Some weeks ago I've build a full-featured XPhase for my father, DK1UO. He is sometimes affected by heavy interference from Plasma-TVs or similar scrap
The function of the XPhase is based on the fact that the interfering signal, received by an auxiliary antenna, is added phase-shifted to the signal received from the main antenna. When the phase-shift is more or less 180°, the interfering signal is canceled out.
My XPhase is based on the work of Hanns, DK9NL. A commercial versions can be found here under the name QRM-Eliminator. Google will provide you some futher informations.