Your radio puts out 50 watts, but how many of those watts actually leave the antenna? Between the radio and the antenna sits your coax cable, and every meter of it turns some of your RF into heat. The Coax Loss Calculator tells you exactly how much: pick a cable type, enter your run length and frequency, and it shows the total loss in dB, the percentage of power that reaches the antenna, and the loss in S-units. It is free, needs no sign-up, and is useful for any amateur radio operator planning a feedline.

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Attenuation: Why Coax Cable Eats Your Signal

A coaxial cable is a transmission line: a center conductor, a dielectric, and a shield carrying RF between your radio and antenna. No cable is perfect — some power is always absorbed as heat in the conductor and dielectric. This loss, called attenuation, grows with both length and frequency, which is why the same 30 meters of cable can be nearly invisible on HF but costly at 446 MHz. The calculator’s values come from typical manufacturer attenuation tables (Times Microwave, Belden class), interpolated to your exact frequency.

The Inputs

Cable — a selector with five types: RG-58, RG-8X, RG-213 (the default), LMR-240, and LMR-400.
Length — a number field (default 30), accepted from 0.1 up to 10000, with a unit selector for meters (default) or feet.
Frequency (MHz) — a number field (default 146), accepted from 0.5 to 6000 MHz.
Calculate button — labeled Calculate / 计算. Pressing Enter inside the length or frequency field runs the calculation too. The tool also runs once automatically when the page loads, so the default result is visible immediately.
Example chips — three presets under the button: RG-213 · 30 m · 146 MHz, LMR-400 · 20 m · 446 MHz, and RG-58 · 15 m · 29 MHz. Tapping one fills all three inputs and runs the calculation instantly.

If an input is out of range, the tool shows a plain-language error (for example, “Enter a length greater than 0” or “Frequency must be 0.5–6000 MHz”) instead of a result.

Walkthrough: 30 Meters of RG-213 at 146 MHz

The default view already answers a very common question: you have 30 meters of RG-213 running to a 2-meter antenna at 146 MHz. Click Calculate (or just read the auto-run result):

The result heading reads RG-213 · 30 m · 146 MHz — a summary of your inputs.
dB / 100 ft: 2.35 — the cable’s attenuation normalized to a standard length.
Total loss: 2.31 dB — the loss across your whole 30-meter run.
Power at antenna: 58.7% — barely half of your transmitter power reaches the antenna.
S-units lost: 0.4 — the loss expressed the way signal reports work (1 S-unit ≈ 6 dB).

The takeaway is immediate: at 146 MHz, 30 meters of RG-213 costs you about 2.3 dB — noticeable but acceptable for FM work. Now scroll to the comparison table and you will see the same run with LMR-400 would lose only 1.46 dB (71.5% reaches the antenna), while RG-58 would lose 5.67 dB (only 27.1% reaches the antenna).

How to Read the Result

dB / 100 ft — attenuation per 100 feet at your frequency. Useful for comparing cable specs and doing quick mental math for other lengths.
Total loss (dB) — the headline number: dB/100 ft scaled to your actual run length. Decibels are logarithmic, so 3 dB means half your power is gone.
Power at antenna (%) — the same loss expressed as a percentage of transmitter power delivered to the antenna, computed as 100 × 10^(−dB/10).
S-units lost — total dB divided by 6, because one S-unit on the receiver’s meter is roughly 6 dB. It answers “will I notice this?” — 0.4 S-units is subtle; 2 S-units is very noticeable.
Compare all cables — a table running all five cable types through your length and frequency, sorted by total loss, with your selected cable highlighted. This is the fastest way to decide whether upgrading the cable is worth it before you buy.

When a Ham Radio Operator Needs This: Feedline Choices for HF and VHF/UHF

Choosing feedline for an HF antenna. Compare RG-58 against RG-213 at 29 MHz for a 15-meter run before you buy: the table shows you whether the cheaper cable is good enough.
Long VHF/UHF runs. At 146 or 446 MHz, attenuation per meter is several times higher than on HF — run your numbers before routing coax across the roof or up a tower.
Deciding between more power and better coax. Losing 5.67 dB on RG-58, switching to LMR-400 (1.46 dB) buys you more signal than doubling your transmitter power — and costs less than a new rig.
Weak-signal and DX work. When you are chasing faint signals, every decibel at the antenna matters. Size the feedline so the loss stays under about 2–3 dB.
Setting expectations with an SWR meter. If your measured signal is weaker than expected, the calculator tells you how much of the difference is simply the coax, before you start suspecting the antenna or the SWR meter reading.

