coax core and braiding

[plugwash]

Coax is about screening

That is one feature of coax but any old screened cable will give you that.

the braid is at 0 volts (earth) and the centre carries the signal.

true at least as far as high frequencies are concerned

attenuation is caused by the fact that the cable is a capacitor

no it is not, provided the signal is correctly matched to the cable the capacitance and inductance do not in themselves cause loss, they only cause delay.

loss comes from the fact that neither the conductors nor the dialectric are perfect.

 

[OLD]

plugwash & Space cat this will explain more fully.
http://www.geocities.com/siliconvalley/2072/elecrri.htm
There is a clue in my profile

 

[plugwash]

that page has absoloutely nothing about transmission line effects.

that page only considers series combinations of resistance, capacitance and inductance and doesn't mention anything about what happens on the middle terminals of the combination, it is totally irrelvent to the discussion at hand.

 

[Space cat]

that page has absoloutely nothing about transmission line effects

Moreover, it's wrong - or at least misleading. Somebody reading that web page who didn't know any better would ADD the inductive and capacitive reactances together then square the result. You can't analyse reactive circuits properly without using the "j" operator.

You can't use the information to find the impedance of a transmission line either. It's not SQRT(Xc^2 + R^2). There is no R in a perfect transmission line. The correct formula for the perfect line is Z = SQRT(L/C) where L and C are the inductance and capacitance per unit length. The resulting Z is a pure resistance.

 

[bernardgreen]

There is no R in a perfect transmission line.

And there is never a perfect transmission line.

 

[OLD]

Looks like i completely missed the point of the original questions this being a diy site i answered in a practical way without going into ac theory , impedance triangles and also maximum power transfer theory as it has no application .Cable has to work in a required frequencys range and has a loss per unit length for that application .

 

[plugwash]

Cable has to work in a required frequencys range and has a loss per unit length for that application

indeed and if you had just said that I would have had no problem

What I do have a problem with is the posting of information or links to information that is highly misleading or even plain wrong. When I see such information posted I will explain why it is wrong even if the information isn't really relavent to the original poster.

 

[johnnyjb]

This is heating up people are throwing the kitchen sink into this one.Im sorry if i asked a question that brought up so much debate

 

[Space cat]

Im sorry if i asked a question that brought up so much debate

There's no need to apologize. There's no slanging match here, just peer review in action. In years to come somebody might wander into this thread by accident and learn something.

Then they might go on to discover a new electromagnetic phenomenon that'll make warp drive possible and then some alien ship might come back through time looking for the idea that sparked the whole thing off so that they can eliminate it ---

But you're still here and so's this thread so it looks like it'll never have happened.