Coax Cable Loss Calculator
Created by: James Porter
Last updated:
Calculate how much signal your feedline absorbs and compare RG-58, RG-8X, RG-213, LMR-240, LMR-400, and LMR-600 at any amateur radio frequency and run length.
Coax Cable Loss Calculator
Amateur RadioCompare total feedline attenuation across common coax types for your frequency, run length, and line SWR.
What is a Coax Cable Loss Calculator?
A coax cable loss calculator tells you how much signal power your feedline absorbs before reaching the antenna. Coaxial cable is not a perfect conductor — resistive losses in the centre conductor and braid, plus dielectric losses in the insulation, convert RF energy into heat rather than radiation. Knowing the actual dB attenuation for your specific cable, length, and frequency lets you make informed decisions about cable type, run length, and whether an upgrade is justified.
Amateur radio operators face a direct trade-off between cable convenience and cable performance. A thick, low-loss cable like LMR-400 or LMR-600 can cut feedline loss by 50 to 70 percent compared to RG-58 on VHF and UHF, but it is heavier, stiffer, and more expensive. For HF below 30 MHz and short cable runs, the difference between cable types shrinks considerably and the lighter choice often works fine. This calculator gives you the actual numbers so you can decide rather than guess.
The POTA and portable operating communities have driven increased interest in feedline efficiency. A portable station running 10 to 25 watts cannot afford to lose half that power in the feedline before the signal even leaves the antenna. Even for higher-power fixed stations, a 3 dB feedline loss effectively halves transmitted power — equivalent to removing one full S-unit from your signal at the receiving end.
Feedline loss also affects receive sensitivity. A feedline with 3 dB of loss raises the effective noise figure at the receiver input by 3 dB, making weak signals harder to copy. This matters most for weak-signal VHF and UHF work, satellite operations, and digital modes like FT8 and WSPR where link margins are measured in single decibels.
How the Coax Cable Loss Calculator Works
Coaxial cable loss is specified in decibels per hundred feet at a given frequency. The calculator uses manufacturer-specified matched loss values at 100 MHz and 400 MHz reference points for each cable type, then interpolates to any amateur frequency using the standard conductor-loss power law. Because conductor skin-effect loss scales as roughly the square root of frequency and dielectric loss increases more linearly, the combined exponent is near 0.5 for most cables — slightly higher for foam-dielectric types where dielectric loss is proportionally larger.
When SWR on the line exceeds 1:1, reflected power bounces from the antenna and travels the cable again, dissipating additional energy in the cable resistance on each pass. The calculator applies the standard additional-loss formula used in the ARRL Antenna Book, which accounts for the interaction between the reflection coefficient and the matched-line attenuation. The comparison chart simultaneously shows all six cable types at your entered length and frequency so you can immediately see which upgrade yields the greatest benefit.
Coax cable loss formulas
Loss exponent n = log(L₄₀₀ / L₁₀₀) / log(4)
Loss per 100 ft at frequency f = L₁₀₀ × (f / 100)ⁿ
Matched line loss ML (dB) = (loss per 100 ft / 100) × cable length in feet
Reflection coefficient Γ = (SWR − 1) / (SWR + 1)
SWR mismatch loss (dB) = 10 × log₁₀[(1 + Γ² × 10^(−ML/5)) / (1 − Γ² × 10^(−ML/5))]
Power at antenna (W) = TX power × 10^(−total loss / 10)
Example Calculations
Example 1: 100 ft RG-213 on 20 metres
At 14.2 MHz, RG-213 loses approximately 0.2 dB per 100 feet, giving only 0.2 dB total loss on a 100-foot run. A 100 W station delivers about 96 W to the antenna — almost negligible loss. This is why RG-213 is a popular and economical choice for moderate HF runs at the typical 50-ohm amateur standard.
Example 2: 100 ft RG-58 on 70 cm
At 446 MHz, RG-58 loses over 7 dB per 100 feet. A 100-foot run throws away more than 80 percent of your transmitter power before the antenna sees it. Switching to LMR-400 on the same run drops loss to around 1.5 dB, recovering the vast majority of that power and dramatically improving both transmit and receive performance.
Example 3: POTA portable with 30 ft of RG-8X
A 30-foot run of RG-8X at 7.15 MHz loses only about 0.08 dB — essentially zero. Even at 145 MHz for a VHF crossband or satellite link, 30 feet of RG-8X loses under 0.6 dB. For short POTA runs, flexible and lightweight RG-8X is a practical choice that does not sacrifice meaningful signal strength.
