MarioMania said:
Null(ing)-a DXing technique. Null does mean zero, however for purposes of DXing, it is reducing the signal strength of a nearby radio station by moving an antenna's position until the local or strong signal is weakened or "nulled" as much as possible in order to make it potentially easier to hear the distant radio station(s)
On AM, the null is created by turning the radio with its built in loopstick antenna to a position (usually one of the ends of the loopstick pointing at the station that the DXer wishes to null) to a point where the undesired station is weakened enough (or completely) in order to hear another station (the DX, or distant station) on the same or an adjacent frequency. On AM it is possible to hear stations underneath or mixed in with each other due to the nature of amplitude modulation. Of course, it's best when the undesired station is not modulating, is weakly modulating, or one of the stations is playing music while the other station is announcing...
On FM, the null is created (in the case of a telescoping antenna) by rotating the antenna in the horizontal plane until the stronger station is weakened enough in order to hear the weaker station. The tip of the telescoping antenna pointed at the stronger station would make it the weakest in theory, whereas the broadside of the telescoping antenna facing the stronger station would make it stronger. Depending on the FM radio used there is also something to consider called the Capture Effect. Read more about that here:
http://en.wikipedia.org/wiki/Capture_effect
In the case of using a rooftop antenna that is rotatable, the antenna is turned until the offending station is "nulled" out and, hopefully the DX station is then heard. A directional FM antenna with a high Front-to-Back ratio will greatly help in nulling out FM stations, provided the station that the DXer wants to null is not particularly strong.
http://www.bext.com/winegard.htm http://www.wtfda.org/index.php?option=com_content&task=view&id=14&Itemid=43
Your results may vary. Some examples: If the AM DX station is 180° with respect to the direction of the stronger signal, it may be very difficult if not impossible to null the stronger signal. On FM, provided the stronger station is not too strong, Tropospheric propagation or E-skip may deliver a distant signal at a higher signal strength than your local or regional station's received signal, a result of the capture effect explained in the Wiki article.
