Reg Edwards wrote:
Obviously, there is more heavy, ferrite material used for sleeves
because the number of turns is only one. But disregarding economics, a
number of sleeves will eventually provide a better very-wide-band
choke than a ferrite ring or a bundle of ferrite rods.

Amidon specifies a typical Z for those toroidal coax sleeves.
For instance, the Z is 215 ohms per core for FB-77-5621
cores at 25 MHz. My store-bought G5RV came with eight of
those cores.
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73, Cecil http://www.qsl.net/w5dxp