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TV Coax--300 vs 75 ohm

Discussion in 'Electronic Basics' started by W. eWatson, Jan 25, 2011.

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  1. W. eWatson

    W. eWatson Guest

    Don't TVs (analog) require 300 ohm coax? Can 300 and 75 be mixed?

    I went to use a old TV the other day that I had hooked up to about 25'
    of coax, but needed an extension of about 12'. There were zero marking
    on the coax, but I'm pretty sure I bought it at The Shack some years
    ago. I asked them the ohmage, but they said it didn't matter. They had
    some nondescript 25' cable, and said it would be fine.

    I did buy it, and it seems OK. Maybe they matched. $24 for 12', so I
    bought the same at K-Mart for $8!
     
  2. Rich Webb

    Rich Webb Guest

    [Oh good, something that I know just enough about to be dangerous ;-) ]

    The 300 ohm stuff that you remember was not coax, it was the old
    twin-lead cable. That configuration has the advantage of somewhat lower
    losses than coax, and also a better impedance match to folded dipole
    antennas (a common TV antenna configuration). A disadvantage is its
    greater susceptibility to interference and also to losses caused by
    neighboring conductive surfaces (e.g., aluminum siding).

    When TV sets went coax, along with the rise of cable systems, twin-lead
    could still be run from an antenna but it needed a balun (balanced
    (twin-lead) to unbalanced (coax)) converter between the two.
     
  3. W. eWatson

    W. eWatson Guest

    Perhaps I should have mentioned that what I have from The Shack uses BNC
    at both ends, and that I'm using a yagi.

    The really old stuff I suspect is flat like a ribbon.
     
  4. 300 ohm coax sounds to me not so easy to get. Coax is usually 50-52
    ohms or 75, much less commonly other values generally less than 300 ohms.
    TV coax is 75. Coax connectors for TV and video are for 75 ohm coax
    cable.

    300 ohm TV cable is twinlead, not coax. What twinlead connects to is
    generally a pair of screws.
     
  5. Guest

    I wired the outlaws' antenna system with that stuff, um, 40 years ago. ;-)
     
  6. Guest

    No. TVs use 75ohm coax. Antiques may have 300ohm twinlead inputs. You can
    convert from twinlead to coax, or verse visa, using a Balun.
    I'm sure they sell RG6. I'd be surprised if RadioShaft sold anything else
    these days.
    Good move! You still got ripped a new one, but not quite as large. ;-)
     
  7. Guest

    Same stuff. RG59 was around but I would have needed two baluns and eaten the
    insertion loss.
     
  8. Jasen Betts

    Jasen Betts Guest

    A BNC coax cable from radio shack was probably an old thin ethernat cable
    50 ohms. http://en.wikipedia.org/wiki/10BASE2
    Yeah this stuff: http://en.wikipedia.org/wiki/Twin-lead
     
  9. Guest

    Half a dB here and a half there and pretty soon you have a snow storm. The
    set only had screws on the back. I wasn't about to hack into the set.
     
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