Coax Cable Loss & SWR Calculator

Every watt of radio frequency energy generated by an amateur radio transmitter must traverse a transmission line before reaching the radiating antenna elements. While operators frequently invest heavily in transceivers, linear amplifiers, and high-gain beams, transmission line losses often quietly consume a substantial fraction of transmitted power. Our online Coax cable loss calculator computes precise decibel attenuation, reflected standing wave losses, and true radiated power across all major coaxial lines.

The Physics of Coaxial RF Attenuation and Frequency Scaling

Coaxial feedline loss is not a constant linear resistance; it consists of two distinct, frequency-dependent physical mechanisms that cause total RF attenuation to rise steeply as frequency increases:

• Conductor Resistive Loss (Skin Effect): At high frequencies, alternating current concentrates within a microscopic layer along the outer perimeter of the center conductor and inner surface of the shield. Because current flows through an increasingly narrow cross-sectional area, effective AC resistance rises proportionally to the square root of frequency (k₁ × √f).
• Dielectric Dissipation Loss: The insulating material separating the center conductor from the shield (polyethylene, foam, or PTFE) undergoes molecular polarization under alternating electric fields, converting RF energy directly into heat. Dielectric loss scales linearly with frequency (k₂ × f).

Cable Selection: Comparing LMR-400 vs RG-8X for Station Installations

The classic dilemma for HF and VHF amateur radio stations is choosing between LMR-400 vs RG-8X. RG-8X (often termed "Mini-8") features a flexible 0.242-inch diameter and stranded center conductor, making it ideal for portable operation, rotator loops, and short interconnect patch cables under 50 feet. However, on long runs or above 30 MHz, its losses escalate rapidly.

In contrast, LMR-400 employs a bonded aluminum foil tape and tinned copper braid shield, gas-injected closed-cell foamed polyethylene dielectric (85% velocity factor), and a 10 AWG solid or copper-clad aluminum center conductor. Across a 100-foot run on the 2-meter band (146 MHz), RG-8X loses roughly 4.5 dB (leaving only 35 watts out of 100 watts transmitted), while LMR-400 loses just 1.5 dB (delivering over 70 watts to the antenna). For VHF, UHF, and serious HF DX stations, upgrading to LMR-400 or hardline yields an immediate, noticeable boost in both transmit power and weak-signal receiver sensitivity.

Reflected Waves and Additional SWR Power Loss

When an antenna load impedance deviates from the nominal 50-ohm characteristic impedance of the coax, a portion of the forward RF wave is reflected back down the line toward the transmitter. As this reflected wave travels backward, it undergoes normal line attenuation. Upon reaching the antenna tuner or transmitter in the shack, it reflects forward again, suffering yet another round of attenuation.

This continuous back-and-forth travel creates additional SWR power loss on top of the matched line loss. For low-loss cables on HF (where matched loss is small), an SWR of 3:1 adds less than 0.5 dB of extra attenuation. However, on already lossy cables (such as RG-58 on VHF or long RG-8X runs), high SWR dramatically magnifies total power dissipation, turning your feedline into an inadvertent heating element.

Strategies for Maximizing Overall Transmitter Efficiency

To achieve peak transmitter efficiency, follow these proven engineering practices:

• Tune at the Antenna: Whenever possible, match non-resonant antennas with an autotuner installed directly at the feedpoint, keeping the coaxial feedline run at a flat 1:1 SWR.
• Size Feedlines by Frequency: Use RG-8X or RG-213 for short HF jumpers, but reserve LMR-400, LMR-600, or 1/2" Heliax for long runs, repeater lines, and VHF/UHF systems.
• Protect from Moisture Ingress: Always weatherproof outdoor PL-259 and Type-N connectors with self-amalgamating silicone tape. Moisture inside foamed dielectric drastically increases attenuation.