Coax Loss Calculator
How much of your power actually reaches the antenna? Pick a cable, enter your run length and frequency. 你的功率到底有多少送到了天线？选馈线型号，输入长度和频率。
Result计算结果
Compare all cables五种馈线对比
Same length and frequency, sorted by total loss. Your selected cable is highlighted.相同长度和频率下五种馈线的损耗对比，按总损耗排序，你选的型号会高亮。
| Cable型号 | dB / 100 ft每百英尺 | Total dB总损耗 | Power %剩余功率 |
|---|
Every meter of coax between your radio and antenna taxes your signal, and the tax rises with frequency. This calculator takes a cable type (RG-58, RG-8X, RG-213, LMR-240 or LMR-400), your run length and your operating frequency, and returns the loss per 100 feet, the total loss, the percentage of power that actually reaches the antenna, and the cost in S-units.
The numbers come from published manufacturer attenuation tables, interpolated on a log-log scale between standard test points — the same method engineers use, since coax loss follows a smooth curve against frequency. The comparison table at the bottom ranks all five cables for your exact length and frequency, which is where the real decisions live: at 146 MHz, 30 metres of RG-213 loses about 2.3 dB, throwing away 41% of a 100 W signal as heat, while LMR-400 cuts that to about 1.5 dB. At 29 MHz the same cables are nearly equal — frequency is what separates them.
Three rules of thumb fall out of the math. First, length beats cable: halving a run always halves the dB loss, while upgrading cable buys a fraction of that. Second, loss hurts both ways — receive signals pay the same tax coming down, which is why a masthead preamp exists. Third, 3 dB is half your power: if the total exceeds 3 dB on your main band, shorten the run or upgrade before touching anything else.
For the numbers behind cable loss — dB, ratios and S-units — the companion tools are the dB Calculator and the SWR Calculator (high SWR adds its own extra loss on top of these figures).
One more practical point: connectors count too. Each adapter or barrel connector adds roughly 0.1–0.3 dB, and a corroded PL-259 on a tower can cost more than ten metres of cable. When a run measures worse than the calculator says, check the ends before blaming the coax itself.
Frequently asked questions
Does doubling cable length double the loss?
Yes. dB loss scales linearly with length, so doubling the run doubles the dB. Halve a 30 m run to 15 m and you halve the loss — often a bigger win than upgrading the cable.
How much better is LMR-400 than RG-213?
Between typical ham cables at HF, the gap is small; at VHF/UHF it grows fast. RG-213 at 146 MHz loses about 2.3 dB per 30 m; LMR-400 loses about 1.5 dB. Choose by your main band and run length — and keep runs short first.
What does 'power at antenna' mean in watts?
Power at antenna = what actually radiates. 100 W into 2.3 dB of coax loss delivers about 59 W to the antenna — the other 41 W warms the cable. Loss also hurts reception the same way on the way down.
How accurate are these loss figures?
The calculator's table uses standard published per-100-ft figures at standard test points and log-log interpolation between them. Your actual cable varies with age, bends, moisture and connectors — treat the result as a planning figure, ±10% is typical.