On 4/16/2012 4:11 AM, Jim Thomson wrote:
> 22 x type 43 beads, of the 1.125 inch long x 1 inch diam x .515 inch ID
> variety, slid over RG-213 or RG-393 coax will also work.
Jim, you are looking at the elephant through a pin hole. You build and
run big power amps, and your measure of the effectiveness of a choke is
that there it doesn't blow up. A string of beads on coax that are not
lossy at the operating frequency CANNOT overheat -- there's not enough
resistive component to their impedance to dissipate power -- but they
are also essentially useless for all of the reasons outlined in my RFI
tutorial.
The primary function of a common mode choke in an antenna that is used
for receiving is to MINIMIZE RECEIVE NOISE, and to do that effectively,
the choke must have a high resistive common mode impedance at the
frequency(ies) of interest. Ferrite chokes operate as parallel resonant
circuits. A single turn through cores made of #31 and #43 ferrite
materials is resonant around 150 MHz, so it is USELESS on the HF bands.
To be useful on the HF bands, we must wind multiple turns through the
material to increase both the inductance and the stray capacitance so
that the resonance moves down to the frequencies where we want to use
them. Until you understand that fundamental concept, you are going to
continue to make incorrect assessments of the value of various
solutions. I refer you to my RFI tutorial, which addresses the
technical issues in detail, including an analysis of dissipation at high
power levels that doesn't "hide behind the math."
http://audiosystemsgroup.com/RFI-Ham.pdf
73, Jim Brown K9YC
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