Coaxial cable--why did a much shorter run degrade my video image? [Archive] - Audio & Video Forums


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03-05-2005, 02:01 PM
Ten years ago, a cable installer left about 40 feet of coaxial cable (labeled "catv") between the wall jack and the TV, on the theory we might move our tv and need the extra length. Well, we haven't (although we got satellite tv five years ago).

Anyway, I decided to replace the big coil behind the TV with 12 feet of Radioshack "gold" series coaxial cable. This resulted in a seriously degraded image from the satellite (directv).

Does anyone have any idea why? I expected either an improvement or, at worst, equivalence. Should I use something else? I would really like to replace the excess coaxial cable, but not at the cost of degrading the picture quality.

Thanks in advance for any help you can provide

03-09-2005, 09:57 AM
One of the magic properties of coax is that length has no effect on the ratio of high to low frequencies - all frequencies are attenuated evenly. So in your case something is going on that isn't really the cable itself. Several possibiliies occur to me:

1. The most likely problem is the RF connectors placed on the ends of the cable. It is very easy to get a short or an open if these are applied with non-specaillized tools.

2. Satellite cables cary DC to power the down converter in the little box aimed at the dish. This DC must not be blocked from getting to the antenna nor can it be allowed to be shorted by any TV set etc. A splitter usually blocks DC.

3. An unlikely possibility is that the shorter cable is overloading the TV input receiver and a weaker signal will actually result in a better picture.

4. Most antenna coax is 50 Ohm but 75 Ohm coax is also available. Usually this mismatch is less severe than you describe and causes "ghosting" but the coax impedance should match the application.

I'd go with likely problem number 1. I have installed many coax connectors and still get it wrong from time to time.

03-11-2005, 02:43 PM
I had no idea that this was so complicated. For now, the big coil of coax is staying behind the TV. I assume that some day we will get a new provider (and/or HDTV) so I guess we'll deal with it then.