One option in determining where to best place your AirPort base station is to measure its signal quality from a wireless client. In theory, where the signal quality is best would make for ideal candidates for placement. This would hold true for a single or multiple base stations in an extended wireless network.
One method to calculate signal quality is to compute the Signal-to-Noise Ratio (SNR). SNR is the signal level (in dBm) minus the noise level (in dBm). Both of these values are typically represented as negative numbers. For example, a signal level of -53dBm and a noise value of -90dBm would yield an SNR of 37dB (i.e., SNR = Signal - Noise = -53 -(-90) = 37)
The calculated SNR value, as measured from a wireless client, would decrease as the range to the base station increases due to applicable free space loss. Also an increase in RF interference from microwave ovens, cordless phones, walls, ceilings, etc, which would increase the noise level, would also decrease the overall SNR value.
- 40+dB = Excellent signal
- 25dB to 40dB = Very good signal
- 15dB to 25dB = Low signal
- 10dB to 15dB = Very low signal
- 5dB to 10dB = Little or no signal
The following are four methods to take signal & noise level readings:
Method 1 - Using iStumbler
Download and install a copy of iStumbler. Use iStumbler's Inspector feature (select Edit > Inspector from the iStumbler's menu) to take the signal and noise level readings.
Method 2 - Using Wi-Fi Explorer
Download and install a copy of Wi-Fi Explorer. Run Wi-Fi Explorer. Select the wireless network that you want to test. Select the Network Details tab. Note the value for Signal-to-Noise Ratio.
Method 3 - Using OS X System Profiler
For OS X Tiger (10.4) through Snow Leopard (10.6):
Click on the Apple icon on the menu bar > About This Mac > More Info... > Contents > Network > AirPort > Interfaces > en1 > Current Network Information > Find your wireless network > Signal / Noise
For OS X Lion (10.7) and Mountain Lion (10.8):
Click on the Apple icon on the menu bar > About This Mac > More Info... > System Report... > Network > Wi-Fi > Interfaces > en1 > Current Network Information > Find your wireless network > Signal / Noise. (Note: Alternatively, for OS X Mountain Lion, you can access Wi-Fi Diagnostics directly from the Wi-Fi icon on the OS X menu bar. Once the application opens, use Command+N to open the Network Utilities window. From there click on the Wi-Fi Scan tab.)
For OS X Mavericks (10.9) through macOS Sierra (10.12):
Click on the Apple icon on the menu bar > About This Mac > System Report... > Network > Wi-Fi > Interfaces > en1 > Current Network Information > Find your wireless network > Signal / Noise
Method 4 - Using Wi-Fi / Wireless Diagnostics
For OS X Mavericks (10.9) through macOS Sierra (10.12) option-click on the AirPort icon. Select Open Wireless Diagnostics ... Do NOT select Continue; Instead from the Wireless Diagnostics menu bar, select Window, and then, select Performance. The Quality value represents SNR.
For OS X Lion (10.7) the Wi-Fi Diagnostics app is located in: /System/Library/CoreServices/Wi-Fi Diagnostics.app
Run Wi-Fi Diagnostics > Select "Monitor Performance" > Continue > The Signal & Noise values will be shown in the Performance table.
For OS X Mountain Lion (10.8) you can access the Wi-Fi Diagnostics directly by Option-clicking the AirPort icon on the OS X menu bar. Once the application open, use Command+N to open the Network Utilities windows. Finally click on the Wi-Fi Scan tab.
Starting in OS X Mountain Lion (10.8.4), Wi-Fi Diagnostics has been replaced with Wireless Diagnostics. To access it, Option-click on the AirPort icon. On the diagnostics menu, select Window > Utilities > Performance
Regardless of which method you use to gather the reading, you would plug them into the SNR formula. The desired goal value is an SNR of 25+dB (35+dB for streaming HD video.) At this SNR value, wireless clients should be getting reasonable throughput performance with the base station. This is also the value you would want at the location where you would want to place a second base station to extend the first if you are planning on extending the wireless range.
If you are getting SNR values of less than 25+dB at the client, either try to locate the source of the Wi-Fi interference or try relocating the base station until they are within a 25+dB range.