[time-nuts] Why a 10MHz sinewave output
cfharris at erols.com
Wed Feb 8 02:49:20 UTC 2012
I think the real relation is 50 ohm coax is just
75 ohm air line with polyethylene dielectric added.
Rick Karlquist wrote:
> Azelio Boriani wrote:
>> Try this for a history about the 50 OHM impedance:
> The reference is full of errors. The lowest loss in coax
> occurs when the ratio of the diameters is 3.6 to 1, regardless
> of dielectric. For air dielectric, this works out to 76.7 ohms.
> For polyethylene, it works out to just over 50 ohms if you assume
> a dielectric constant of 2.3. It is also worth noting that the
> difference in loss between 3.6 to 1 and 2.3 to 1 is very slight
> and not worth worrying about in most practical cases. What
> is probably more important is that higher impedance cable
> uses much less copper in the center conductor for the same loss.
> The story about 2 inch and 3/4 inch pipe might be true.
> The thinnest copper tubing, type M, has an ID for trade size
> 2 inch of 2.009 inches and and OD for trade size 3/4 inch of
> 0.875 inch, which is very close to 2.3 to 1. Of course there
> is also type K and type L and lots of other sizes, so this
> story may be a case of "data mining" (IE you can always find
> some combination of pipes to support any ratio you claim).
> Rick N6RK
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