关于 Wi-Fi 天线作用的实验
关于 Wi-Fi 天线作用的实验
背景
在这学期返校以后，我就立刻霸占了组里的一台 AMD R9 5950X 的机器当作我的工作站（之前的 Dell Vostro 老是编译爆内存烦死我了）。然而，我们很早就发现这台机器有一个问题：Wi-Fi 速度很慢，这对于一张插上这么高档 CPU 的主板来说是很不正常的。此前我们尝试了调整各种软件参数，也实验了改变主机和路由器的角度和距离，但是都没有什么效果。后来我学习了一点业余无线电，在看到了机箱屁股上的 SMA 端子后，我立马意识到，这可能是因为没有天线的问题。
当时我们没有天线去验证这个假设，但是现在它成为了我的主力工作站，我就有动机去自费买两根天线来做这个实验了，于是就有了这篇文章。我买的天线是下面样子：
根据我学到的无线电知识，上面这种天线应该算是半波长偶极子天线，也称为全向增益天线。这种天线的长度非常关键，一般是波长的一半，对于 2.4GHz 的 Wi-Fi 信号来说，波长是 12.5cm，所以天线的长度应该是 6.25cm。知道这一点后，再想一想没有天线时全靠两个 SMA 端子发射的情况，效率是可想而知的低啊！那么到底会差多少呢？趁此机会我们做个实验看一看。
实验
我主要观测了两组数据，一是 nmcli dev wifi 的输出，而是 iperf3 -c 的网速测量结果。
无天线
nmcli dev wifi
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IN-USE BSSID SSID MODE CHAN RATE SIGNAL BARS SECURITY
* 64:79:F0:84:02:D3 GUYUHAO-UBUNTU Infra 6 65 Mbit/s 65 ▂▄▆_ WPA1 WPA2 WPA3
04:E0:B0:F7:51:B5 wr1200js-openwrt Infra 36 270 Mbit/s 49 ▂▄__ WPA3
54:A7:03:77:1F:A5 你工人爷爷_5G Infra 153 405 Mbit/s 45 ▂▄__ WPA1 WPA2
F8:8C:21:49:26:17 xukong Infra 153 270 Mbit/s 42 ▂▄__ WPA1 WPA2
32:13:8B:F9:DE:F5 DIRECT-F5-HP 4104dw LJ Infra 157 65 Mbit/s 40 ▂▄__ WPA2
F4:B7:8D:4F:6D:70 nscc-guest Infra 60 260 Mbit/s 32 ▂▄__ WPA2
F4:B7:8D:4F:6D:64 nscc-team Infra 149 130 Mbit/s 32 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:6D:72 nscc-employee Infra 60 260 Mbit/s 30 ▂___ WPA1 WPA2
F4:B7:8D:4F:6D:74 nscc-team Infra 60 260 Mbit/s 30 ▂___ WPA1 WPA2
F8:8C:21:49:26:15 xukong Infra 11 270 Mbit/s 29 ▂___ WPA1 WPA2
D0:EE:07:54:6F:24 home_5G Infra 44 130 Mbit/s 29 ▂___ WPA1 WPA2
F4:B7:8D:4F:6D:62 nscc-employee Infra 149 130 Mbit/s 27 ▂___ WPA1 WPA2
D4:83:04:D1:DC:A0 FAST_DCA0 Infra 6 270 Mbit/s 20 ▂___ WPA1 WPA2
F4:B7:8D:4F:6D:50 nscc-guest Infra 6 130 Mbit/s 15 ▂___ WPA2
iperf3 -c
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Connecting to host 10.42.0.189, port 5201
[ 5] local 10.42.0.1 port 60872 connected to 10.42.0.189 port 5201
[ ID] Interval Transfer Bitrate Retr Cwnd
[ 5] 0.00-1.00 sec 2.38 MBytes 19.9 Mbits/sec 1 226 KBytes
[ 5] 1.00-2.00 sec 2.00 MBytes 16.8 Mbits/sec 0 317 KBytes
[ 5] 2.00-3.00 sec 2.38 MBytes 19.9 Mbits/sec 0 382 KBytes
[ 5] 3.00-4.00 sec 1.88 MBytes 15.7 Mbits/sec 0 440 KBytes
[ 5] 4.00-5.00 sec 2.12 MBytes 17.8 Mbits/sec 0 542 KBytes
[ 5] 5.00-6.00 sec 2.50 MBytes 21.0 Mbits/sec 0 626 KBytes
[ 5] 6.00-7.00 sec 1.75 MBytes 14.7 Mbits/sec 0 713 KBytes
[ 5] 7.00-8.00 sec 3.12 MBytes 26.2 Mbits/sec 0 766 KBytes
[ 5] 8.00-9.00 sec 1.50 MBytes 12.6 Mbits/sec 0 766 KBytes
[ 5] 9.00-10.00 sec 1.38 MBytes 11.5 Mbits/sec 0 803 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 21.0 MBytes 17.6 Mbits/sec 1 sender
[ 5] 0.00-10.11 sec 18.0 MBytes 14.9 Mbits/sec receiver
iperf Done.
