CABL Forum - Re: Standing waves, IMP. Mismatches, CPD, CMI?

Cpd is easy to spot because you will see a video spike every six mhz...Standing waves have no pattern to them. If you move your marker to the audio spike and turn your modulation on you can hear the broadcast.

> A Question here for you reverse path geniuses. Can't standing waves also appear from Common Path Distortion, or Common Mode Interference? How can you tell the difference, or what will CPD look like in your reverse spectrum? I know CPD will manifest from corrosion at dissimiliar metals junction, but not too sure what CPD looks like on a spec-an or sweep trace. ANY HELP?
> Thanks
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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 waves, IMP. Mismatches, CPD, CMI?