Physical cause of reflected waves on terminated electrical transmission line?

[tim1861]

I understand that reflected waves on a transmission line are a result of the constraints of Ohm's Law and Kirchhoff's Law at the transition points.

However, I'm having trouble visualizing the physical cause for this. What is actually causing the electrons to flow in the opposite direction? Is it that when they encounter the terminating impedance some are "bounced" backwards?

 

[Bob S]

The forward wave has a particular relation between the voltage and current in a transmission line (e.g., coaxial cable). For 50-ohm cable like RG-8, the voltage (+volts) is 50 times the current (+amps). The forward wave travels at about 0.6 to over 0.95 times the speed of light. If the termination is a short, it can absorb no power, so all of the power has to be reflected. The reflected power flows back through the incident power (wave) without interacting with the forward wave. For a short, the reflected wave has to have the opposite sign voltage, and for an open end (which also can absorb no power), the reflected wave has to have the opposite sign current. In general, a termination of R ohms can only absorb power that is in the correct ratio of R=volts/amps. All the mismatch power has to be reflected. The reflected waves also have the same relation between volts and amps, except for the sign of either the voltage or current.

The impedance of the transmission line is sqrt(L/C), and the signal velocity is sqrt(1/LC), where L and C are the inductance and capacitance per unit length.

 

[Tisfurs]

Contact separation, between the male and female center pins, is a contributing factor to the mismatch between a source and load. For a good visualization go to www.phy.davidson.edu/instrumentation/neets.htm and check out the modules on wave propogation, and microwave principles.

 

[f95toli]

I understand that reflected waves on a transmission line are a result of the constraints of Ohm's Law and Kirchhoff's Law at the transition points.

However, I'm having trouble visualizing the physical cause for this. What is actually causing the electrons to flow in the opposite direction? Is it that when they encounter the terminating impedance some are "bounced" backwards?

It is not quite correct to say that the electrons are "traveling backwards", I suspect the confusion here stems from the fact that we often model transmission lines as if they were standard circuits (with distributed impedances) with voltages and current; but in order to understand the physics you really have to use the full EM wave description, i.e. you need to consider the electric and magnetic fields.

Hence, what is reflected is an electromagnetic wave, not electrons (the electrons will of course be affected by the field distribution but that is a secondary effect).

 

[Bob S]

If you ever take a physics course on Electricity and Magnetism, you will learn that the energy flowing in a transmission line, like RG-8, is flowing in the electric and magnetic fields between the conductors. The currents in the conductors are just supporting this energy flow, like the walls in a waveguide. The electric field lines in RG-8 are radial, while the magnetic fields are azimuthal.

 

[Bob S]

Here is a visualization of how the mismatch causes a reflection. Suppose we had a forward signal in a shorted RG-8 coax (Z = 50 ohms) of V = +50 volts and I = +1 amp. Because the cable is shorted, the reflected signal voltage has to be V_R= -50 volts at the point of reflection. because Z =50 ohms, the reflected current has to be I_R= +1 amp. So the current is doubled at the reflection. The sign of V_R/I_R = -50 ohms indicates it is a reflected signal.