I know the advantages of coax, and the reasons that it is superior to twisted pairs, but, is there any way or a specific length of cat 5 that would be close to the same impedance? i need to make a 30-35' run, and it doesn't have to be anything amazing as far as picture quality, it's going to my 42' plasma, and i just want something better than s-video. any ideas?
The trick, really, is that the inputs and outputs of your devices are unbalanced and twisted pair is a balanced configuration, as well as being the wrong impedance. In this case, the issue really isn't the limitations of twisted pair as such, but is that the devices in question aren't configured to use balanced, unshielded 100 ohm lines. This means:
(1) No shielding, so there's quite a bit of potential for EMI, RFI, and crosstalk;
(2) Impedance mismatch (100 ohms +/- 10 or 15 for CAT5, 75 ohms for your component video signals) which will give you some signal reflection; this may result in unacceptable ringing or ghosting.
Characteristic impedance isn't related to length, so no, there's no particular length of CAT5 that would give you the right impedance.
To know whether you'd consider the image quality issues significant, the only way to be sure would be to hook it up and see; but I wouldn't be optimistic about it looking good at 30-35 feet. If it were only a few feet, you'd likely get away with it just fine, especially if there isn't a lot of EMI or RFI to pick up.
There is a way to do this, however, which some people seem to find acceptable, and that's to use baluns to convert your 75 ohm unbalanced signals to balanced 100 ohm; I'm not familiar with the particular brands, but I know there are a number of people who make that sort of thing.
If you're just looking to save on budget, my suggestion would be to consider buying some bulk RG-59 coax at home depot or the like, and soldering on some RCAs.
