How does a coaxial transmission line work?

[cragar]

On a coaxial transmission line is there AC current running through the center wire? And this AC current produces EM radiation that bounces down the wave guide. So is the purpose of this transmission line to send the signal via radio waves. And is this how cable tv works, are radio waves being sent to my tv and picked up?

 

[davenn]

trying to keep it simple :) ...

the RF current is constrained to the centre conductor, more so to the outer skin of that conductor. RF currents have a skin effect traveling down a conductor that DC currents dont. ie ... DC current will flow through the complete conductor where RF currents flow on the outer surface, only penetrating into the conductor minimally. the higher the freq, the greater that effect (shallower the depth).
tho there is some EM field around the centre conductor, the conductor is carrying the principal current, not really acting as a waveguide. There is no bouncing down a waveguide, when referring to a coax cable. In fact that applies for the waveguide as well :)
Yes, the EM fields are kept contained inside the tube, but the RF currents are flowing along the metal walls

In a waveguide, there is no centre conductor...
A wave guide is a combination of a twin lead transmission line and 1/4 wave metallic insulators. In a rectangular waveguide the RF currents down the centre of the 2 long sides, commonly called the H plane.

here's my drawing taken from one of my amateur radio books some time ago
I admit upfront, I'm not an artist ;)

https://www.physicsforums.com/attachment.php?attachmentid=36035&stc=1&d=1306749618

so a waveguide is basically 2 parallel conductors held separated by an infinate number of 1/4 wave stubs ( that is.. a solid metal wall)

hope that helps a bit :)

Dave

 

[cragar]

ya that helps, thanks for your answer