Near Field RF probes can be very helpful when tracking down sources of unwanted RF emissions or radiation. This video presents the basics of the E-Field and H-Field probes, how they work, and how to use them. Here are some useful links on near field probing, including plans and discussions on how to make your own near field probes:
My show notes pages:
http://www.qsl.net/w2aew/youtube/NearFieldProbes.pdf
Good thread on EEVBlog:
http://www.eevblog.com/forum/projects/diy-magentic-field-probes/
Easy to build H-field probe:
http://www.emcesd.com/tt120100.htm
Some additional articles and references:
http://www.compliance-club.com/archive/keitharmstrong/emc_testing1.html
http://www.edn.com/design/test-and-measurement/4380475/Near-field-probes-sniff-circuits
http://www.eng.mu.edu/~richiej/seminar/aidi.pdf
http://www.ets-lindgren.com/pdf/ee_0287_berger.pdf
http://www.compliance-club.com/archive/old_archive/030718.htm
Here are the "yellow" probes I used in this video:
http://www.beehive-electronics.com/probes.html
There are many references you can find - Google is your friend.