On Mon, 31 May 2010 16:53:18 -0700, Kevin Normoyle wrote:
>Wouldn't it be better to use less of the balun (to avoid capacitance
>between the start and end turns) and not do the crossover to get to the
>other side?...I'm thinking double stacking type 31 allows me to get away
>with only using 2/3 of the balun, which is good...
>Doesn't the crossover introduce physical closeness of two turns that
I've tried W1JR's crossover technique, and I can't measure any difference
between it and a continuous winding. Other good engineers have also tried,
and can't see any difference. My measured data is for continuous windings.
Remember that the capacitance of the winding is part of the design!
>Everything I've read seems to say there's strong capacitive coupling
between the turns and the toroid itself, (in addition to turn-to-turn
capacitance).... which is why using enamelled or stranded wire turns are
sometimes worse, since they wrap tighter to the toroid and get increased
capacitance...I'm wondering if some of the benefits people see with coax
turns is decreased coupling to the toroid...
Again, capacitance is part of the design. Don't try to avoid it unless you
have too much!
>So insulated wire, or wrapped toroids, gets the turns up off the toroid,
which is good. same thing when you see people doing these loose-turns
around the balun.
Nope. See above.
>See here's what I'm planning with my toroids. I have some teflon
insulated
>18 awg stranded, which I think is fine for 1500 watts if reasonable SWR.
Teflon is fine for voltage, but the capacitance will be DIFFERENT from
THHN. If you want to wind some and get them to me, I'll measure them.
But there's another important difference -- characteristic impedance.
Enamel is pretty close to 50 ohms. It's roughly 100 ohms for THHN.I don't
know what it is for teflon. Mismatch can introduce a bit of loss at higher
frequencies. On the other hand, if you're using two chokes in series-
parallel to get a 4:1 transformer, 100 ohms is what you're looking for. :)
>By using 18 awg, I can space the bifilar turns to get some separation
turn to turn to help increase voltage breakdown between turns.. and I
won't use the full toroid..So I won't do a crossover winding, but still be
able to get to the SO-239's on either side of the 4x4x2 NEMA6 box without
too much distance or extra capacitance.
But by increasing the spacing, you also raise the characteristic
impedance, so you could easily go too far. :) Close-spaced THHN is
already too far for 50 ohms.
>I also can't understand when people say they try to get a bifilar pair to
>be 50 ohms based on wire spacing. From what I read, it seems like no
>matter what you do, you're going to get 75-100 ohms for the impedance of
>any wire pair.
No, the dielectric changes both Zo and choking impedance significantly.
>Has anyone actually measured 50 ohms for a real bifilar pair?
YES! Sevick published on that many years ago, and my testing is consistent
with his. He said that close spaced THHN #14 will be 100 ohms.
>I was also wondering if a triple core stack with just 8 turns might be
>even better. I can space the bifilar turns more, and less of the balun is
>used, so it's more of a straight run from so-239 to so-239 on either side
>of the box.
Again, spacing the turns is not necessarily a good thing.
>The inductance vs wire length tradeoff, I think, is better with the stack
>of two or three?
Inductance depends both on wire length and N-squared. But you're also
changing the C, which moves the resonance of the choking impedance. I have
not measured either of those configurations.
>I have to check that again, but since wire length is closely related to
>capacitance which causes the resonance issues, it seems like that's what
>we want: the optimal wire length to inductance tradeoff.
Don't obsess on wire length.
>I know Jim K9YC has measured THNN #14 awg on type 31, but I don't want to
>wrap pvc insulated wire turns that close for 1500 watts (his turn count
>is close the max you can get due to the inner core circumference limit?)
What bothers you about the close spacing? THHN is rated for 600V in AC
wiring, and that's in steel conduit. So conductor to conductor rating is
at least 1200V. 1.5kW at 50 ohms is 275 volts. That's quite a bit of
headroom for voltage breakdown.
73,
Jim K9YC
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