On 1/11/2014 5:24 AM, Jeff wrote:


As for handling more power - rubbish. The current in 75 ohm coax is
lower than that in 50 ohm coax, for the same power rating.


Can you not see the stupidity if that remark???

Jeff


It is perfectly true. P=I^2xR. As R increases, I MUST decrease to
handle the same power. And since current is the limiting factor in
wire, you don't need as large a gauge of wire to handle more power.

You really should learn what you're talking about before opening your
"mouth". You only continue to show your ignorance.

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On 1/11/2014 11:33 AM, Jeff wrote:
On 11/01/2014 14:12, Jerry Stuckle wrote:
On 1/11/2014 5:24 AM, Jeff wrote:


As for handling more power - rubbish. The current in 75 ohm coax is
lower than that in 50 ohm coax, for the same power rating.


Can you not see the stupidity if that remark???

Jeff


It is perfectly true. P=I^2xR. As R increases, I MUST decrease to
handle the same power. And since current is the limiting factor in
wire, you don't need as large a gauge of wire to handle more power.

You really should learn what you're talking about before opening your
"mouth". You only continue to show your ignorance.


Perhaps you should actually look at the theory of coax transmission
lines. If you did you would find that the optimum impedance for max
power handling peaks at about 30 ohms and falls away either side of that
impedance, 50 ohms being better than 75 ohms. It is a little more
complicated than just Ohms Law.

It would appear that it is you who are showing their ignorance, and
inability to even look at the article that was linked in post that you
were replying to!!!
http://www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm


Jeff

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.

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On 1/12/2014 5:51 AM, Jeff wrote:
Jeff

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.


Well let me think; who am I doing to believe; text books on transmission
line theory, data sheets for coax cables, and an article by well
respected cable manufacturer), or your personal (and incorrect) theory??

Perhaps you should also point out to Belden that their article is wrong
as well!! If you think that sending power over a matched cable is a
specific requirement then you obviously do not understand the article.

Jeff

I go by my textbooks and professors. I don't believe someone who read
an article about a specific installation and tried to apply that to the
entire world.

And I didn't say their article was wrong. I said YOUR APPLICATION of
what the article said is wrong.

Two entirely different things.

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On 1/12/2014 12:00 PM, Jeff wrote:


I go by my textbooks and professors. I don't believe someone who read
an article about a specific installation and tried to apply that to the
entire world.

And I didn't say their article was wrong. I said YOUR APPLICATION of
what the article said is wrong.

Two entirely different things.


The point that are missing is that the graphs in that article are
independent of application. They represent the true picture for
correctly terminated coax. You also seem not to understand how those
graphs are derived.

Jeff

OK, I give up. From reading one chart, you know more than all of the
physicists, professors and engineers in the world. And you can write
the book that contradicts every physics textbook out there.

You're THE MAN! I bow to your expertise.

NOT!


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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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