I'll defer to Jim Brown and Henry Ott, both suggest that any wire much longer than 1/2 can be an interference antenna.
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Jim Brown, past AES committee co-chair on EMI/RFI.
'
RFI, Ferrites, and Common Mode Chokes For Hams'
http://www.audiosystemsgroup.com/RFI-Ham.pdf
about 50 more Jim Brown papers:
http://www.audiosystemsgroup.com/publish.htm
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Henry Ott industrial EMI/RFI expert.
'Electromagnetic Compatibility Engineering'
by Henry W. Ott,
publisher: John Wiley & Sons, hardcover 872 pages, 566 figures, 65 tables.
Publication date: August 2009, ISBN#: 978-0-470-18930-6.
http://www.hottconsultants.com/EMCE_book_files/emce_book.html