Measuring Signal Strength
I considered a number of tools for measuring signal strength before settling on NetStumbler. NetStumbler provides a nice signal strength vs. time plot that can easily be stopped when moving between locations. It also has pretty good WLAN chipset / card support, including the two cards that I wanted to use: the D-Link WNA-2330 (Atheros based) and Linksys WPC54G (Broadcom based).
WLAN cards aren't the most accurate signal meters around. But they are good enough for what we're trying to do because we need only relative signal strength. I started by using the D-Link WNA-2330 card associated with a Linksys WRT54G wireless router. I didn't run any traffic—these measurements were made using the WRT54G's beacons only.
Figure 3 shows signal strength vs. location for my five test locations, plus a few others that are described in the figure callouts.
Figure 3: Signal Strength vs. Location - D-Link WNA-2330 card
Note that the largest signal drop occurs between the first two measurement points—"Antennas touching" and Location 1 (about 3 ft. away). Since RF power follows the inverse-square law, you can see that we're dealing with very low levels of energy pretty quickly as we move away from the access point.
Table 2 takes the Signal Level readings from Figure 3 and relates them to an equivalent path loss, which is then overlaid on a downlink throughput vs. path loss plot for the card in Figure 4.
|Test Location||Signal Level (dBm)||Delta||Equiv.
Table 2: D-Link WNA-2330 Signal Levels
This is simply done by taking Location 5 as a reference and assigning it to the last plotted point. The last point is used as the reference instead of Location 1 because it's easy to tell when you lose signal. The values in the Delta column in Table 2 are then subtracted from the Path Loss value of the last plotted point to get the path loss values for the other Locations.
Figure 4: Path Loss vs. Location - D-Link WNA-2330 card
Figure 4 shows the results, with the locations overlaid on a downlink throughput vs. path loss plot.