Not sure where to post this but this forum has a lot of traffic so...

I have a bunch of new, unused Cat 6 cable that says it's good to 250 MHz. Would it be any good for running a line from the RF output of my dig cable box (i.e. Ch. 3) to the other end of the house for a bedroom TV? We're talking probably 40-50 ft. Or do I need regular coax for that?

NO, its twisted pair, not 75/300 ohm and not shielded. Using the proper cable is as important as a good antenna. rg59 if you want to go low budget or rg6 is good for home use.

OK, I figured it might have shielding problems. I don't know exactly why data doesn't need shielding when TV does, but I believe ya.

The data in Cat 5/Cat 6 cables is a balanced signal in twisted pairs within the cable, so it is somewhat self-shielding.

Looks as if it's feasible to distribute RF over cat5/6 cable though I've never tried. http://www.klicktv.co.uk/tv-and-video-distribution-solutions/tv-over-cat5/rf-over-cat6.html

You would certainly need a balum of some sort at each end. Balanced connections work by having equal and opposite polarity signals on each of the 2 wires. Because they are in very close proximity any interference is induced equally on both wires and can be rejected at the receiver. An overall shield can give additional interference protection but this is far from essential in most applications.

The other requirement for RF is controlled impedance. The twist of cat5/6 is very tightly controlled to give a well defined impedance. The wikipedia article isn't marvellous but gives some idea of how it all works. http://en.wikipedia.org/wiki/Twisted_pair

On a historical note there were a number of early TVs (more in the US than the UK) that had balanced antenna inputs which were connected by flat 300 ohm cable. Because the cores were so far apart and not twisted I would have thought that immunity to interference was poor.

Looks as if it's feasible to distribute RF over cat5/6 cable though I've never tried. http://www.klicktv.co.uk/tv-and-video-distribution-solutions/tv-over-cat5/rf-over-cat6.html

You would certainly need a balum of some sort at each end. Balanced connections work by having equal and opposite polarity signals on each of the 2 wires. Because they are in very close proximity any interference is induced equally on both wires and can be rejected at the receiver. An overall shield can give additional interference protection but this is far from essential in most applications.

The other requirement for RF is controlled impedance. The twist of cat5/6 is very tightly controlled to give a well defined impedance. The wikipedia article isn't marvellous but gives some idea of how it all works. http://en.wikipedia.org/wiki/Twisted_pair

On a historical note there were a number of early TVs (more in the US than the UK) that had balanced antenna inputs which were connected by flat 300 ohm cable. Because the cores were so far apart and not twisted I would have thought that immunity to interference was poor.

The balanced input does reject common mode signals. The other thing we used to do was to put a twist in the 300 ohm every couple of feet to help cancel any inductive pickup.

John

Thanks for the explanation. I finally get what balanced actually means.

I thought all TVs used 300 ohm flat cable up until cable tV started to take over in about the 1980s.

Now I'm curious what a common mode signal is.

Cat-5 cable has 100 ohm impedance, and can cause reflections (ghosts or frequency suckouts) when used with TV gear, which is 75 ohms unbalanced coax or 300 ohm balanced twinlead. Not worth it to save a few $$ by installing possible problems.

Edit: in addition, unshielded leads can have a problem if routed along metal, like electrical conduit. Coax, with a 75-300 ohm "balun" transformer if needed for your TV, is the best way by far.

("Balun" is a contraction of "balanced to unbalanced" and it works in either directiion, but in TV gear the unbalanced side is always 75 ohms and the balanced is 300. Professional signal analyzing equipment is often 50 ohm unbalanced, and needs different matching transformers or pads to connect to common 75 ohm cable. I see RS has a balun for sending baseband video (NOT RF) over CAT-5, but it costs $25. Again, better to put the money into cable and use the less expensive common 75-300 ohm balun if you need one.)

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Thanks for the explanation. I finally get what balanced actually means.

I thought all TVs used 300 ohm flat cable up until cable tV started to take over in about the 1980s.

Now I'm curious what a common mode signal is.

Common mode vs Differential (http://www.dxengineering.com/TechArticles.asp?ID=%7B3F341778-BFE9-4988-AD87-142FD1E04EB2%7D)

The above link gives a fairly good explanation.

If both conductors are picking up the same signal, current trying to go in the same direction in both, that's common mode. That gets cancelled at the balun for obvious reasons. But, with baluns at both ends and a straight run of 300 ohm twinlead you have a loop that can have current induced in it though magnetic induction if you don't put a few twists in it to cause it to cancel out.

That's not a very careful explanation, but I'm sure you get the idea.

It's easy enough to make the proper baluns, but it may not be worth the effort. If you do it would be interesting to hear the results.

John

I ended up selling these dumpster-dived partial spools of Cat 6 cable on CL to a guy who was redoing his house and installing a network. I had 1600 ft. of cable some doofus discarded (in 150 to 600ft. pieces). Sold it for 5 c/ft., $80. Some days you do OK!

Thanks for talking me out of a bad idea, BTW. And thanks jeyurkon for the link.

More kit for doing RF over network cable:

http://www.hypex.co.uk/product.php?productid=16531&cat=0&page=