently!) Well, turns out, it doesn’t really make a difference. Go back and take a look at the oscilloscope findings for the crossover; it has the same waveform pattern as shown below. The number one reason that I know of to cross over the windings is mechanical…on more crowded toroids, it puts the antenna output on the opposite side of the toroid from the feedline input and ground. This configuration makes it easier to stick it in a box.

Bottom Line: Do what you’d like with your ununs kiddos!

Hope this clears some things up. This testing is limited by the use of 1X oscilloscope probes into a 1M oscilloscope at RF frequencies. I am sure there are better ways to test these things, but I think this shows interesting results regardless.

Scientifically yours,

KM1NDY