Yes indeed; a ferrite bead transformer can add impedance. Typically, small
snap-on EMI ferrites run about 850 permeability (e.g.: FairRite 43 or
Steward 28 material) and come in at 100 MHz around 80-150 ohms or so,
depending on the ratio of ferrite to air in occupied volume, falling off
rapidly in the HF region. They can have some effect on measurements at HF.
Other materials have higher permeability; Steward 35 is an excellent
high-loss ferrite choke at HF and could be a poor choice for the
transformer. Steward 27 material gives about 100 ohms at 25 MHz while
Steward 25 material is rated at 300 MHz, and will have least effect at HF.
(For our low frequencies, the Steward 35A024-0A bead -- as one example --
is listed at 93 ohms impedance at 4 MHz. Could be JUST what the doctor
ordered for 80 meters or even 160 meter fixes. But it would certainly
change the current in the wire under test if used as a transformer core.)
I generally use a one turn secondary made by bringing a piece of RG-141 (or
other small coax) back around and connecting the center conductor to the
shield - and the receiver, analyzer or detector terminates it. This gives
plenty of signal for receivers and sensitive analyzers, even with
relatively low-mu cores.
For MEASURING, I use clamp-on current probes made for the purpose, whose
transfer function is known.
Cortland
Ian White writes:
> If you don't terminate the secondary (or terminate it only in a high-Z
> detector) then the same core looks like a plain old ferrite bead with a
> series impedance of tens to hundreds of ohms.
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