I am quite new in this field and I started recently by reading the first books in electrical engineering where I learned the concept of current, voltage, impedance, reactance (inductive and capacitive).

I was reading yesterday about coaxial cables where the conductive elements are the core and the shield. I though that the the core and the shield are the "extension" of plus and minus poles of the power source (that in the ac source alternate).

The reason I am asking is that the shield is typically connected to the ground ("The cable is protected by an outer insulating jacket. Normally, the shield is kept at ground potential and a signal carrying voltage is applied to the center conductor."). So somehow I am a bit confused on if I understand precisely how one can think a coaxial cable being a part of a simple RLC circuit schematic.

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    \$\begingroup\$ I am a bit confused on what you are actually asking... \$\endgroup\$ – PlasmaHH Sep 12 '17 at 11:21
  • \$\begingroup\$ Can you help me perhaps understand if I have an AC power source a coaxial cable and a lamp how I will connect the AC power to the core and the shield. Perhaps this might help me apprehend better the concept. Alex \$\endgroup\$ – Alex P Sep 12 '17 at 11:29
  • \$\begingroup\$ I think he's asking how coax can be used for antennas, which transmit from AC. And the ground bit seems to be confusion about whether it's okay to let the sleeve become '+'. \$\endgroup\$ – user2497 Sep 12 '17 at 11:29
  • \$\begingroup\$ @AlexP Don't use coax for lamps. The shield is there to filter out interference, coax isn't for anything except radio/audio signals, in- or outgoing. Nice coax can handle over 100W, but that's not the point. \$\endgroup\$ – user2497 Sep 12 '17 at 11:30
  • \$\begingroup\$ I do not use coax for lamps. Lets assume that I indeed have an antenna connected to some AC source. How one can think of connecting core and shield to the two "pins" of the AC source? Regards Alex \$\endgroup\$ – Alex P Sep 12 '17 at 13:59

first you have to understand that R, C and L are electric parameters that can be built up from special structures and materials. For example, a capacitor is a couple of paralel plates isolated with a dielectric, and its capacitance is determined from plate's area (A) and separation (d), kind of isolation (E), so C=EA/d, in general from material and geometry. In this idea one can divide any transmission medium like a coax into several segments, a very short segment whose length tends towards zero, if you look carefully to one segment you will find out that its structure in esence is quite similar to that of a capacitor, plates no planed but curved, and dielectric, so that short segment should have capacitance too. Similarly, one can say the same for the other electric parameters: resistance and inductance, and deduce that what has in a cable is really a AC circuit with RLC components.

Well, the RLC parameters in a cable are significant only if the transmitted signal through it has a wavelength comparable to the cable length, otherwise one can just look down on them, and this occurs generally at low frequencies in communication applications. So when you are working a low frequencies and need RLC parameter you have to work with resistor, inductor, and capacitor components (lumped elements) but at high frequencies you just need to consider only the RLC component in the line itself (distributed elements), and some times you can use a hybrid approach: lumped and distributed elements in order to built a AC circuit with RLC.

Hope you can understand the concept.

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  • \$\begingroup\$ thanks for the reply. Unfortunately my upvotes do not count still. Thanks for the reply. How then the core and the shield relate to the two polarities of an power source? Is it too wrong to thing of the shield and the core as two simple cables transfering energy.? \$\endgroup\$ – Alex P Sep 13 '17 at 6:16
  • \$\begingroup\$ Not at all, you can think of shield and core in that way, nevertheless coax cable are designed to transfer low power signals not power energy, though there is a cathegory of coax, outdoor coax, that can transfer both low and high power signal. In such a way you can relate core to line and shield to ground, but trying to make the experiment can damage the coax cable due to its power limits \$\endgroup\$ – archiesette Sep 14 '17 at 13:23

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