In this work, we developed an analytical estimation of the performance loss due to multipath propagation for a narrowband OFDM system. The propagation characteristics required for this loss estimation, are experimentally determined by virtual SIMO measurements in a large conference room where repeated reception problems were reported for an IEEE 802.11 system, as well as in 2 other large conference rooms for comparison. The resulting losses due to multipath are calculated for IEEE 802.11a/n and related to the propagation characteristics.