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Coax Downlead

Transmission Cable
Twinlead (ribbon cable) used to be common for TV antennas. It has its advantages. But due to its unpredictability when positioned near metal or dielectric objects, it has fallen out of favor. (Such objects, even if not touching the cable, cause a portion of the signal to bounce, return to the antenna, and get retransmitted.)

Coaxial cable is recommended. It is fully shielded and not affected by nearby objects. Coaxial cable has a feature called its characteristic impedance, which for TVs should always be 75 ohms. (50 ohm coaxial cable is also common. Avoid that cable.) Although rated in ohms, this has nothing to do with resistance. A resistor converts electric energy into heat. The “75 ohms” of a coaxial cable does not cause heat. Where it comes from is mathematically complicated and beyond our scope here. But coax also has ordinary resistance (mostly in the center conductor) and thus loses some of the signal, converting it into heat. The amount of this dissipation (loss) depends on the frequency as well as the cable length.

Type: Center conductor: Cable diameter: Coax Cable
RG-59 20-23 gauge 0.242 inches Image
RG-6 18 gauge 0.265 inches Image

The above table is only approximate. There are many cable manufacturers for each type and there is no enforcement of standards. If the mast-mounted amplifier gain exceeds the cable loss then it shouldn’t matter what cable you use. But there are two problems with this: Some cable has incomplete shielding. The cheapest is RG-59 cable. When the cable run is longer than 200 feet, the low-numbered channels can become too strong relative to the high-numbered channels. In this case, an ultra-low-loss RG-6 is recommended. (These alternatives are not expensive.) Alternatively, frequency compensated amplifiers will work. We recommend RG-6 for all TV antennas. It can be stapled in place using a staple gun with common 9/16” T25 staples. How long the cable lasts depends solely on how long you can keep water out of it and on weather conditions.

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This article was published on Tuesday 14 December, 2004.
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