Limitations

Theoretical estimates, not measurements. Values come from typical manufacturer attenuation tables, interpolated to your frequency. Your actual cable varies with age, bending, water ingress, and how the connectors were installed.
Connectors are not included. Every connector in the run adds its own small loss; the calculator models the cable alone.
No mismatch loss. The tool assumes a matched system — it does not add extra loss from high SWR. Measure SWR separately with an SWR meter or antenna analyzer.
Input limits. Frequency is limited to 0.5–6000 MHz and length to 10000 units; the result assumes the whole run is one cable type.

Frequently Asked Questions

How accurate is the Coax Loss Calculator for ham radio feedlines?

It is a solid planning figure, not a lab measurement. The tool interpolates typical manufacturer attenuation values (Times Microwave, Belden class) to your exact frequency and scales by your run length. Real-world loss varies with connectors, installation, and measurement conditions — to know your actual numbers, measure your own run.

Which of the five cables has the lowest loss?

At any given length and frequency, LMR-400 loses the least of the five, followed by RG-213 and LMR-240, then RG-8X, with RG-58 losing the most. Scroll to the comparison table to see the exact ranking for your inputs — the answer flips less than you might think at HF, where all five are fairly close.

Does cable length matter as much as cable type?

Yes — loss scales linearly with length, so doubling your run doubles the dB loss. A short run of RG-58 can easily outperform a long run of LMR-400. Enter your real length before judging a cable.

Why does the same cable lose more at higher frequencies?

Attenuation in coax rises with frequency — the signal’s energy concentrates closer to the conductor surfaces and dielectric losses increase. That is why a run that is fine on HF can be a real problem at UHF, and why the calculator needs your frequency, not just the cable.

Does the calculator include connector loss or SWR effects?

No. It models the cable alone, assuming a matched feedline. Connectors add a small amount of loss each, and high SWR adds mismatch loss on top — for those, measure with an SWR meter or antenna analyzer.

Related Tools

Repeater Lookup — feedline loss bites hardest at VHF/UHF, exactly where repeaters live; find a repeater, then check whether your coax will deliver enough signal to reach it.
Band Conditions — after sizing your HF feedline, check live propagation before you get on the air.
Greyline & Sun Times — when your feedline is already costing you decibels, time your operating around the greyline, when low-band DX propagation peaks.

Disclaimer

Loss figures are theoretical estimates based on typical manufacturer attenuation values (Times Microwave, Belden class), provided for reference and education only. Real-world loss varies with connectors, installation, and measurement conditions. Always measure your own feedline run — for example with an SWR meter or antenna analyzer — before judging your antenna system or changing your station setup.

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馈线损耗计算器使用指南：你的功率到底有多少送到了天线

你的电台输出 50 瓦，但真正从天线发射出去的有多少瓦？电台和天线之间是你的同轴馈线，每一米都会把一部分射频变成热量。馈线损耗计算器能告诉你具体损多少：选馈线型号，输入走线长度和频率，它会给出总损耗（dB）、到达天线的功率百分比，以及换算成的 S 值。免费使用，无需注册，任何规划馈线的业余无线电爱好者都用得上。

👉 试用馈线损耗计算器

想看看其他工具？去工具箱总览页浏览全部工具。

衰减：为什么同轴馈线会吃掉你的信号

同轴电缆是一条传输线：中心导体、绝缘介质、屏蔽层，负责把射频在电台和天线之间传送。没有完美的电缆——一部分功率总会被导体和介质吸收变成热量。这种损耗叫衰减，随长度和频率一起增长：同样 30 米的电缆，在 HF 上几乎可以忽略，在 446 MHz 上却可能代价不小。计算器的数值来自典型厂家衰减表（Times Microwave、Belden 同级），按你输入的频率插值算出。

输入项说明

馈线型号（Cable）——下拉选择五种型号：RG-58、RG-8X、RG-213（默认）、LMR-240、LMR-400。
长度（Length）——数字输入（默认 30），范围 0.1 到 10000，单位可选米（默认）或英尺。
频率（MHz）——数字输入（默认 146），范围 0.5 到 6000 MHz。
计算按钮——标着 Calculate / 计算。在长度或频率框里按回车也会直接计算。页面加载时会自动算一次，所以默认结果一打开就能看到。
示例 chips——按钮下方三个预设：RG-213 · 30 m · 146 MHz、LMR-400 · 20 m · 446 MHz、RG-58 · 15 m · 29 MHz。点一下会自动填好三个输入并立即计算。

