On 1/12/2014 11:14 AM, Jeff wrote:


Try again. I had it back in college in the early 70's (as an EE major),
and I don't think the laws of physics have changed.

And the best impedance for a coax is that which matches the input and
output impedance of the system, or if the input and output are of
different impedances, acts as a matching stub between the two.

And yes, I read the article. But you obviously don't understand it. The
30 ohms they are talking about was for ONE SPECIFIC REQUIREMENT. That
does NOT mean it is true in different situations.

I suggest you learn what you're talking about before you make yourself
look even sillier.

Jerry, let me put you out of your misery.

The peak power capability of coax is not set by the ohmic losses but by
the breakdown voltage, that is where the ~30 ohms for best max power
handling comes from.


Which has absolutely nothing to do with what we were discussing. And
breakdown voltage of the coax is dependent on the dielectric only
(material and thickness). While these will affect impedance, saying 30
ohm impedance is the "best" impedance is not true.

The max *average* power capability is governed by heating of the cable
due to the losses per until length (both resistive and reactive). The
losses being proportional not the the cross sectional area of the
conductors but to *surface* area (due to skin depth considerations).


Which is directly related to the square of the current through the coax.
The lower the impedance of the coax, the higher the current for the
same power.

Now whilst it is true that 75ohm cable will have marginally lower loss
than 50 ohm cable, this is only true when the inner conductor diameter
is kept constant and the 75 ohm cable hase a larger shield diameter. For
cables with the same outside diameter the losses for 75ohm are greater
due to the smaller diameter (and hence lower surface area) of the inner.

So in practical terms for the same size cable 50 ohm has lower loss,
greater average power capability, and greater peak power capability.

If you don't believe me these figures can be easily verified by looking
at the specs of 50 & 75 ohm cable of the same shield diameter.

Jeff

But as I said - you're the man. You are able to interpret this chart
and contradict every physics textbook, engineer and professor in the
world. You are the expert!

NOT!

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Jerry Stuckle

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