John Popelish wrote:
At low frequency, I understand how this is a good approach (though
this discussion was about the W2DU style choke balun). But at higher
frequencies, I am concerned that the turn to turn capacitance might
provide a low impedance path that parallels the choke. A string of
beads does not have this problem.

Not a valid point.

Say we have 1000 ohms XL in parallel with 1000 ohms Xc at 10MHz. At 14
MHz we have 1400 ohms XL in parallel with 714 ohms Xc. The combined
reactance is just over 1400 ohms.

Capacitive reactance in parallel with inductive reactance increases the
impedance, until we are over 1.414 times the resonant frequency.
Anyplace below that, the shut C decrases unwanted coupling.

If you look at actual chokes, you'll see that is a totally unfounded
concern at HF.

That toroid showing the flipped winding that is supposed to reduce
stray C? It actually makes a choke work slightly worse...not
bettter...until we get way up in VHF or UHF.

I always try to not bend any cable to a smaller radius than the
manufacturer recommends, and I am not familiar with this
recommendation for common coax types used by amateurs. I would have
to look that up before wrapping a core.

Manufacturers are very conservative. I use RG400 in very tight coils
without any issue, and my main baluns are all air core RG213, LMR400,
or RG8X. I do some very tight radius bends without problems. My 160
vertical array uses a phase inverting transformer made with RG-400, and
it is two parallel 3" tall stacks of 61 material 2" OD cores wound as
tight as possible with RG-303. It handles huge voltage across the
winding without any issues.

The only part of the cable that significantly increases CM impedance is
the part inside the core window. Why care how tightly the cable is
pulled against the core outside??

The weight of the cable will tighten the turns on the core.

Simple to fix. Don't hang anything from the cable leaving the core.

I have over 300 feet of RG8X hanging vertically from a high dipole, but
I have a rope lacing the cable as a strain relief. Been up since 1999.

73 Tom

wrote:
John Popelish wrote:

At low frequency, I understand how this is a good approach (though
this discussion was about the W2DU style choke balun). But at higher
frequencies, I am concerned that the turn to turn capacitance might
provide a low impedance path that parallels the choke. A string of
beads does not have this problem.


Not a valid point.

Say we have 1000 ohms XL in parallel with 1000 ohms Xc at 10MHz. At 14
MHz we have 1400 ohms XL in parallel with 714 ohms Xc. The combined
reactance is just over 1400 ohms.

Capacitive reactance in parallel with inductive reactance increases the
impedance, until we are over 1.414 times the resonant frequency.
Anyplace below that, the shut C decrases unwanted coupling.

If you look at actual chokes, you'll see that is a totally unfounded
concern at HF.

That toroid showing the flipped winding that is supposed to reduce
stray C? It actually makes a choke work slightly worse...not
bettter...until we get way up in VHF or UHF.

All sounds good to me. I just hadn't computed the turn to turn
capacitance for wound coax shield to figure out how high you would
have to go in frequency before the capacitance would be a limit. Glad
to hear that there is lots of range where it is no problem.

http://www.w8ji.com/toroid_balun_winding.htm

73 Tom