Coax Loss Calculator
Estimate how much power a coaxial feed-line loses at a given frequency and length, including the extra loss from SWR at the antenna.
Matched loss
Every coaxial cable turns some of the signal into heat, in the conductors and in the dielectric. The loss is quoted in decibels per 100 ft or per 100 m with the cable terminated in its own impedance, the matched loss, and it rises with frequency: conductor loss grows with the square root of frequency and dielectric loss in proportion to it. Typical figures for new cable per 100 ft at 14.2 MHz are 1.7 dB for RG-58, 1.3 dB for RG-8X, 0.7 dB for RG-213 and 0.5 dB for LMR-400. At 146 MHz in the 2 m band, the same cables lose 5.6, 4.5, 2.4 and 1.5 dB. The calculator interpolates between published values for each amateur band, from 160 m to 23 cm.
How SWR adds loss
With a mismatch at the antenna, the standing wave makes the current and voltage on the line higher at some points, and the line loses more. The ARRL handbook gives the total loss as 10 log₁₀[(a² − |Γ|²) ÷ (a(1 − |Γ|²))], where a is the matched loss as a power ratio and Γ the reflection coefficient at the load. For 100 ft of RG-8X at 14.2 MHz with a 2:1 SWR at the antenna, the matched loss is 1.30 dB and the total 1.54 dB, so the SWR costs an extra 0.24 dB. Of 100 W at the radio, 70.2 W reaches the antenna. On 2 m the picture is very different: 100 ft of RG-58 at 146 MHz loses 5.76 dB at a 1.5:1 SWR, delivering only 26.5 W, while LMR-400 delivers 69.4 W. The cable matters far more than the SWR.
The SWR you see is not the SWR at the antenna
Line loss attenuates the reflected wave twice, on its way up and back, so an SWR meter at the radio reads lower than the SWR at the antenna. With 1 dB of matched loss and a 3:1 SWR at the antenna, the radio sees 2.32:1. With a long run of lossy cable, a badly mismatched or even broken antenna can show a deceptively good SWR at the shack end, because the cable is acting as an attenuator. A common mistake is to treat a low SWR on a long, thin cable as proof of a good antenna; check the matched loss before trusting it.
Choosing a cable
For short HF runs of 50 ft or less, RG-8X or even RG-58 loses well under a decibel and is light and flexible. For long HF runs, or anything at VHF and UHF, low-loss cable such as RG-213 or LMR-400 pays for itself: every 3 dB saved doubles the power at the antenna and the received signal. Weather matters as well: water in the cable or connectors raises loss dramatically over time, so seal outdoor connectors and replace cable that has been wet.
How to use it
- Choose the cable type and enter the operating frequency.
- Enter the cable length in feet.
- Enter the SWR at the antenna end and your transmitter power.
- Read the matched loss, the extra loss from SWR, the power at the antenna and the SWR seen at the radio.
Questions from the shack
How much power do I lose in 100 feet of coax?
It depends on the cable and frequency. At 14.2 MHz, 100 ft of RG-8X loses about 1.3 dB, so roughly 74 W of 100 W reaches a matched antenna. At 146 MHz, 100 ft of RG-58 loses about 5.6 dB, leaving only about 28 W. Low-loss cable such as LMR-400 loses about 1.5 dB at 146 MHz.
How much extra loss does high SWR cause in coax?
It depends on the matched loss. On 100 ft of RG-8X at 14.2 MHz, a 2:1 SWR adds about 0.24 dB to the 1.30 dB matched loss. On a lossy line at VHF the extra loss is larger in decibels, but the matched loss is usually the bigger problem.
Why does my SWR look better at the radio than at the antenna?
Because cable loss attenuates the reflected signal on its way back. With 1 dB of matched loss, a 3:1 SWR at the antenna reads about 2.3:1 at the radio. A long, lossy cable can make even a faulty antenna look well matched.
Is RG-58 good enough for ham radio?
For short runs on HF, yes: 25 ft of RG-58 loses well under half a decibel on 20 m. For long runs or VHF and UHF, its loss becomes significant, more than 5 dB per 100 ft at 146 MHz, and RG-213 or LMR-400 is a much better choice.
Disclaimer: Radio results are calculations from handbook formulas and typical cable data for the values you enter. They are not a substitute for measuring your installation, an RF exposure evaluation or your licence conditions. Trim antennas with an analyser, transmit only within your licence privileges, and keep antennas well clear of power lines; installing antennas and towers can be dangerous.