The AirPort Signal tool is designed to help you measure and compare wireless performance to facilitate adjustment. Leave the "Network Name" field empty to scan for open networks in range. Specify a Network Name to scan for stations in a "closed" network or limit the scan to matching stations. Use the "Merge Similar Names" field to list a single entry for each network showing the strongest base station detected. Use the Limit field to specify how many times to scan (0 for unlimited). Use the Delay field to specify the delay in seconds between scans. Use "Delete" to remove selected entries from the table.

Interpreting the Results

During a wireless scan, the AirPort Signal tool creates a table row for each station detected with information about the signals it received.

First, the signals are identified by the time of the scan, the network name (SSID), the channel (1-14 [2.4 GHz (802.11b/g/n)], 36-165 [5 GHz (802.11a/h/j/n)]), and type of signal. The type allows you to distinguish between normal infrastructure Access Points (AP), computer-to-computer (adhoc) networks, and networks using Wired Equivalent Privacy (WEP) or Wi-Fi Protected Access (WPA).

Next, the signal strength and noise are displayed along with the signal to noise ratio (SNR) in dB (deciBels) for comparing relative performance. "dB" is a logarithmic scale for measuring relative system gain (or loss) and corresponds to the ratings used for directional antennas and other radio components. You can use this to measure relative interference (SNR), antenna performance, or evaluate whether an external antenna with a rated gain will provide the signal coverage desired.

Intuitively, 3 dB corresponds to a just noticeable improvement in signal strength from doubling the power. 10 dB is a significant improvement in signal strength from 10 times the power. If you are considering an external antenna to improve AirPort range, a directional antenna with a gain of 3 dB will provide only a modest increase. Relocating or re-orienting your existing base station antenna may provide similar improvement.

The plot column shows the relative performance (SNR dB) at a glance. Each blip represents 3dB. As additional scans are performed, any previous data scrolls down the table.

Finally the Notes column allows you to annotate individual entries to document the configuration tested. Simply click in the corresponding field to enter any notes.

The last transmit rate will be shown along the bottom if your Mac is currently associated with a WiFi base station.

The Antenna Is The Thing

Experienced radio professionals know the antenna is the most important component for improving the range of two way radio systems. While space craft designers pay careful attention to hi-gain antennas, you don't have to be a rocket scientist to have basic antenna intuition. Here are a few factors to consider:

Additional Features

You can select the results of a test and copy them to the clipboard as tab separated values for pasting into email messages or a spreadsheet. If you include the first row of the table in your selection, the table headings will also be copied.

You can save test results to a file using "Save As..." and reload them again using "Open..." to compare previous results with current readings. When you open previous results, they are appended to the bottom of the table. Test results are saved in ".plist" format with additional scan information including the base station MAC address.

A popup menu keeps a list of Recent Targets or history. The contents of the target field are added to the list when a test to that target is invoked. If the recent targets menu becomes full (10 entries), the least recently used item will be removed. To add or remove an item, or clear the entire list, use the corresponding selections from the History menu.

How is the AirPort Signal Tool Different from MacStumbler or iStumbler?

MacStumbler and iStumbler are designed to help you locate wireless hot spots. The AirPort Signal tool is designed to help you measure and compare wireless performance. Key features:

A common use for the AirPort Signal tool is to find a radio channel with the least amount of interference for your own AirPort base station. The SNR (Signal to Noise Ratio) reflects the relative merit of the channel in use by the associated base station. Observing what other base stations are visible from your location and trying other channels can help you find one that offers acceptable performance in your location. Notice the SNR reflects the relative merit of the corresponding channel even if there are "closed" base stations that are not visible in the scan results becuase they do not broadcast their SSID at regular intervals.

Known Issues

The first scan may not report all the base stations in your area since it takes a little time to record the beacons of stations that are not associated with your computer.

Some older AirPort Extreme cards always report a noise value of zero when scanning for available stations. This is due to a bug in the AirPort Extreme driver (AirPort 3.3 software). When the noise value is reported as "0", the Signal to Noise Ratio is just the reported signal strength. If you have AirPort software version 3.4 or later, the tool can read and display the noise value for the base station you are currently connected to (if any).

Technical Notes

The "signal" is a quality figure roughly proportional to the SNR. The "noise" or "silence" value is based on the energy observed before the frame, while the "signal" is based on the average power of the received frame. The values are approximate because they are not calibrated for the nonlinearities of the individual receiver circuitry. A rough approximation is that the signal - 100 is close to the level in dBm.


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