输入超出范围时，工具会显示通俗的报错（比如”请输入大于 0 的长度””频率范围 0.5–6000 MHz”），而不是给出结果。

实操演示：30 米 RG-213 在 146 MHz

默认视图回答的就是一个很常见的问题：30 米 RG-213 接到 146 MHz 的 2 米波段天线。点计算（或直接看自动算好的结果）：

结果标题显示 RG-213 · 30 m · 146 MHz——这是你输入的汇总。
dB / 100 ft：2.35——该电缆在此频率下、折算到标准长度的衰减。
总损耗（Total loss）：2.31 dB——整条 30 米走线的损耗。
到达天线功率（Power at antenna）：58.7%——发射机功率只有一半多一点真正到了天线。
损失的 S 值（S-units lost）：0.4——按信号报告的习惯换算（1 个 S 单位约 6 dB）。

结论很直观：146 MHz 下 30 米 RG-213 损耗约 2.3 dB，做 FM 通联可以接受。再往下拉看对比表：同样走线用 LMR-400 只损 1.46 dB（71.5% 到达天线），用 RG-58 则损 5.67 dB（只有 27.1% 到达天线）。

怎么看懂计算结果

dB / 100 ft——该频率下每 100 英尺的衰减。方便对照电缆规格书，也方便心算其他长度。
总损耗（dB）——核心数字：按你的实际长度折算后的损耗。分贝是对数单位，3 dB 就意味着一半功率没了。
到达天线功率（%）——同一损耗换算成百分比：送到天线的功率占发射机功率的比例，公式是 100 × 10^(−dB/10)。
损失的 S 值——总 dB 除以 6，因为接收机表头上 1 个 S 单位约等于 6 dB。它回答的是”影响大不大”：0.4 个 S 单位几乎察觉不到，2 个 S 单位就很明显了。
五种馈线对比（Compare all cables）——把五种型号按你的长度和频率各算一遍，按总损耗从小到大排序，你选的型号会高亮。买线之前，这是最快判断”换粗线值不值”的方法。

什么时候用得上：HF 与 VHF/UHF 的馈线选择

给 HF 天线选馈线。 买线之前，先把 RG-58 和 RG-213 在 29 MHz、15 米走线下比一比：对比表会告诉你便宜的线够不够用。
长距离 VHF/UHF 走线。 146 或 446 MHz 下每米衰减是 HF 的好几倍——在屋顶布线或上塔之前，先把数字算出来。
加功率还是换好线。 RG-58 损 5.67 dB 的情况下，换成 LMR-400（1.46 dB）带来的信号提升，超过把发射功率翻倍——花的钱还比买新电台少。
弱信号与 DX 通联。 追微弱信号时，天线端的每一分贝都重要。把馈线损耗控制在 2–3 dB 以内再上机。
配合驻波表判断。 如果实测信号比预期弱，先用计算器算出馈线本身该损多少，再去怀疑天线或驻波表读数。

局限性

理论估算，不是实测。 数值来自典型厂家衰减表，按频率插值、按长度折算。实际电缆的损耗会随老化、弯折、进水、接头做工而变化。
不含接头损耗。 走线上的每个接头都有自己的小损耗，计算器只算电缆本身。
不含失配损耗。 工具假设系统是匹配的——高驻波带来的额外损耗不在内。请用驻波表或天线分析仪单独测量驻波。
输入范围限制。 频率限 0.5–6000 MHz，长度限 10000；默认整条走线是同一种电缆。

常见问题

这个计算器算馈线损耗准吗？

做规划够准，做实验不够。它按典型厂家衰减表（Times Microwave、Belden 同级）插值到你的精确频率，再按走线长度折算。真实损耗随接头、施工、测量条件变化——想知道准确数字，请实测自己的馈线。

五种线里哪种损耗最小？

同样长度和频率下，LMR-400 损耗最小，其次是 RG-213 和 LMR-240，再次是 RG-8X，RG-58 损耗最大。拉到对比表看你输入下的精确排名——在 HF 频段五种线差距比想象中小。

长度和线型，哪个影响更大？

两个都大——损耗和长度成正比，走线翻倍、dB 数就翻倍。短距离的 RG-58 完全可能跑赢长距离的 LMR-400。评判一条线之前，先填上你的真实长度。

为什么同样的线，频率越高损耗越大？

同轴电缆的衰减随频率上升——信号能量越集中在导体表面附近，介质损耗也越大。所以 HF 上没问题的走线，到 UHF 可能就是大问题；计算器要你填频率，原因就在这里。

计算器包含接头损耗和驻波影响吗？

不包含。它只算电缆本身，且假设馈线是匹配的。接头各有小损耗，高驻波会额外增加失配损耗——这两项请用驻波表或天线分析仪实测。

免责声明

损耗数值是基于典型厂家衰减表（Times Microwave、Belden 同级）的理论估算，仅供参考与学习。真实损耗随接头、施工、测量条件变化。评判天线系统或改动电台配置之前，请实测自己的馈线走线——比如用驻波表或天线分析仪。

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Coax Loss Calculator: How Much of Your Power Reaches the Antenna