Using the shape of the magnetic field

ADDED NOTES December 2011:
    The procedures described here are for the long range "DQ" receiver and high-power beacons.  With the Basic-1 and 2 radiolocators, the physics of locating ground zero remains the same, but some considerations are different. 

    Radiolocation requires practice. Even though this gear often gives a useable signal close to 1 km away, always try to position yourself as close as possible to the expected location by using topographic overlays, GPS, compass course and distance from an entrance, etc. Try to always be higher than the cave passage, ie wait uphill rather than downhill. If you are within a horizontal distance of about 1.4 times the expected depth of the beacon, the locating will be very easy.

Causes of Poor Nulls