Coaxial Cable: Types, Ratings, and Specifications

What Is Coaxial Cable?

Coaxial cable ("coax") carries RF signals along a center conductor surrounded by a dielectric insulator, a braided or solid outer conductor (shield), and a protective jacket. Because the signal travels inside the shield, coax rejects external interference and contains the RF field internally. Every coaxial cable has a characteristic impedance — the ratio of voltage to current for a wave traveling along the line under matched conditions. In RF and telecom work, two impedances dominate: 50 Ω (used in two-way radio, cellular, and most RF test equipment) and 75 Ω (used in cable television, broadcast distribution, and satellite systems). Mixing impedances without proper matching creates reflections and signal loss.

Signal Loss in Coaxial Cable

Attenuation is the reduction of signal power as it travels through the cable. It is expressed in decibels per unit length (dB per 100 feet or dB per meter) and increases with frequency. The higher the frequency, the greater the loss per foot. Cable manufacturer datasheets list attenuation at several spot frequencies — for example, at 100 MHz, 400 MHz, and 1000 MHz. You always look up the value at the highest operating frequency to find the worst-case loss.

Total feedline loss = (attenuation per 100 ft) × (run length in feet ÷ 100)

If total feedline loss exceeds the system loss budget, the link will fail.

Common Coaxial Cable Types

RG-6

RG-6 is a 75 Ω cable designed for residential and light-commercial CATV, satellite, and over-the-air antenna wiring. Its outer diameter is roughly 0.270 inches. The dielectric is foam polyethylene, which lowers attenuation compared with solid polyethylene. RG-6 is inexpensive, flexible, and widely available, but its small center conductor limits power-handling capability. Maximum recommended run lengths for CATV are typically up to about 150–200 feet before amplification is needed. It is not used on 50 Ω transmitter outputs.

RG-11

RG-11 is also 75 Ω but uses a larger center conductor and outer diameter (roughly 0.405 inches). Its attenuation is significantly lower than RG-6 at every frequency, making it suitable for longer cable runs in CATV trunk lines and antenna feed runs where signal loss must be minimized. RG-11 is stiffer and more expensive than RG-6. Use it when run length exceeds what RG-6 can support within the loss budget.

LMR-240

LMR-240 (by Times Microwave Systems; "LMR" is a product line designation) is a 50 Ω flexible coaxial cable with an outer diameter of approximately 0.240 inches. The dielectric is foam polyethylene. LMR-240 is a workhorse for short-to-medium antenna cable runs in two-way radio, Wi-Fi, and cellular installations. It is far lower loss than RG-8X or RG-58 and handles moderate power levels. Its flexibility allows routing through conduit and around obstacles.

LMR-400

LMR-400 is a 50 Ω low-loss cable with an outer diameter of approximately 0.405 inches. At 450 MHz, manufacturer data shows roughly 1.5 dB of loss per 100 feet — substantially less than LMR-240 or RG-8. LMR-400 is widely used for base-station antenna feedlines, cellular installations, and commercial two-way radio towers where long runs must keep loss low. It is semi-flexible: it can be bent but requires more effort than smaller cables, and minimum bend radius specifications must be respected to avoid kinking the foam dielectric.

7/8-Inch Hardline (Rigid or Semiflexible)

Hardline uses a solid or corrugated copper outer conductor rather than braid. The 7/8-inch designation refers to the outer-conductor outer diameter. Hardline has extremely low attenuation — typically less than 0.5 dB per 100 feet at VHF/UHF — and high power-handling capability. It is used on broadcast transmitter outputs, large cellular base stations, and high-power repeater sites. Hardline is rigid or semi-rigid; it cannot be bent freely and requires special connectors and weatherproofing at every junction.

Power Ratings

Every cable has a maximum continuous power rating that depends on frequency and ambient temperature. Exceeding this rating overheats the dielectric, causing impedance changes, gas evolution, and eventual failure. Always verify that the transmitter's output power — reduced by any connector losses — is within the cable's rated limit. Hardline and LMR-400 handle kilowatts at lower frequencies; RG-6 is rated for low power only.

Safety Considerations

RF exposure: Energized feedlines radiate at damaged or loose connector points. Never work on a feedline carrying RF power without first confirming the transmitter is off and locked out. Follow your site's lockout/tagout (LOTO) procedure.

Tower and roof work: Installing hardline or long runs of LMR-400 on towers requires proper fall-protection PPE — harness, lanyard, and anchor — in compliance with applicable OSHA regulations. Never carry large reels of heavy coax up a structure without mechanical assistance.

Sharp braid edges: Cut coax ends have sharp wire ends that can puncture skin. Wear gloves when handling cut ends and dispose of braid trimmings safely.

Weatherproofing: Outdoor connectors that are not properly weatherproofed allow moisture ingress that degrades the dielectric, raises attenuation, and can cause arcing at high power. Use self-amalgamating tape or the weatherproofing method specified by the manufacturer.

Selecting the Right Cable

To choose a cable, gather four pieces of information: (1) system impedance (50 Ω or 75 Ω), (2) highest operating frequency, (3) run length, and (4) maximum allowable feedline loss from the system link budget. Look up attenuation per 100 feet from the manufacturer's datasheet at the target frequency. Multiply by run length to get total loss. If it exceeds the budget, select a lower-loss cable and recalculate. Also confirm the power rating meets the transmitter output.