CABL Forum - Re: standing wave -GOOD QUESTION

The 492 in this formula is a constant representing 1/2 speed of light, the pulse travels at speed of light to the fault and back so 1/2 that is the distance to the fault itself. using this X the Vp [varies by manufacturer] is the correction factor as signals pass thru a span of cable less than the speed of light. I will go to my reference and post a better explainition of how this number is derrived.

> Good question. I believe the NVP of all coax is the same as long as it's 75 ohm. Believe the only difference is going to be your attenuation per foot, but I am willing to be corrected. The 492 I believe is also constant.
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> > Does this very on cable size? That part should be stated also.
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> > > 492 x (times) the velocity of propagation (.87) / 4
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> > > > What did the 492* repersent?
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> > > > > Try using this formula, this will give you a distance in feet to the standing wave 492 * Vp/ frequency. the Vp is most likley .87% sweep your plant measure the distance between standing waves in MHz. An example would be waves are 4 MHz apart then 492 * .87 / 4 = 107' to the fault, try this and let me know if it halps you.
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> > > > > > Standing waves are impedance mismatches usually caused by damaged cable. A cable gets it's impedance from the ratio between the center conductor and the sheath. 75 ohms is used for cable tv, if the cable is damaged it changes the ratio between the sheath and the center conductor and that will change the ohms from 75 to ....whatever. The best way to find the problem is to sweep it....I know that everyone dosen't have a sweep meter if you have a meter that will scan the entire spectrum that is almost as good as a sweep meter...go back to a point where you have a flat response then find the first place where you have standing waves....The problem is between thoes two points...Also Taps, Actives and just about anything can cause standing waves.......most of the time I have found it to be cable.
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> > > > > > > I could really use some pointers in troubleshooting standing wave on an active system (large cascades 450 system, many thanks in advance

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Re: standing wave -GOOD QUESTION