# Physical cause of reflected waves on terminated electrical transmission line?

 P: 1 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?
 P: 4,663 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.
 P: 6 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.