 and the CDF for selection combining, it is possible to estimate the "apparent" diversity gain (i.e., how much it is possible to gain in the deepest fading dips), normally at a probability level of 1%, by choosing the best antenna. The most relevant value, though, should be how much one gains in comparison to using an ideal antenna (i.e., to compare the CDF for a single antenna with 100% antenna efficiency with the CDF for selec- tion combining). This is the "effective" diversity gain [30]. If one compares with the CDF for a real antenna (i.e., with losses), this is the "actual" diversity gain. For antenna configurations with large mutual coupling, such as two dipoles moved very close to each other, antenna efficiency will become very low, SO what may look like a very good diversity gain (i.e., "apparent" diversity gain) is in reality compared to using just a single antenna that is not at all very good. In Figure 12.9 below one can see that at 11-mm distance between two 900-MHz dipoles, the "effective" diversity gain is only 1.5 dB at the 1% probability level. In this example, more than 90% of the time the diversity configuration will lose signal strength compared to just using a single antenna. In the case of using dipoles, the "effective" and "actual" diversity gains are very similar because a dipole typically has an antenna efficiency of about 95%. The measurements in Figure 12.9 take only 1 minute to perform in the Bluetest high-performance chamber using a multiport network analyzer. In addition to the 11 mm Dipole Spacing 50 mm Dipole Spacing Branch 1 Branch 1 Branch 2 Branch 2 Reference Reference Selection comb. Selection comb. Relative Received Power (dB) Relative Received Power (dB) Figure 12.9 "Apparent" and "effective" diversity gain for two parallel dipoles at distances of 11 and 50 mm at 900 MHz. The dashed line corresponds to the fad- ing of a single dipole normalized to 100% efficiency. The distance between the dashed line and the line for selection combining are the "effective" diversity gain, here