Common Amateur Radio Uses
- Determine total feedline loss before purchasing cable to avoid over-specifying expensive low-loss cable where a lighter cable performs nearly identically.
- Compare whether upgrading from RG-58 to LMR-400 is worth the cost for a given cable run and frequency — it is rarely necessary on short HF runs but is highly effective at VHF and UHF.
- Calculate power actually reaching the antenna when planning EIRP and link budget calculations for satellite, VHF weak-signal, or long-path HF contacts.
- Evaluate whether a high-SWR antenna with an ATU at the transmitter is more efficient than adding a remote tuner or matching network at the feed point.
- POTA and SOTA planning — confirm that a portable coax run is efficient enough for the power level of your field station before heading to the park.
- Receive-path planning for weak-signal and SDR stations, where feedline loss directly raises the system noise floor and reduces sensitivity to faint signals.
Tips for Better Ham Radio Planning
Connector quality matters more than many hams expect. Poorly crimped or soldered PL-259 connectors, or moisture-contaminated N-type joints, can add more loss at UHF than 20 feet of quality cable. At VHF and above, use weatherproof connectors, self-amalgamating tape at all outdoor joints, and drip loops to keep moisture out. Inspect connectors annually because corrosion inside a PL-259 can take a good cable run from under 1 dB loss to 3 or 4 dB over several seasons.
For POTA and portable operations, shortening the cable run is the single most cost-effective strategy. A 20-foot cable run is usually better than a 50-foot run even if the shorter run means awkward transmitter placement. At HF this difference may be trivial, but at 2 metres or 70 cm it can easily be 1 to 2 dB — the difference between a copied signal and a missed contact. Plan your antenna placement to minimise cable between the transmitter and the radiating element.
Frequently Asked Questions
Why does coax loss increase with frequency?
Higher frequencies drive current into a thinner skin-depth layer on the conductor surface, raising resistive loss. Dielectric loss in the insulation also increases with frequency. Together these effects mean a given run of RG-58 loses roughly twice as much signal at 400 MHz as it does at 100 MHz, which is why low-loss cables matter far more on VHF and UHF than they do on 40 metres.
How much feedline loss is acceptable for a ham station?
A widely used guideline is to keep total feedline loss under 1 dB on the primary operating band. Every 3 dB of loss halves the power reaching the antenna, so a 100 W transmitter delivering only 50 W to the antenna already suffers a meaningful penalty. For VHF and UHF weak-signal work, even 2 or 3 dB of cable loss is noticeable on a marginal path, making low-loss cable worthwhile.
What is matched line loss and how does SWR change it?
Matched line loss is cable attenuation when the antenna presents a perfect 50-ohm load. In practice, SWR greater than 1:1 causes reflected power to travel back and forth on the line before being fully radiated, increasing the total power dissipated in the cable. A 2:1 SWR on a 1 dB matched-loss line adds only a fraction of a dB, but the same mismatch on a lossy long run compounds quickly.
Is LMR-400 worth the cost for a short HF feedline?
For HF below 30 MHz and cable runs under 50 feet, the loss difference between LMR-400 and RG-213 is so small that the heavier cable is rarely worth the cost or handling difficulty. LMR-400 starts earning its price at VHF and UHF and on long cable runs where the per-hundred-foot loss difference compounds substantially. Use the calculator with your specific frequency and length to see the actual numbers.
What feedline should I use for a POTA portable station?
For HF POTA operations on 40 and 20 metres, a 25 to 50 foot run of RG-8X loses well under 1 dB, which is entirely adequate for a typical 10 to 100 watt portable station. Keep the run as short as the site allows, use good-quality connectors, and avoid coiling extra cable tightly because coiled coax can act as an RF choke on some bands.
Does the calculator account for connector losses?
The calculator covers only the cable jacket loss based on manufacturer data for matched conditions. Each PL-259, N-connector, or BNC splice adds typically 0.05 to 0.2 dB per connector depending on quality and frequency. For a two-connector HF run this is negligible, but at UHF a chain of four mediocre connectors can add close to 1 dB on top of the cable loss shown here.
Sources and References
- ARRL Antenna Book, 24th edition — Chapter 24, Transmission Lines, cable loss tables and SWR correction methods.
- ARRL Handbook for Radio Communications — Feedline selection guidance and coaxial cable specifications.
- Times Microwave LMR cable series technical datasheets — matched line loss specifications at multiple frequencies.
- Belden cable technical data — RG-58, RG-8X, and RG-213 attenuation curves at VHF and UHF.