Resonant loops are absolutely the way to go with portable radios. My simple
80-15m design, quite similar to that of W0IVJ, is included in the extra files
with the current ARRL Handbook. Many people have built this from their junk
boxes. http://wb9jps.com/Gary_Johnson/RFI_files/Handheld_DF_Antenna.pdf
Sensitivity is outstanding on this antenna. For instance, I have no problem
standing in my CA back yard copying all of NA on 20 or 40m. Bandwidth is
sufficient to see most of a single band with my portable spectrum analyzer
(Anritsu MS2711) though its sensitivity is not as high as a cheap shortwave
radio.
As others have reported, nulls are sharp. This little lollypop antenna, at
nearly zero cost, and with no batteries or other stuff to cart around, is the
mainstay of my RFI hunting. Every ham should have something like this as a
starting point.
-Gary NA6O
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