RadioBanter - View Single Post

March 30th 06, 03:53 AM posted to rec.radio.amateur.antenna

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.