[TowerTalk] Type 31 baluns

[Kevin Normoyle]

jim VE7RF mentioned the common use of crossover windings in toroid baluns, which is the area of some things I'm wondering about before I wind....

Questions:
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 have a larger capacitance between them, then the adjacent turns...i.e. it's making things worse for introducing capacitance that lowers the resonance?

I thought the standard is to leave a 1/3rd gap on the toroid between start and end turns for this reason. So it would seem the crossover method violates this (halfway) ??? or am I misunderstanding?

Another question:
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...

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.


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.

I was going to double stack and teflon tape type 31, and wrap 10 turns of that wire (bifilar).

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.

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.
Has anyone actually measured 50 ohms for a real bifilar pair?

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.

The inductance vs wire length tradeoff, I think, is better with the stack of two or three?
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.

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?)


-kevin
ad6z