1 根天线
nmcli dev wifi
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IN-USE BSSID SSID MODE CHAN RATE SIGNAL BARS SECURITY
* 64:79:F0:84:02:D3 GUYUHAO-UBUNTU Infra 6 65 Mbit/s 100 ▂▄▆█ WPA1 WPA2 WPA3
D4:EE:07:54:6F:24 home Infra 9 130 Mbit/s 100 ▂▄▆█ WPA1 WPA2
54:A7:03:77:1F:A3 你工人爷爷2_4 Infra 1 405 Mbit/s 97 ▂▄▆█ WPA1 WPA2
04:E0:B0:F7:51:B5 wr1200js-openwrt Infra 36 270 Mbit/s 95 ▂▄▆█ WPA3
54:A7:03:77:1F:A5 你工人爷爷_5G Infra 153 405 Mbit/s 94 ▂▄▆█ WPA1 WPA2
20:76:93:56:5D:AE wyf-wifi Infra 11 130 Mbit/s 92 ▂▄▆█ WPA2
F8:8C:21:49:26:15 xukong Infra 11 270 Mbit/s 85 ▂▄▆█ WPA1 WPA2
D0:EE:07:54:6F:24 home_5G Infra 44 130 Mbit/s 85 ▂▄▆█ WPA1 WPA2
F4:B7:8D:4F:6D:70 nscc-guest Infra 60 260 Mbit/s 85 ▂▄▆█ WPA2
F4:B7:8D:4F:6D:72 nscc-employee Infra 60 260 Mbit/s 85 ▂▄▆█ WPA1 WPA2
F4:B7:8D:4F:6D:74 nscc-team Infra 60 260 Mbit/s 85 ▂▄▆█ WPA1 WPA2
F8:8C:21:49:26:17 xukong Infra 153 270 Mbit/s 85 ▂▄▆█ WPA1 WPA2
60:3A:7C:25:30:64 Sakana Infra 11 405 Mbit/s 82 ▂▄▆█ WPA1 WPA2
F4:B7:8D:4F:6D:50 nscc-guest Infra 6 130 Mbit/s 80 ▂▄▆_ WPA2
20:76:93:56:5D:B0 wyf-wifi_5G Infra 36 270 Mbit/s 80 ▂▄▆_ WPA2
5E:EA:1D:32:AB:39 DIRECT-39-HP M281 LaserJet Infra 6 130 Mbit/s 70 ▂▄▆_ WPA2
F4:B7:8D:4F:6D:64 nscc-team Infra 149 130 Mbit/s 69 ▂▄▆_ WPA1 WPA2
D4:83:04:D1:DC:A0 FAST_DCA0 Infra 6 270 Mbit/s 67 ▂▄▆_ WPA1 WPA2
24:CF:24:ED:CA:D8 Xiaomi_CAD7 Infra 6 130 Mbit/s 64 ▂▄▆_ WPA1 WPA2
F4:B7:8D:4F:6D:A0 nscc-guest Infra 56 260 Mbit/s 64 ▂▄▆_ WPA2
F4:B7:8D:4F:6D:A4 nscc-team Infra 56 260 Mbit/s 64 ▂▄▆_ WPA1 WPA2
F4:B7:8D:4F:6D:A2 nscc-employee Infra 56 260 Mbit/s 64 ▂▄▆_ WPA1 WPA2
F4:B7:8D:4F:6D:94 nscc-team Infra 165 130 Mbit/s 64 ▂▄▆_ WPA1 WPA2
74:EA:C8:86:3F:D8 PARATERA-GZ Infra 6 270 Mbit/s 60 ▂▄▆_ WPA1 WPA2
F4:B7:8D:4F:6E:50 nscc-guest Infra 64 260 Mbit/s 55 ▂▄__ WPA2
F4:B7:8D:4F:6E:54 nscc-team Infra 64 260 Mbit/s 54 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:6E:52 nscc-employee Infra 64 260 Mbit/s 54 ▂▄__ WPA1 WPA2
D0:05:E4:76:AE:D7 HUAWEI-B311-AED7 Infra 13 130 Mbit/s 49 ▂▄__ WPA2
F4:B7:8D:4F:67:A2 nscc-employee Infra 44 260 Mbit/s 49 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:6E:E2 nscc-employee Infra 52 260 Mbit/s 49 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:68:50 nscc-guest Infra 40 260 Mbit/s 47 ▂▄__ WPA2
F4:B7:8D:4F:68:52 nscc-employee Infra 40 260 Mbit/s 47 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:67:A0 nscc-guest Infra 44 260 Mbit/s 47 ▂▄__ WPA2
F4:B7:8D:4F:67:A4 nscc-team Infra 44 260 Mbit/s 47 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:6E:E4 nscc-team Infra 52 260 Mbit/s 47 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:6E:E0 nscc-guest Infra 52 260 Mbit/s 47 ▂▄__ WPA2
