如果你准备在路由器上添加天线以扩展家庭的Wi-Fi范围，你可以使用多长时间的电缆？电缆的长度是否重要？今天的超级用户问答帖子回答了一位好奇的读者的问题。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

照片由泰勒·尼恩豪斯（Flickr）提供。

问题

超级用户阅读器Searock想知道每英尺天线电缆长度会损失多少Wi-Fi信号强度：

I am thinking about buying an antenna for my router so that I can extend the Wi-Fi range in my home. I have been looking at some products like this one, for example: TP-Link TL-ANT2405C Indoor Desktop Omni-directional Antenna

The length of the cable is 130 centimeters (~51 inches). Is it OK if I increase the length of the cable or will it affect the potential range? What is the maximum length of cable that I can use?

每英尺天线电缆长度实际损失多少Wi-Fi信号强度？

答案

超级用户贡献者Jamie Hanrahan为我们提供了答案：

There is no arbitrary limit, but any increase in cable length will reduce signal strength. The connectors required to add another section of cable to the one shown will also have the same effect. As other commenters have noted, how much the signal strength is reduced for a given length depends on the cable and the frequency.

A common, relatively inexpensive cable for short runs to a Wi-Fi antenna is LMR100. At 2.4 GHz (the common Wi-Fi band), 15 feet of LMR100 will result in a signal loss of about 6 dB. That is equivalent to dropping power down to just about 25 percent of what it was (each 3 dB equals a 50 percent gain or loss in power). With LMR400 cable, your loss would only be about 1 dB, but it is more expensive and also a lot less flexible (more difficult to install).

The loss in dB is linear with a cable’s length. If you use 30 feet of LMR100 cable, the loss will be 12 dB (the signal will be about 1/16 of what it was). With 7.5 feet of LMR100 cable, the loss will only be 3 dB (about half of the signal’s strength). All of these numbers are for the 2.4 GHz Wi-Fi band. For the 5 GHz Wi-Fi band, it will be much worse.

Do not even think about using RG59 (the older, thinner coax cable that was used for TV cable/antennas and is commonly seen with “F” or “BNC” connectors attached; it is not even the right impedance) or RG58 (which has the right impedance, but is still very “lossy” at these frequencies). These cable types are not rated at all for use above 1 GHz.

You can find data sheets with signal loss graphs and calculators for various types of microwave coax cable all over the Internet. Here is a calculator (found at a cable dealer’s website) that covers a wide variety of cable types. And to convert dB to power ratios (or back), try this decibel calculator. Keep in mind that since this is signal loss we are talking about, make sure to enter the dB as a negative number before pressing the calculate button. Also note that you want the power ratio, not the voltage.

One last tip. Do not try to assemble cables yourself. Buy cables with the right connectors already attached. What may seem like very minor mistakes with connector assembly can cause huge losses at these frequencies. And absolutely do not cut the connectors off and try to splice the coax. You might as well throw the antenna away at that point.

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