# Reflected Waves

Discussion in 'General Electronics Chat' started by junyan, Jun 16, 2008.

1. ### junyan Thread Starter New Member

May 13, 2008
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How do you compute the time required for a reflected wave to travel FROM the load? Do you take the square root of LC or do you take LC times 2?

2. ### Wendy Moderator

Mar 24, 2008
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2,540
I think a little more information and background is in order here. If it is a signal in a wire it is somewhere around .98C (speed of light). What kind of signal are you talking about, and why is it reflected? A properly matched load on an amplifier doesn't reflect much back, 60 db is about 1 million times less than the original signal strength.

Apr 20, 2004
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4. ### junyan Thread Starter New Member

May 13, 2008
7
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I apologize for not explaining myself fully. I am trying to compute the time it takes a voltage wave to reflect from the load back to the source. I cannot remember if you take the square root of L (inductive) C (capacitance) or if it is simply 2 times LC. For some reason I remember the TIME as being 2 times the LC.
Thank you for taking the time out of your busy day to help me.

5. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
The time for a reflected signal to bounce is the same reguardless of what the reactance is. Reactance affects loading, the amount of signal absorbed into the load, by making the antenna (if that's what we're talking about) resonant.

6. ### silencer New Member

Jun 3, 2008
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it depends on the length of the line and the phase velocity of the wave. by L and C, I assume you are referring to the per unit length inductance and capacitance of your transmission line. the phase velocity would then be (assuming a TEM wave): 1/sqrt(LC)

7. ### roddefig Active Member

Apr 29, 2008
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Seconded. However, if you want to get the time then wouldn't you need to multiply that by 2 * the length of the line to get the round-trip time?

8. ### theamber Active Member

Jun 13, 2008
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Are you are talking about the standing wave returning to the load?. When you refers to L and C are these reactances?, reactances are imaginary so you need Impeadance to do any meaningful calculation.
The speed inside a transmission line is close to the speed of light. To calculate the exact speed you need Integral calculus. You can also calculate group delay or phase distorsion by differentiating the insertion phase response of the DUT versus frequency. Usually engineers are more concerned with measuring the ratio of loss of signal to the load SWR, return loss, Scattering parameters, impeadance and admittance and things of that nature in order to transmit the maximun posible without losses.

9. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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It is the square root. Here is some good reading: http://literature.agilent.com/litweb/pdf/5988-6505EN.pdf

10. ### pebe AAC Fanatic!

Oct 11, 2004
628
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The speed of a signal travelling through a cable equals the Speed-of-light * Velocity Factor of the cable.

The Velocity factor depends on the dielectric used and is 1.0 for air spaced cable going down to 0.6 for solid polythene. Typically, semi air-spaced or foamed dielecetric (the two most common ones) have a VR of about 0.8 to 0.85

11. ### theamber Active Member

Jun 13, 2008
318
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And the velocity factor of the cable is calculated:
Vp = 1 / SQRT (dielectric constant)