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No.

The LNB (or multiswitch and LNBs) is powered as he says from the receiver through the coax. That same voltage also serves to tell the LNB or multiswitch whether to send down on that one cable the right hand polarized or the left hand polarized signals by changing back and forth from 13 to 18 volts as you go through the channels. (The RH and LH signals are duplicate frequencies so they cannot be on the one cable at the same time.) The LNB doesn't care whether it gets 13 or 18 volts for power.

That said, this is one instance where power is carried by the coax in addition to the RF signal. Its understandable then that you would want to keep the DC voltage drop on that cable within acceptable limits. That is not something you would be concerned about with coax running to an antenna or a CATV drop and wiring in a building because it doesn't handle any power.

Most coax has a center conductor that is made of steel that has a thin layer of copper plated on the outside (called "copperweld"). The steel is used for strength and the copper provides conductivity at RF frequencies because of the "skin effect". The skin effect says that the higher the frequency the more to the outside of a conductor the signal will travel. Therefore at CATV and higher RF frequencies the signal travels entirely on the outside of the center conductor through the copper plating. So using a solid copper center conductor for a cable that only handles RF is also a waste of money.

What happens when you use a cable with a steel copperweld center conductor to handle power also? As said, when it has to handle any current you need to be concerned with making sure that what ever is on the other end gets ample voltage to operate. If you look at the DC resistance specs for a copperweld cable vs a solid copper center conductor, the solid copper obviously will be lower due to the fact that copper is a better conductor than steel.

What does this mean? Really not much unless you are installing long runs where voltage drop becomes a factor. Then you would want to use a solid copper center conductor.

As far as a copperweld center conductor causing the cable to short, think about it. The center conductor would have to somehow contact the shield through the dielectric that separates the two. Usually this happens because of a staple shot through the cable. The cable can work for awhile then fail. But lets say there were no staples used. Could the center conductor become hot enough because of the current to possibly melt through the dielectric? Nope. Even with a dead short at the LNB, the receiver LNB power is current limited to probably no more than 250ma. That would hardly cause even an 18 ga steel wire to get warm.

What would cause the cable to short? An improperly grounded dish and lightning is the first thing that comes to mind. Wouldn't even have to be a close strike. Then comes crummy cable itself, as well as water migration within the cable.

If you really want to know remove the shorted cable and start examining it. Look for the obvious stuff first like pinholes, staples, etc. Then start cutting it into sections and using your ohmmeter until you find the shorted section. Then cut that apart to see what is causing it.

-Hal

Well, I've learned more about Coax cable in the last minute right here than in the last 5 years :bow: :bow: to Hal. That is why I love this board!!

I have seen a lot of this. Using coax ends that aren't sealed allows water into the coax and it sits there at the low spot. I have actually seen a center conductor totally corroded off due to water.

I have also removed coax and held it differently (upside down) and had water pour out like a garden hose.

Hal - thanks for the very informative reply. I know that there were no staples involved in this installation. These 2 cables were the last to be run and were tie wraped to other existing cable runs. When I'm in the area I'll look at/for grounding issues. Thanks again. This is such a great place for information of all types.

Steve

Here I go!

Steve,

Hal is RIGHT ON with his information here. I would like to add a few things to his comments.

I have worked with coax installations since the early 80's when 23 channels on CATV or 300mhz was about it. A lot has changed since then. But is seems some "Sparkies" still resort to substandard specifications.

The weakest link in a coax installation is its connectors/connections. With frequencies reaching the 1-Gig mark on Cable TV, and 2-Gig for Satellite, your connectors must be top-notch and terminated correctly.
This is why they make proper "Stripping" tools for all sizes of coax. If "Sparky" is still using a razor knife or even his pocketknife, he is probably "Scoring" the skin of the center conductor causing the signal to have to jump over the scratch. This can cause major signal problems. Also, use a good quality connector, and always tighten all outdoor connectors with a wrench.

Hal is also correct about a short causing the center conductor to melt. WHAT??? :rofl: There is no way. We install Commercial Direct TV here in Jacksonville, FL. and I have never seen or heard that one. Lightening, Yes could be, but we have dishes on the roofs up to 36 stories high, and never have service calls for lightning hits. Most of our grounds are done in the communications closet where our multi-switches are located, using ground blocks.

From your post, I am assuming there is only 1 TV off of this dish? You are testing the cable from the basement to the receiver? Did you disconnect the receiver before you tested? Could it be possible someone has come in behind you and thought they could spit off your cable to another TV? A splitter can cause or act like a short when doing a continuity test. Have you tested the voltage output of the receiver to make sure it’s not faulty? Are you using any Diplexers, and if so, are they connected properly?