Coaxial cable attenuation is the reduction in signal power as it travels through the cable. It is measured in decibels (dB) and increases with both frequency and cable length. Every coaxial cable has some inherent loss due to conductor resistance (copper losses) and dielectric absorption in the insulating material. Attenuation is typically specified in dB per 100 feet (or dB per 100 meters) at specific frequencies. Higher quality cables like LMR-400 or hardline have significantly lower attenuation than basic cables like RG-58.

For most coaxial cables, attenuation scales approximately with the square root of frequency. The formula used is: Attenuation(f) = A_ref × √(f / f_ref), where A_ref is the known attenuation at reference frequency f_ref (typically 100 MHz). For example, if a cable has 4.1 dB/100ft loss at 100 MHz, at 400 MHz the loss would be approximately 4.1 × √(400/100) = 4.1 × 2 = 8.2 dB/100ft. Total cable loss is then: (dB/100ft) × (cable_length / 100). This square-root relationship holds well for most solid and foam dielectric cables up to several GHz.

Attenuation in coaxial cables is caused by two main factors: resistive (ohmic) losses in the copper conductors (both center conductor and shield), and dielectric losses in the insulating material between them. Resistive losses increase with frequency due to the skin effect, which forces current to flow only near the surface of conductors. Dielectric losses also increase with frequency, especially in lower-quality insulation materials. Additional minor contributors include radiation leakage through imperfect shielding, connector losses, and impedance mismatches causing reflections.

Cable loss increases with frequency, approximately following a square-root relationship (∝ √f). This means if you double the frequency, the loss increases by about 41% (×√2). For example, if a cable loses 3 dB/100ft at 100 MHz, it will lose approximately 6 dB/100ft at 400 MHz (4× the frequency, 2× the loss). At microwave frequencies (above 1 GHz), attenuation becomes significantly higher, which is why low-loss cables like LMR-400 or hardline are essential for long runs at UHF and higher bands.

The best low-loss coaxial cable depends on your application. LMR-400 is widely considered the sweet spot for amateur radio and general RF applications, offering excellent performance (1.5 dB/100ft at 100 MHz) at a reasonable cost and flexibility. For even lower loss, LMR-600 (0.95 dB/100ft) or 1/2″ hardline (1.1 dB/100ft) provide superior performance but are thicker, heavier, and more expensive. For tower installations, 7/8″ or 1-5/8″ hardline offers the absolute lowest loss. Always balance cost, flexibility, and attenuation needs for your specific frequency range and cable run length.

To minimize cable loss: (1) Use the shortest cable run possible. (2) Choose a cable with lower specified attenuation at your operating frequency (e.g., upgrade from RG-58 to LMR-400). (3) Use high-quality connectors properly installed to avoid additional insertion loss. (4) Keep cables away from sharp bends that can distort the impedance and increase reflections. (5) For very long runs at high frequencies, consider using hardline or even moving the transmitter closer to the antenna (e.g., using a remote-controlled radio at the tower base). (6) Ensure good impedance matching to minimize SWR-related additional losses.

RG-58 is a basic 50Ω flexible cable with 4.1 dB/100ft loss at 100 MHz. It uses a solid polyethylene dielectric and a single-braid shield. LMR-400 is a premium low-loss 50Ω cable with only 1.5 dB/100ft at 100 MHz — nearly 3× less loss. LMR-400 uses foam polyethylene dielectric for lower loss, has a bonded aluminum foil + braid shield for better shielding (>90 dB isolation), and handles higher power. At 430 MHz, RG-58 loses ~8.5 dB/100ft while LMR-400 loses only ~3.1 dB/100ft. For any run over 25 feet at VHF/UHF, LMR-400 is strongly recommended.

Here are typical attenuation values at 100 MHz (dB/100ft): RG-174: 8.5 (ultra-thin, high loss), RG-58: 4.1 (general purpose), RG-8X: 3.5 (mini-8), RG-59: 3.4 (75Ω video), RG-6: 2.8 (75Ω CATV), RG-213: 2.7 (heavy-duty), RG-8: 2.0 (HF base), LMR-240: 3.0, LMR-400: 1.5 (popular low-loss), LMR-600: 0.95, 1/2″ Hardline: 1.1, 7/8″ Hardline: 0.6, 1-5/8″ Hardline: 0.35. All values are approximate and vary by manufacturer.

Impedance matching ensures maximum power transfer from the transmitter through the cable to the antenna. Most amateur radio equipment uses 50Ω impedance, while cable TV systems use 75Ω. If there's an impedance mismatch, some signal power reflects back toward the source, creating standing waves (measured as SWR). High SWR not only wastes power but can also damage transmitters, cause additional cable heating, and create misleading measurements. Always match your cable's characteristic impedance to your system (typically 50Ω for ham radio, 75Ω for video/CATV).

Yes. As temperature increases, conductor resistance rises, which increases attenuation. Typical temperature coefficient is approximately +0.2% per °C for copper conductors. In extreme environments (e.g., attic installations in summer reaching 60°C/140°F), attenuation can be 6–8% higher than at room temperature. Conversely, in very cold conditions, attenuation decreases slightly. For critical outdoor installations, especially in hot climates, consider this additional loss when designing your system. High-quality foam-dielectric cables like LMR series are slightly more temperature-stable than solid polyethylene types.

This calculator uses the square-root-of-frequency model, which is accurate for most coaxial cables from about 1 MHz to 6 GHz. Below 1 MHz, conductor losses become more dominant and the √f model slightly underestimates loss. Above 6 GHz, many cables approach or exceed their cutoff frequency where higher-order propagation modes can appear, and the √f model becomes less reliable. For critical microwave applications above 3 GHz, consult the manufacturer's detailed specification sheet. The calculator also shows warnings if the selected cable is being used near or above its recommended maximum frequency.

This calculator provides good engineering estimates based on the √f scaling model and typical manufacturer reference data at 100 MHz. Actual attenuation can vary by ±10–15% depending on the specific manufacturer, batch, cable age, connector quality, and environmental conditions. For precise system design, always refer to the manufacturer's official datasheet for your specific cable model and production lot. The values here are most accurate for new, properly installed cables at room temperature (20–25°C / 68–77°F) with high-quality terminations.