# Coaxial cable and reflected waves

Discussion in 'Homework Help' started by Niles, Jan 8, 2009.

1. ### Niles Thread Starter Active Member

Nov 23, 2008
56
0
Hi all.

If I send an electromagnetic wave down a coaxial cable which is short-circuited, I have found that the reflected complex amplitude is related to te complex incoming amplitude by:

$
V_{\text{reflected}} = -V_{\text{incoming}}.
$

Should I interpret this as all the wave is reflected, but it is not upside down, i.e. the phase has changed by 180 degrees?

Best regards,
Niles.

2. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,047
295
Hi Niles:

I hope you get a chance, if you haven't already, to study the Smith Chart. Transmission lines become abundantly clear when you learn how to use this powerful tool.

When looking at the the power without regard to phase, the power absorbed in the termination is simply the forward power minus the reflected power.

When looking at VOLTAGE, however, this can be either in phase or out of phase. For example, if I have a Time Domain Reflectometry setup (TDM) and send a short pulse down a long (lossless) transmissioin liine, several things can happen. If the transmission line is perfectly terminated, that is the termination resistance is perfectly matched to the characteristic impedance of the transmission line, there will be no reflection . If the termination resistance is HIGHER than the transmission linie impedance, the reflected pulse will be in phase. If the termination resistance is LOWER than the line impedance the reflected pulse will be OUT of phase.

Now, if the forward and reflected pulses happen to overlap, whether they add or subtract depends entirely on the timing. It also results in the creation of standing waves.

Transmission line theory is a big subject, and really needs to be studied comprehensively...not in bits and pieces. The ARRL Handbook has a fabuouls chapter on transmission line theory, and a great tutorial on the Smith Chart.

Hope this helps some,

eric

3. ### Niles Thread Starter Active Member

Nov 23, 2008
56
0
Hi Eric

I haven't heard about the Smith Chart before, but I'll google it. Perhaps it really will help.

So from what you have told me, I am correct to say that the reflected wave (when the cable is short circuited) is totally out of phase with the incoming wave, i.e. the reflection coefficient is -1.

Last edited: Jan 8, 2009