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I have got 2 questions on transmission line. In coax line, the centre wire is Copper, and the outer line is Aluminium, as far as I can see in the coax that I have. I have got couple of questions on that.

Question 1:

enter image description here

Here this model assumes uniform resistance, conductance, capacitance ... per unit length of both wires, and then went on to derive the equations and intrinsic impedance. But this is not the case for coax line, as centre wire (Copper), and the returning wire (Aluminium) both have different resistance, capacitance ... per unit length. Say top wire and bottom wire in the diagram are Copper and Aluminium respectively.So how come we are allowed to use $$Z_0 = \frac{R + j\omega L}{\gamma}$$. Where R can either be the per unit resistance of Copper wire OR Aluminium wire, same goes for the value of inductance. So what I am reading wrong here?

The second question is on the application side of this question.

Question 2:

enter image description here

Say I am feeding the signal through centre wire (lets assume antenna, and coax are perfectly matched). Braid is at ground. Now as antenna is a transmission line, the upper antenna gets the entire signal, but the bottom antenna is connected to ground. In this particular case the upper half radiates, but the bottom half of the antenna does not radiate as it is at ground. But this a topology which I have seen in some of the places. This does not make sense to me, as the antenna is intended to be a half wavelength dipole antenna, and same V/I characteristic should produced in both the parts of the antenna. Instead this design puts the bottom half at flat Voltage. I must be interpreting something wrong here. Even if we add a BALUN at the connecting point to the antenna, the signal proceeds to the top half of the antenna through Copper, and the signal to the bottom half through Aluminium. This does not make sense to me as both of them have different electrical characteristic.

Your guidance will be greatly appreciated.

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  • $\begingroup$ upper half of question is duplicate of your question on EE.SE, which already got answers there? $\endgroup$
    – Marcus Müller
    Commented Feb 17, 2017 at 10:29
  • $\begingroup$ If you have two questions, it's usually better to ask two separate questions on the site so you can get detailed answers to each. $\endgroup$
    – Phil Frost - W8II
    Commented Feb 17, 2017 at 12:12

1 Answer 1

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The resistance in that lumped element model does not represent the physical resistance of the center conductor. Rather, it represents all losses of the coax. That is, resistive losses in the center conductor, plus resistive losses in the shield, plus leakage, plus dielectric losses (which will dominate at high frequency).

Your second question looks like a duplicate of your previous question: Dipole Antenna Current Distribution at any Time Instant

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