F4:B7:8D:4F:68:54 nscc-team Infra 40 260 Mbit/s 44 ▂▄__ WPA1 WPA2
32:13:8B:F9:DE:F5 DIRECT-F5-HP 4104dw LJ Infra 157 65 Mbit/s 40 ▂▄__ WPA2
F4:B7:8D:4F:6D:52 nscc-employee Infra 6 130 Mbit/s 20 ▂___ WPA1 WPA2
F4:B7:8D:4F:6D:80 nscc-guest Infra 1 130 Mbit/s 19 ▂___ WPA2
iperf3 -c
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Connecting to host 10.42.0.189, port 5201
[ 5] local 10.42.0.1 port 40158 connected to 10.42.0.189 port 5201
[ ID] Interval Transfer Bitrate Retr Cwnd
[ 5] 0.00-1.00 sec 6.00 MBytes 50.3 Mbits/sec 0 296 KBytes
[ 5] 1.00-2.00 sec 4.88 MBytes 40.9 Mbits/sec 0 345 KBytes
[ 5] 2.00-3.00 sec 5.00 MBytes 41.9 Mbits/sec 0 345 KBytes
[ 5] 3.00-4.00 sec 4.25 MBytes 35.7 Mbits/sec 0 345 KBytes
[ 5] 4.00-5.00 sec 4.62 MBytes 38.8 Mbits/sec 0 382 KBytes
[ 5] 5.00-6.00 sec 3.88 MBytes 32.5 Mbits/sec 0 382 KBytes
[ 5] 6.00-7.00 sec 1.62 MBytes 13.6 Mbits/sec 2 290 KBytes
[ 5] 7.00-8.00 sec 2.75 MBytes 23.1 Mbits/sec 1 225 KBytes
[ 5] 8.00-9.00 sec 1.62 MBytes 13.6 Mbits/sec 8 124 KBytes
[ 5] 9.00-10.00 sec 2.62 MBytes 22.0 Mbits/sec 1 102 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 37.2 MBytes 31.2 Mbits/sec 12 sender
[ 5] 0.00-10.04 sec 35.5 MBytes 29.7 Mbits/sec receiver
iperf Done.
2 根天线
nmcli dev wifi
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IN-USE BSSID SSID MODE CHAN RATE SIGNAL BARS SECURITY
54:A7:03:77:1F:A3 你工人爷爷2_4 Infra 1 405 Mbit/s 100 ▂▄▆█ WPA1 WPA2
* 64:79:F0:84:02:D3 GUYUHAO-UBUNTU Infra 6 65 Mbit/s 100 ▂▄▆█ WPA1 WPA2 WPA3
D4:EE:07:54:6F:24 home Infra 9 130 Mbit/s 100 ▂▄▆█ WPA1 WPA2
04:E0:B0:F7:51:B5 wr1200js-openwrt Infra 36 270 Mbit/s 100 ▂▄▆█ WPA3
D0:EE:07:54:6F:24 home_5G Infra 44 130 Mbit/s 89 ▂▄▆█ WPA1 WPA2
F8:8C:21:49:26:15 xukong Infra 11 270 Mbit/s 85 ▂▄▆█ WPA1 WPA2
F4:B7:8D:4F:6D:70 nscc-guest Infra 60 260 Mbit/s 82 ▂▄▆█ WPA2
F4:B7:8D:4F:6D:74 nscc-team Infra 60 260 Mbit/s 82 ▂▄▆█ WPA1 WPA2
F4:B7:8D:4F:6D:72 nscc-employee Infra 60 260 Mbit/s 80 ▂▄▆_ WPA1 WPA2
20:76:93:56:5D:B0 wyf-wifi_5G Infra 36 270 Mbit/s 77 ▂▄▆_ WPA2
F4:B7:8D:4F:6D:80 nscc-guest Infra 1 130 Mbit/s 74 ▂▄▆_ WPA2
5E:EA:1D:32:AB:39 DIRECT-39-HP M281 LaserJet Infra 6 130 Mbit/s 74 ▂▄▆_ WPA2
24:CF:24:ED:CA:D8 Xiaomi_CAD7 Infra 6 130 Mbit/s 72 ▂▄▆_ WPA1 WPA2
F4:B7:8D:4F:6D:64 nscc-team Infra 149 130 Mbit/s 72 ▂▄▆_ WPA1 WPA2
74:EA:C8:86:3F:D8 PARATERA-GZ Infra 6 270 Mbit/s 67 ▂▄▆_ WPA1 WPA2
F4:B7:8D:4F:6D:A0 nscc-guest Infra 56 260 Mbit/s 64 ▂▄▆_ WPA2
F4:B7:8D:4F:6D:A4 nscc-team Infra 56 260 Mbit/s 64 ▂▄▆_ WPA1 WPA2
