Scott -- Due to skin effect, the current carrying ability of round
conductors at HF is c. 20% of that at DC.
On Mar 10, 2006, at 12:15 PM, Scott Townley wrote:
> A rambling set of observations on Pi-L tank design and realization:
> Proposed amp is for 17m/12m, single 3-500Z, 650W output. It fills the
> gaps...
> First come up with design values for the Pi-L from TAP. Then I built
> a spreadsheet to design the coils using the Nagaoka formulation rather
> than the Wheeler approximation (in every ARRL handbook since Wheeler
> published it, no doubt).
> In playing with the proposed inductor diameter I started wondering
> what the lower limit was. With a loaded Q of around 12 then I would
> need the unloaded Qo to be ~200 to keep the tank losses below 0.5dB
> (70W). I could accomplish that with 1/4" tubing or with some existing
> 2" B&W coilstock, but it’s only 14AWG. Both will easily give me
> Qo>200. So what’s the difference? I’d rather use the coilstock (less
> work for me).
> Some number-crunching gave inductor losses of 34W and 5.6W (L1 and L2,
> respectively) for a Qo of 300 (perhaps optimistic). To get an idea of
> the temperature rise, I went through a simple DC analogy for the 12m
> coil:
> Total wire in 12m coil (L1)=71" (of 1/4" tubing)
> DC resistance of the equivalent wire (2AWG)=973micro-ohms
> If it dissipates 34W, then the analogous DC current is
> sqrt(Pdiss/DCR)=190A!!
> A quick look at a wire manufacturers chart (Alpha and Belden both have
> nice hookup wire charts) implies the wire temperature reaching
> 100degC. This inference made from the published current limit for a
> 2AWG wire with a rated 100C insulation being 190A or thereabouts.
> Might be more since tubing has less thermal mass than solid wire.
> A similar run for L2, but using 18AWG for the much smaller inductor
> required gave an equivalent DC current of 21A and a temperature of
> 125C.
> This should be the worst case temperature rise, since at lower
> frequencies, even though Q decreases approximately as the sqrt(f), the
> total wire required for the same Xl is roughly ~1/f so equivalent
> currents will decrease roughly 1/sqrt(f).
> Tank inductors get hot! I can see why Rich mentions forced-air
> cooling of tank components from time to time. 40W doesn’t sound like
> much (we don’t force-air cool our house lights, right?). I’m guessing
> that the ultimate limit is probably around 180C (solder melting
> point)…any comments? Probably less than that given that metal
> resistance has a positive temperature coefficient.
> I always thought the decreasing inductor wire size with decreasing
> frequency (e.g., on a B&W850 and many others) was more for compactness
> when realizing the higher inductances involved…is it as much as
> thermal consideration?
> Comments? Flames? Further observations? (already hot enough here as
> it is…)
>
>
> Scott Townley NX7U
> Gilbert, AZ DM43di
>
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>
Rich Measures, 805.386.3734, AG6K, www.somis.org
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