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  • Why doesn't anyone use that flat 300 ohm antenna wire anymore?

    You remember this stuff, right? Two wires with a fat piece of plastic between them. Bare wire or lugs at the end. If you're old enough to remember knobs on TVs, you've probably seen your share of it. No one uses that 300 ohm flat antenna wire anymore, though. Did you ever wonder why?

    Here's the simple answer: coaxial cable is better. Coaxial cable is much more resistant to interference. It's more durable, too. Not that the old flat wire is unsafe or anything -- if you're using it now there's no reason to stop. If you're wiring a new antenna though, you'll find that the connection you get from that antenna is always coax.

    Which leads us to the question, if coax is so much better, why did anyone ever use twin-lead cable at all? It's cheaper to make, and it does actually perform better over long distances than coax if there isn't any interference. The big reason though, is impedance. Impedance is that hard-to-explain characteristic of wire that represents how much a circuit opposes the introduction of current into it. One way of imagining it is to think of a clogged pipe, and how much the clog in the pipe stops the water from flowing. (That's not technically accurate but it gives you some idea.)

    Whether or not you can wrap your head around impedance, the point here is that twin-lead wire is much more similar in impedance to the antenna itself. You don't need a transformer to go from the antenna to the wire. With coax, you do. That's one more part that was hard to make in the old days, and expensive too.

    Today, we don't have to worry much about the cost of a matching transformer that converts a signal from 300 ohms to 75 ohms (which is what coax uses.) So, we're free to use coax cable and enjoy its superior shielding and freedom from interference. It's a little more expensive, but not enough to make a huge difference.

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    Comments 4 Comments
    1. PhoenixAZ's Avatar
      PhoenixAZ -
      Didn't they use coax for VHF and flat wire for UHF? UHF inputs always seemed to have the two screws for the flat wire.

      Do antennas still use the 300 ohm to 75 ohm adapters at the antenna?
    1. Stuart Sweet's Avatar
      Stuart Sweet -
      The adapters to which you refer are called baluns and they are always going to be present at the antenna side (even if you can't see them.) The impedance of the antenna itself is closer to 300 ohm (the same as that flat wire) and needs to be converted to 75 ohm for signals to travel down the coa line.
    1. PhilK's Avatar
      PhilK -
      The 300-ohm twin lead was used for both VHF & UHF. It still has better loss factors for every frequency than most coax on the market. However, Stuart is correct about interference rejection being better with coax but really correct about the impedance matching. Also, now that we've gone to digital TV and cars are much less electrically noisy the twin lead would probably work better than most coax. In fact, I would use it if I could buy it. The remaining twin lead I do have is used now to get from the antenna into the attic, there I convert to coax and add an amplifier so I can split the signal up & get it through the coax, which has a higher loss factor than the twin lead, to the TVs.

      The biggest reason that we've moved to coax, as far as I can determine, is that it is far more expensive and the profit margin on it is much better than the twin lead. Amateur radio operators like 300-ohm, 450-ohm, and 600-ohm twin lead, window line, or ladder line (all pretty much the same - look them up if you don't know the difference) because of the three things, 1. the lower losses in the feed line, 2. wider frequency response, and 3. much lower cost.

      I do believe that we finally have coax with lower losses in the UHF frequency range, but this coax is even more expensive than the normal TV coax.

      Bottom line is that the loss of twin lead, on a cost/loss factor basis, is still much better than any coax on the market.
    1. PhilR's Avatar
      PhilR -
      I currently have 300 ohm from the antenna to the DTV box (the TV tuner must have gotten fried in a storm) and convert it to 75 ohm coax at the DTV converter box.

      I do not think I have interference issues due to the 300 ohm cable, but DTV really is subject to the weather.
      At least with the old analog signal you could fuzzily view the video and the audio might be a bit garbled, but the DTV signal is useless and the audio drops out worse. At least if you can hear what's happening the video isn't as important - for short patches.
      I'll be looking for some new baluns as the DTV box has a 75 ohm coax input but it's too close to the 75 ohm coax output so it doesn't fit right. Either a short coax extension or a smaller balun is what I need. (Mine is the old right-angle, 1"x3/4"x3/4" push-in balun, instead of screw-in, and abuts the coax-out so the balun sits to the outside at an angle. It works, but it's not very stable if I need to move things around.)

      PhilK - why can't you buy 300 ohm twin lead? A google search brings up all kinds of results. Sometimes people throw out antennas and the cable too - which is where I got my last antenna, on the street before trash day. I thinking of adding it to point toward Milwaukee as my current antenna is pointing toward Chicago.