Marine VHF & AIS Coaxial Cable Loss Calculator
Calculate high-frequency RF transmission loss (dB attenuation), real power delivered to the antenna, and Effective Radiated Power (ERP) across marine coax lines.
Quick Engineering Answer: How Much Signal Does Marine Coax Lose?
ITU-R M.493 • USCG Marine VHF StandardsCoaxial cable attenuation increases directly with operating frequency and physical run length. On a typical 65-foot sailboat masthead run at marine VHF frequency (156.8 MHz), standard thin RG-58 loses roughly 3.6 dB (wasting over 56% of your radio's power as heat before it reaches the antenna). Upgrading to low-loss RG-8X reduces attenuation to 2.3 dB (saving 4 Watts), while LMR-400 drops loss to just 0.9 dB (delivering 20.3 Watts of a 25W transmission directly to the antenna). For masthead runs exceeding 50 feet, RG-8X or LMR-400 is essential for safety of life at sea.
RF Circuit & Antenna Parameters
RF Power Transmission & Radiated Signal
Select your coaxial cable type and length, then click Calculate Coaxial Attenuation & ERP.
Engineering Physics: Coaxial Attenuation & Signal Margin
The Skin Effect and Dielectric Dissipation Factor
At marine VHF frequencies (156 MHz), alternating current does not flow evenly through the entire copper conductor. Instead, electromagnetic repulsion drives current into a microscopic outer skin layer—a phenomenon known as the skin effect (skin depth ≈ 5.3 μm at 156 MHz).
- Conductor Resistive Loss: Proportional to the square root of frequency ($\sqrt{f}$). Smaller center conductors (such as 20 AWG in RG-58) suffer severe skin-effect resistance compared to 10 AWG center conductors in LMR-400.
- Dielectric Dissipation Loss: Caused by molecular agitation in the plastic insulation between the center conductor and the shield. Solid polyethylene dielectrics (used in generic RG-58) have higher dielectric loss factors than closed-cell gas-injected foam polyethylene (used in RG-8X and LMR-400).
- Connector Insertion Loss: Every connector interface introduces minor impedance bumps and contact resistance, averaging 0.10 dB to 0.15 dB for clean, soldered silver-plated PL-259 connectors.
Losstotal (dB) = [(α × √fMHz + β × fMHz) × (L / 100)] + (Nconn × 0.15 dB)Pantenna (Watts) = Ptx × 10-(Losstotal / 10) | ERP = Pantenna × 10(GaindBi - 2.15) / 10
Application: Cruising sailboat installing a 25W fixed marine VHF transceiver connected to a 3 dBi stainless steel whip antenna at the top of a 65 ft mast with 2 connectors.
1. Baseline RG-58: Attenuation is 5.2 dB/100ft at 156.8 MHz. For 65 ft plus 2 connectors ($0.30\text{ dB}$), total loss is $3.38 + 0.30 = 3.68\text{ dB}$. Power delivered to antenna: $25\text{W} \times 10^{-0.368} = 10.7\text{ Watts}$ (a 57.2% power loss dissipated as heat inside the mast).
2. Upgraded RG-8X: Attenuation drops to 3.4 dB/100ft. Cable loss is $2.21 + 0.30 = 2.51\text{ dB}$. Power delivered: $25\text{W} \times 10^{-0.251} = 14.0\text{ Watts}$ (a 31% increase in transmitted RF power).
3. Premium LMR-400: Attenuation is only 1.4 dB/100ft. Cable loss is $0.91 + 0.30 = 1.21\text{ dB}$. Power delivered: $25\text{W} \times 10^{-0.121} = 18.9\text{ Watts}$ (delivering 76% of total transmitter power).
4. Effective Radiated Power (ERP): With the 3 dBi antenna ($+0.85\text{ dBd}$ gain), ERP reaches $18.9\text{W} \times 1.216 = 23.0\text{ Watts}$, doubling emergency broadcast range over RG-58.
Coaxial Cable Attenuation & Specification Comparison Matrix
Published nominal attenuation (dB per 100 ft) and physical dimensions across common marine coaxial cables:
| Coax Type | Outer Diameter (mm / in) | Dielectric Core | Shielding Coverage | Loss @ 156 MHz (VHF) | Loss @ 450 MHz (UHF) | Loss @ 850 MHz (LTE) | Loss @ 2400 MHz (Wi-Fi) |
|---|---|---|---|---|---|---|---|
| RG-58A/U | 5.0 mm (0.195 in) | Solid Polyethylene | 95% Bare Copper Braid | 5.2 dB / 100 ft | 9.6 dB / 100 ft | 14.0 dB / 100 ft | 26.5 dB / 100 ft |
| RG-8X (Mini-8) | 6.1 mm (0.242 in) | Foam Polyethylene | 95% Bare Copper Braid | 3.4 dB / 100 ft | 6.6 dB / 100 ft | 9.8 dB / 100 ft | 18.5 dB / 100 ft |
| LMR-240 | 6.1 mm (0.240 in) | Gas-Injected Foam PE | 100% Foil + 90% Tinned Braid | 2.8 dB / 100 ft | 5.2 dB / 100 ft | 7.5 dB / 100 ft | 13.8 dB / 100 ft |
| RG-213 / RG-8U | 10.3 mm (0.405 in) | Solid Polyethylene | 97% Bare Copper Braid | 2.6 dB / 100 ft | 4.9 dB / 100 ft | 7.2 dB / 100 ft | 14.5 dB / 100 ft |
| LMR-400 | 10.3 mm (0.405 in) | Gas-Injected Foam PE | 100% Foil + 90% Tinned Braid | 1.4 dB / 100 ft | 2.6 dB / 100 ft | 3.8 dB / 100 ft | 6.8 dB / 100 ft |
Frequently Asked Questions
Why is RG-58 coaxial cable discouraged for sailboat masthead VHF antennas?
RG-58 has high attenuation at VHF frequencies (approx 5.2 dB per 100 ft at 156 MHz). Over a typical 65 ft masthead cable run, RG-58 loses roughly 3.6 dB (over 56% of your transmitter's power) as heat before reaching the antenna. On a standard 25W VHF radio, less than 11 Watts reaches the antenna tip, severely compromising emergency broadcast range.
What is the difference between RG-8X and RG-213 or LMR-400 for marine communications?
RG-8X (0.242" OD) offers an optimal balance of flexibility and moderate attenuation (approx 3.4 dB/100ft at 156 MHz) for runs under 50 feet. RG-213 and LMR-400 are thicker 0.405" cables with half the loss (approx 1.5 dB and 0.9 dB/100ft), but their stiffness and weight require larger conduit and gentle bend radiuses inside mast conduits.
How much RF loss does a PL-259 (UHF) connector add to a VHF antenna circuit?
A properly installed, high-quality silver-plated PL-259 connector with soldered center pin adds approximately 0.10 dB to 0.15 dB of insertion loss at 156 MHz. Poorly crimped or corroded deck-gland barrel connectors can introduce 1.0 dB to 3.0 dB of loss or dangerous impedance mismatches (high VSWR).
What is Effective Radiated Power (ERP) and how is it calculated?
Effective Radiated Power (ERP) measures the true directional RF power emitted by your antenna system relative to a half-wave dipole. It is calculated by multiplying the transmitter's power by the line transmission efficiency, and then scaling by the antenna's linear gain factor (dBd = dBi - 2.15). A 3 dBi antenna provides approximately 1.22x gain over a dipole.