Coax Common Mode Noise Filter
Common mode current induced by radiated sources (plasma TV, routers, computers, transmitters, etc) can be picked up by the outside of the coax braid from the antenna feed point back to the receiver. This portion of the coax braid acts like a receiving antenna, picking up common mode noise signals that override weak signals making them difficult or impossible to hear!
The solution? A common mode noise filter with high choking impedance at the receiver end of the coax.