On Mon, 07 Jan 2008 16:00:33 GMT
"John KD5YI" wrote:

"Roy Lewallen" wrote in message
...
I've made a program to help visualize some transmission line phenomena.

(snip program description)

Thanks, Roy. It is very interesting.

I'm having a hard time getting my head around Demo 5. The waves go away
periodically (every 8 wavelengths?), sort of like a squegging oscillator.
(By the way, it appears that the line is two wavelengths long rather than
three.) This was totally unexpected. I will think more about this and try to
picture the reason.

Nice tool.

John



Demo 5 is a demonstration of the pitfalls of dealing with discontinuities. The demonstration portrays equal periods of sequential open circuit conditions and short circuit conditions.

The discontinuity here is that a reflection factor of 1 is used if the returning voltage is less than the initial voltage, but a reflection factor of -1 is used for any returning voltage exceeding the initial voltage.

Demo 5 is a demonstration of why a Thevenin voltage source is much to be prefered over a simple ideal voltage source for continuous running circuits.

Another way of illustrating the problem is to say that the ideal voltage source overwhelms any voltage from a returning wave. If an external voltage less than the initial voltage is applied to the ideal voltage source, vr = 0, vf = videal. If the external voltage exceeds the initial voltage, then vr = 0, vf = 0 This is a discontinuity, which shows up in Demo 5.

A third way of illustrating the discontinuity problem is to say that you can not apply a voltage to two wires that are short circuited at the input. In Demo 5, the reflected wave returns to find the circuit short circuited. On the other hand, the ideal voltage source applied voltage to an open circuit. We can not have it both ways.

73, Roger, W7?WKB