F4:B7:8D:4F:6D:A2 nscc-employee Infra 56 260 Mbit/s 64 ▂▄▆_ WPA1 WPA2
54:A7:03:77:1F:A5 你工人爷爷_5G Infra 153 405 Mbit/s 55 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:6E:54 nscc-team Infra 64 260 Mbit/s 54 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:6E:50 nscc-guest Infra 64 260 Mbit/s 52 ▂▄__ WPA2
F4:B7:8D:4F:6E:52 nscc-employee Infra 64 260 Mbit/s 52 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:68:54 nscc-team Infra 40 260 Mbit/s 47 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:68:52 nscc-employee Infra 40 260 Mbit/s 47 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:6E:E2 nscc-employee Infra 52 260 Mbit/s 47 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:6E:E0 nscc-guest Infra 52 260 Mbit/s 47 ▂▄__ WPA2
F4:B7:8D:4F:6E:E4 nscc-team Infra 52 260 Mbit/s 47 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:6E:32 nscc-employee Infra 1 130 Mbit/s 45 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:68:50 nscc-guest Infra 40 260 Mbit/s 45 ▂▄__ WPA2
F4:B7:8D:4F:67:A0 nscc-guest Infra 44 260 Mbit/s 44 ▂▄__ WPA2
F4:B7:8D:4F:67:A2 nscc-employee Infra 44 260 Mbit/s 44 ▂▄__ WPA1 WPA2
F4:B7:8D:4F:67:A4 nscc-team Infra 44 260 Mbit/s 42 ▂▄__ WPA1 WPA2
32:13:8B:F9:DE:F5 DIRECT-F5-HP 4104dw LJ Infra 157 65 Mbit/s 40 ▂▄__ WPA2
20:76:93:56:5D:AE wyf-wifi Infra 11 130 Mbit/s 32 ▂▄__ WPA2
iperf3 -c
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Connecting to host 10.42.0.189, port 5201
[ 5] local 10.42.0.1 port 47524 connected to 10.42.0.189 port 5201
[ ID] Interval Transfer Bitrate Retr Cwnd
[ 5] 0.00-1.00 sec 4.50 MBytes 37.7 Mbits/sec 0 608 KBytes
[ 5] 1.00-2.00 sec 2.75 MBytes 23.1 Mbits/sec 11 301 KBytes
[ 5] 2.00-3.00 sec 5.25 MBytes 44.0 Mbits/sec 0 335 KBytes
[ 5] 3.00-4.00 sec 5.00 MBytes 41.9 Mbits/sec 0 351 KBytes
[ 5] 4.00-5.00 sec 5.00 MBytes 41.9 Mbits/sec 0 359 KBytes
[ 5] 5.00-6.00 sec 5.12 MBytes 43.0 Mbits/sec 0 361 KBytes
[ 5] 6.00-7.00 sec 4.88 MBytes 40.9 Mbits/sec 0 361 KBytes
[ 5] 7.00-8.00 sec 5.12 MBytes 43.0 Mbits/sec 0 362 KBytes
[ 5] 8.00-9.00 sec 5.38 MBytes 45.1 Mbits/sec 0 368 KBytes
[ 5] 9.00-10.00 sec 5.12 MBytes 43.0 Mbits/sec 0 379 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 48.1 MBytes 40.4 Mbits/sec 11 sender
[ 5] 0.00-10.02 sec 45.2 MBytes 37.9 Mbits/sec receiver
iperf Done.
结论
可以非常显著地看到，有天线和没天线是质的区别：在没天线的时候，几乎搜不到什么 AP，少数几个 AP 连接质量也很差，网速只有区区 17.6 Mbits/sec；在有 1 根天线的时候，AP 列表立马就变长了，质量也变满了，网速也达到了 31.2 Mbits/sec；2 根天线的时候相较于 1 根天线提升没那么大，但是也能明显看到信号质量的普遍提升，网速也达到了 40.4 Mbits/sec。综上，这个实验证实了我此前的猜测，说明 Wi-Fi 天线对于信号质量和网速的提升是非常重要的，一定要买！
本文由作者按照 CC BY 4.0 进行授权