Mike Thompson wrote:
Ok I feel I need to interject here. Anyway being a n00b I was pondering the statement of having a secondary resonate to a 1/4 wavelength and was wondering why #1 is viewed as a correct statement?
You're speaking of Myth #1? Combination of circumstances:For a half wavelength long dipole antenna, or a quarter wavelength vertical over ground, the voltage is high at the ends (and low at the feedpoint).
For transmission lines, a quarter wavelength transforms a low impedance (low voltage, high current) at one end to a high impedance at the other (high voltage, low current)
Both are "sort of" similar to the typical tesla coil (high voltage at one end, etc.), but note that lengths of both are 1/4 wavelength in free space (many hundreds of meters for typical TC frequencies)
ANDOne can use some distributed LC transmission line equations to model a long skinny inductor (the secondary) as transmission line with a very slow propagation speed, so that 1/4 wavelength is MUCH shorter (meter scale).
ANDIt happens that for some common coil dimensions, the secondary turns out to require about 1/4 free space wavelength of wire, particularly for the longer skinny secondaries popular 20-30 years ago. (that is, a 3 foot long secondary with 800-1200 turns on a 4" form is about 800-1200 feet, which is 1/4 wavelength for 200-300 kHz)
NONE of which have anything to do with the real physics going on, which is pretty well modeled by a lumped inductor and capacitor, or, to go the next sophisticated step, by a funny shaped distributed capacitor along the secondary and including the topload. (Note that a simple lumped approximation gets you to within 5% of real values, which is tighter than most construction tolerances)
So you can see the myth takes a lot of busting, because there's 4 or 5 ways that "seem" to justify it.
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