There was some discussion of the effectiveness of various ferrite
chokes a few days ago. I have access to a HP 4194A Impedance
Analyzer, so I put a couple of popular ferrites through a
swept-frequency impedance analysis.
I measured two of Radio Shack's products: the first is a cylindrical
ferrite in a plastic holder, about 1.25 inches long by .75 inches in
diameter. It opens up like a clamshell and clamps down on the wire.
The opening is about 0.25 inches. The values shown are for a single
wire through the ferrite.
FREQ INDUCTANCE ESR (Equivalent Series Resistance)
300 kHz 2.07 uH 0.073 ohms
2.3 MHz 2.25 uH 6.9 ohms (the inductance peaks at this freq)
10 MHz 1.18 uH 54 ohms
20 MHz 0.80 uH 89 ohms
40 MHz 0.50 uH 130 ohms
The second Radio Shack ferrite is a rectangular device which opens up
and allows the ferrite halves to be separated so multiple turns can be
wrapped around it. It is 1.675 inches long by 1.125 inches wide by
.375 inches thick. The opening is much larger in this ferrite, so
more turns can be wrapped around it than the cylindrical one. The
values shown are for a single wire through the ferrite. Unlike the
cylindrical ferrite, there is no inductance peak.
FREQ INDUCTANCE ESR
300 kHz 0.39 uH 0.044 ohms
2.3 MHz 0.38 uH 0.6 ohms
10 MHz 0.29 uH 7 ohms
20 MHz 0.21 uH 11 ohms
40 MHz 0.18 uH 10 ohms
As with all coils, increasing the number of turns increases the
inductance and ESR by the square of the increase. For example, if one
turn gives 1 uH and 10 ohms, two turns will give 4 uH and 40 ohms,
three turns would give 9 uH and 90 ohms and so on.
One word of caution: Radio Shack is known for changing their product
line at the drop of a hat, so use caution in applying these
measurements.
Unfortunately, the 4194A only measures up to 40 MHz. If anyone has an
impedance analyzer capable of VHF measurements, that information would
certainly be worth posting.
Hope this helps someone.
73, Bill W7TI
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