Hello,
I've been reading through the first few sections of this website on transmission lines and just wanted to clarify something.

I'm doing an assignment where I'm hooking up an infinite length of RG-58/U coax cable to the output of a logic gate. basically i just want to work out the current that flows into the cable so I'm thinking i'll need to know the voltage at the end of the infinite length of cable (nothing is hooked up to it at the end).
So is the voltage at the end 0 V, because there is inductance in the transmission line causing voltage drops... ?
I'm a little confused as I was reading some info that made it sound like the voltage at the end of the line would be the same as voltage at the beginning of the transmission line. I wouldn't think this would be very useful though, as then the current through the line would be nearly 0.

Thanks.

 

Hi,

RG-58 has an inherent capacitance of 22 pF/foot. An infinite length means you are charging an infinitely large capacitor. Added with the infinate amount of inductance, it means you have to wait a very long time before any amount of current into the one end produces a voltage at the other (even at the speed of light, it will take an infinate length of time for any electrical effect to traverse the distance).

 

Originally posted by beenthere@Oct 2 2004, 10:48 AM
it means you have to wait a very long time before any amount of current into the one end produces a voltage at the other (even at the speed of light, it will take an infinate length of time for any electrical effect to traverse the distance).

[post=2752]Quoted post[/post]​

Cool, that makes sense.
So, if the signal never reaches the other end (or takes an infinite time to do so), then what current is sourced by the coax cable if there is, say, 4V applied at its starting end? I was thinking it would just be 4V/50ohm = 80mA...

Thanks

 

Hi,

Not quite so simple. The 50 ohms is a reactive impedance for the coax. At DC, all you have is the resistance of the inner conductor and an increasing back EMF from the inductance of the field surrounding the inner conductor as the current keeps flowing. Initial current might be very high, but has to taper off. Not only does building the magnetic field take energy, but the current has to flow along a conductor with an inherent resistance. Eventually, the overall resistance of the inner conductor will become infinite and everything stops. Even though the electric field can propagate at the speed of light, it still takes an infinite time for it to traverse the infinite length of RG-58

At least you don't have to factor in charge time for the conductor-to-shield capacitance.