MathBench

Coax Cable Loss Calculator

Pick a cable, enter length and frequency, and see the dB loss, the watts that reach the antenna, and the percentage lost along the way.

Result

—total loss (dB)
—power out (W)
—power delivered
—dB per 100 ft

How this calculator works

Every coax type has a characteristic attenuation that rises with frequency. The calculator models it with the industry-standard two-term formula (a square-root-of-frequency term for conductor (skin-effect) loss plus a linear term for dielectric loss), scales it by your cable length, and converts the resulting decibels into the watts that arrive at the far end.

The presets cover the cables hams and installers buy, from thin RG-174 patch cable to LMR-600 hardline substitute. Real cable varies by manufacturer (bargain "RG-58" can run noticeably worse than the catalog value), so for critical links, pick "Custom" and type the dB/100 ft figure from your cable's datasheet at your operating frequency.

The formula

dB per 100 ft = K1 × √F + K2 × F      (F in MHz)
Total loss    = dB/100ft × length ÷ 100
Power out     = power in × 10^(−loss ÷ 10)

K1 captures conductor loss (grows with √F as skin depth shrinks) and K2 dielectric loss (grows linearly). LMR coefficients come from Times Microwave's published datasheet model. RG figures are fits to typical catalog values and vary a few percent between manufacturers.

Worked example

Say you're feeding a UHF antenna through 100 ft of LMR-400 at 446 MHz with 50 W:

  1. Per-100 ft loss: 0.12229 × √446 + 0.00026 × 446 = 2.58 + 0.12 = 2.70 dB
  2. Total for 100 ft: 2.70 dB
  3. Power ratio: 10^(−0.270) = 0.537
  4. Power out: 50 × 0.537 = 26.9 W

Even low-loss cable gives up nearly half the power on a 100-foot UHF run, and the same run in RG-58 would deliver about 5 W. This is why serious UHF stations put money into feedline before putting it into amplifiers.

Assumptions & tips

  • Loss works on receive too. The same decibels that shrink your transmit signal shrink incoming signals ahead of your receiver's preamp: on weak-signal paths, feedline is often the cheapest "amplifier" you can buy.
  • Shorter beats thicker. Before upgrading cable, see if the run can be shortened: mounting hardware or a relocated entry point that saves 30 feet often outperforms the next cable tier at lower cost.
  • Match the connector to the band. Above ~440 MHz, prefer N or properly installed crimp connectors over soldered UHF (PL-259) types, and weatherproof every outdoor joint: water in the braid ruins good cable permanently.
  • Check VSWR before blaming the cable. The numbers here assume a matched antenna. If your match is poor, total line loss rises above the matched value. Quantify it with the VSWR calculator first.
  • 75-ohm RG-6 is a legitimate trick. For receive-only antennas (scanners, SDRs, TV), cheap RG-6 has respectable loss figures and the 50↔75 ohm mismatch costs only ~0.18 dB, often the best value per dollar going.

Frequently asked questions

How much coax loss is acceptable?

A common target is to keep total feedline loss under 3 dB, the point where half your power is heating cable instead of radiating. For weak-signal work (satellites, microwave, marginal repeaters), builders often aim for 1 dB or less. For casual local FM use, even 5 dB may be tolerable. The right answer depends on how close to the noise floor you operate.

Why does loss increase with frequency?

Two mechanisms. Conductor loss grows with the square root of frequency because current crowds into a thinner skin of the conductor as frequency rises. Dielectric loss grows linearly with frequency. So the standard cable model has a √f term plus an f term, and cable that is fine at HF can be unusable at 1.2 GHz.

Is RG-58 good enough for my antenna run?

For short runs at low frequencies, yes: 25 feet of RG-58 at 28 MHz loses about 0.7 dB. The same cable in a 100-foot run at 446 MHz loses roughly 10 dB, over 90 percent of the power. As a rule of thumb, RG-58 belongs in patch cables and mobile installs, not in long fixed runs above VHF.

How much loss do connectors add?

A clean, properly installed pair of quality UHF or N connectors adds on the order of 0.05 to 0.2 dB at VHF/UHF, small compared to cable loss. The practical danger is not good connectors but bad ones: corroded, water-contaminated, or poorly soldered joints can add several dB and create intermittent faults that look like anything but a connector.

Sources

  1. LMR-400 Flexible Low Loss Communications Coax — product data sheet. Times Microwave Systems. Publishes the two-term attenuation model dB/100 ft = K1·√F + K2·F, and the coefficients 0.122290 and 0.000260 used verbatim by the LMR-400 preset; the companion LMR-240 sheet supplies 0.242080 and 0.000330.
  2. Belden 8259 technical data sheet — 50 Ω wireless transmission coax, RG-58 type. Belden. catalog.belden.comA worked example of published catalog attenuation against frequency for RG-58: the class of data the RG-58 preset approximates, and a reminder of how far branded cable can sit from a generic fit.
  3. Belden 9258 technical data sheet — 50 Ω wireless transmission coax, RG-8X type. Belden. catalog.belden.comCatalog attenuation against frequency for a representative RG-8X cable, of the kind the RG-8X preset is fitted to.
  4. Belden 8267 technical data sheet — 50 Ω mil-spec coax, RG-213 type. Belden. catalog.belden.comCatalog attenuation against frequency for a representative RG-213 cable, of the kind the RG-213 preset is fitted to.
  5. Belden 8216 technical data sheet — 50 Ω wireless transmission coax, RG-174 type. Belden. catalog.belden.comCatalog attenuation against frequency for a representative RG-174 cable, the lossiest type in the list.
  6. Belden 1694A technical data sheet — 75 Ω SDI coax, RG-6 type. Belden. catalog.belden.comCatalog attenuation against frequency for a representative RG-6 cable, and confirmation that RG-6 is a 75 Ω cable: the mismatch the last tip weighs against its low loss.
  7. Microwave Engineering, 4th edition. David M. Pozar, Wiley, 2011. wiley.comDerives why coaxial conductor loss grows as √f through the skin effect while dielectric loss grows linearly with f: the physical reason the model needs two terms.

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