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this post was submitted on 22 Feb 2024
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They're still a thing, kind of. TV Antennas are still inherently wired this way, and need the part in the picture (a balun) to convert the signal from the "balanced" 300 ohm twin antenna wire to "unbalanced" 75 Ohm coax cable.
Most TVs used to have the twin screw connections to hook directly to a roof antenna. But at some point (I'm guessing the 90s), more people got their TV directly from cable providers, delivered over Coax, so it made more sense for manufacturers to provide a direct coax input for the antenna.
So now, if you do have a roof antenna it probably has the balun integrated right into it, so you can take the coax (hopefully through some lightning protection) directly inside.
300 ohm is only for the folded dipole antennas which have a four times higher impedance than a single dipole. So you don't need to use a balun with a roof antenna, just the right type of dipole.