I did some tests on cardboard tubes and wirewool, and found that the method has
its uses. It's only for reducing the amount of signal on the outside of the
coax, and with a 1 metre length, I could get a reduction of about 10 dB at
400MHz on a signal on the outside of the coax, and about 20dB at 2.4GHz. The
measurement is subject to all sort of variations, and the whole effect can be
negated by capacitive coupling. Below 200MHz, it's 'tolerably useless'. Because
the outside of the coax is not a matched transmission line, then the
effectiveness is also a variable depending on where the sleeve is in relation to
the SWR pattern. I did find it helpful when doing some tests on a 2.4 GHz WLAN
in an anechoic chamber, to keep the 2.4GHz out of the connecting cables.
My experience has been that ferrite chokes aren't so effective above 200MHz or
so, although baluns using ferrite beads over solid coax are use in professional
mixers and couplers up to 2GHz. I probably had the wrong ferrite.
I'd summarise it as a potentially usefull approach in certain circumstances,
(like all else failed?) although limited in its effectiveness.
73
Peter G3RZP
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