[TowerTalk] Ferrite beads for common mode current

[Jim Brown]

On Thu, 26 Jul 2007 16:02:41 +0100, Ian White GM3SEK wrote:

>That can easily  happen, as the equivalent lumped capacitance is only a 
>few pF so the resonant frequency is extremely sensitive to small 
>variations in strays. 

Yes. I've devised a method of winding transmitting baluns using coax the 
size of RG8 and RG8X that minimizes the stray capacitance and results in 
multi-turn chokes that have relatively high choking impedance to 30 MHz. 
Details are in the tutorial. 

For example, four turns of RG8X through a stack of 5 #31 cores results in a 
resonance around 19 MHz, and parallel equivalent R, L, and C of 4,400 ohms, 
56uH, and 1.3 pF. Note that this capacitance INCLUDES 0.4 pF from my test 
fixture, so the actual C is less than 1 pF and the actual resonance is above 
20 MHz. These chokes are VERY lossy -- the circuit Q of this one is 0.67, 
which is typical -- so the resonance is quite broad. 

The measurement method and setup are documented in the tutorial, and there's 
a lot of measured data.  

73,

Jim Brown K9YC