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Ham Radio 101: Everything You Wanted to Know About Coax But Were Afraid to Ask
Coax cable has its roots dating back to the mid-1800s, when engineers developed transatlantic cable communication. These early telegraph cables were made up of a center conductor encased in a cylindrically shaped rubber-like insulator. They had no outer shield—the seawater surrounding them completed the circuit.
The coax cable we now use was based on a design developed by Lloyd Espenschied and Herman Affel at Bell Telephone Laboratories in 1916. In the 1930s, coaxial cable was used to connect radio networks, television networks, and long-distance telephone networks.
When World War II came along, military contractors cranked out coax for the war effort. The development of polyethylene made it possible to produce cheap, flexible coaxial cables for easy and quick deployment. After 1945, military surplus coax cable could be obtained cheaply, making it popular among hams in the postwar years. It became an easy-to-use alternative to open wire feed lines.
You’ve Got Coax Questions? We Have Answers
Let us help you unravel the mysteries and myths behind the cable that helps connect you to the rest of the world.
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What Are RG, M17, and LMR Numbers?
RG stands for Radio Guide, which was the original military specification for coax cable. The number that follows the RG was just a page in the radio guide book. In reality, these are just general descriptions of the cables available.
M17 refers to the military specifications set by the U.S. Department of Defense. Mil-DTL-17, or M17, is their current standard for coaxial cable. This mil-spec has replaced the old RG numbers and ensures that the new M17 coaxial cables will stand up to tough military requirements.
LMR stands for Land Mobile Radio, which indicates its original intended use for mobile radio systems. The 400 in LMR 400 refers to the cable’s nominal diameter, which is 0.405 inches.
Every manufacturer has their own variations, including differences in shielding material, insulation, outer jackets, and other traits. Transmission loss, power handling, and other specs will vary somewhat from one brand to another. So an RG-8 cable from one manufacturer may be slightly different from that of another.
For example, the Belden 8214 version of RG-8/U has an 11 AWG stranded center with bare copper braid and an outside diameter of .403 inches. DX Engineering RG-8/U has an 11 AWG stranded center, bare copper braid, and an outer diameter of .405 inches. Loss figures and power ratings are similar, within a few tenths of a dB.
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What Does “Velocity Factor” Mean?
The velocity factor is the speed at which an RF signal travels through a material compared to the speed this same signal travels through a vacuum. It’s expressed as a percentage. Cables with a lower velocity factor are more lossy, but that’s because they use solid insulation instead of a low-density foam or air with small spacers to keep the center conductor roughly centered inside the outer conductor. Velocity factor is also used in calculating a particular cable’s wavelengths for matching or phasing antennas.
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Can You Bury Coaxial Cable Without Using a Conduit?
Some coax cable is meant to be buried directly in the ground and other cables are not. If you look at the cable and find the words “direct bury” on the cable then you can bury it without a conduit. Cables that are not designed to be buried will deteriorate with exposure to soil and moisture.
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Why is Coax 50 ohms?
The best coaxial cable impedances were experimentally determined at Bell Laboratories in 1929 to be 77 Ω for low-attenuation, 60 Ω for high voltage, and 30 Ω for high power.
The arithmetic mean between 30 Ω and 77 Ω is 53.5 Ω; the geometric mean is 48 Ω. The selection of 50 Ω is a compromise between power-handling capability and attenuation. Also, 50 Ω works out well because it’s close to the feed point impedance of a 72 Ω half-wave dipole, mounted approximately a half-wave above normal ground.
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Can I Coil Up Extra Coax?
For HF frequencies, it’s not a problem if you need to coil up small amounts of excess cable. Wrap loosely to avoid cable damage. For VHF/UHF, higher losses come into play—longer cable, more loss. The best option is to cut the coax to fit or at least buy the shortest cable you can that still fits.
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Why Do I Get Better SWR by Adding Coax Cable?
The shield of the coax line can actually become part of the antenna. If you’re feeding a dipole that doesn’t have a balun or choke, then adding a short piece of cable will add to the antenna length, affecting the SWR reading.
Another reason is the coax can act as an impedance transformer. The impedance changes along the transmission line with standing waves. By adding some cable, you’ve changed the impedance to something your tuner can more easily match.
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I’ve Got Leftover 75 Ω TV Coax Cable. Can I Use It with My Ham Antennas?
Sure, you can use 75 Ω coax instead of 50 Ω. If you’re feeding a dipole, 75 Ω coax is a better match since dipole impedance is about 72 Ω. In other cases, like feeding a typical 1/4 wave vertical (36 Ω depending on radials), it would be a worse match.
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Can You Mix Different Types of 50 Ω Coax Cable to Extend a Feed Line?
For example, you have 25 feet of RG-8/U and need to extend it 50 feet to your antenna. You have 50 feet of LMR-400 available. No problem.
The only downside would be losses caused by linking the cables and differences in the cable. If you used RG-8X instead of the LMR-400, you’d likely experience a small loss of signal to the antenna.
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With CB Radio, I Was Told to Feed My Antenna with Multiples of a Half-Wavelength—18 feet for CB. Does This Also Apply to Ham Radio?
The 18 feet length is a myth. It might be about half wavelength in free space at CB frequencies but that does not take the velocity factor of the coax into consideration. If an antenna has a perfect match at the feed point, a half wavelength in coax will repeat that match but with velocity factors running between about .66 and .85 for most modern coax, a half wavelength in coax would be between 11.88 and 15.3 feet, not 18 feet.
The proper length of coaxial cable is just long enough to connect the radio and antenna together. There’s no special length needed, no magic measurement so your radio will work better.
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