On 10/10/2010 9:29 AM, Grant Saviers wrote:
> Why use 9913 to make chokes when smaller diameter PTFE cables are
> available? At 30Mhz, RG's 142 (3Kw), 223, 304(6Kw), all handle max
> legal power plus with some VSWR headroom, and 316(1Kw @ 30Mhz, 2.26Kw @
> 7Mhz) for lower frequencies and power levels. (RG213 is 2Kw at 30Mhz).
> It seems to me that the smaller braid OD has less capacitance per turn,
> assuming they are spread out around the core. Or at the same self
> resonant frequency get more choke Z with a few more turns - N squared
> sure helps. Even at a couple of bucks per foot, only need a few feet
> are needed. You might have one more coax to coax connection up in the
> air and lose another 0.05db , which is a little downside.
You're correct about the relationship between conductor diameter and
stray C. That's why you'll find two sets of measured data for chokes in
my tutorial -- one for RG8-size cable and another for RG59-size cable.
BUT -- for all practical purposes, especially at 50 MHz, the only
advantage of using smaller coax would be that you could use smaller i.d.
cores. But smaller i.d. cores also tend to be shorter, and both L and R
are proportional to core LENGTH, so you would need more chokes in series
to get the desired choking Z. Hence my advice to simply wind turns of
9913 around bigger cores. :) It's a simpler solution, no additional
connectors to get wet.
A LOT of serious thought and measurement went into my having settled on
Fair-Rite's #31 material, the 2.4-inch toroids, and the "biggest
clamp-on" as being most useful for ham use. I also find clamp-ons of 0.4
- 0.5 in i.d. to be quite useful for suppressing RFI radiated on smaller
cables by noisy equipment. Above 5 MHz, Fair-Rite #43 is just as good,
and it's cheaper. But if you're buying only a few part numbers, you can
buy in quantity and take advantage of quantity discounts, which makes
them even cheaper.
73, Jim K9YC
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