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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|