What are the uses and limitations of transmission lines?

[hasan_researc]

"Transmission lines are specialized cables and other media designed to carry alternating current and electromagnetic waves of high frequency (radio frequency or higher), high enough that its wave nature must be taken into account. Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas, distributing cable television signals, and computer network connections." Quoted from Wikipedia

1. "other media" - such as?

2. "alternating current and electromagnetic waves of high frequency" - I understand that a transmission line will carry an alternating current due to a voltage across the line. But how can such a line carry em waves, and why do the current and wave have to be of high frequency?

3. "high enough that its wave nature must be taken into account" - why is the wave nature a dominant influence on the analysis of the passage of current/wave if its frequency is high?

Thanks in advance for any help!

 

[tiny-tim]

hi hasan_researc!

1. "other media" - such as?

2. "alternating current and electromagnetic waves of high frequency" - I understand that a transmission line will carry an alternating current due to a voltage across the line. But how can such a line carry em waves, and why do the current and wave have to be of high frequency?

3. "high enough that its wave nature must be taken into account" - why is the wave nature a dominant influence on the analysis of the passage of current/wave if its frequency is high?

such as optical fibres, carrying light

i think the wave nature is referring to things like reflection, which we ignore in ordinary Kirchhoff-type circuit analysis

(not sure about the high frequency )

 

[jjmontero9]

Electromagnetic waves can also be transmitted using http://en.wikipedia.org/wiki/Waveguide_(electromagnetism)" , for example. They are not strictly propagated through cables: the RF waves are emitted through the air and can be reflexed and guided with many other elements.

The frequency is important because transmission lines generally have an intrinsec impedance, and also the connected load can be purely resistive or have an inductive/capacitive component. So, frequency is important because it will modify the angle between current/voltage or magnetic/electric field.

 

[litup]

Electromagnetic waves can also be transmitted using http://en.wikipedia.org/wiki/Waveguide_(electromagnetism)" , for example. They are not strictly propagated through cables: the RF waves are emitted through the air and can be reflexed and guided with many other elements.

The frequency is important because transmission lines generally have an intrinsec impedance, and also the connected load can be purely resistive or have an inductive/capacitive component. So, frequency is important because it will modify the angle between current/voltage or magnetic/electric field.

Plus one more effect: Skin conduction. The higher the frequency, the more the conduction is on the outer layer of conductors.

 

[Robot B9]

The skin conduction also implies that the wires be hollow since current would flow only near the surface of the conductors. A nice idea, would save significant weight and material but is difficult to do effectively.