At 04:41 PM 7/11/2003 -0400, Pete Smith wrote:
>At 11:55 AM 7/11/03 -0700, Jim Lux wrote:
>>And even better, there is a detailed data sheet with derating and i
>Coincidentally, I have recently done some measurements, using an MFJ259B,
>on a solenoid choke (18 turns (closewound) of RG-8X on a 4 /2 inch piece
>of PVC pipe, versus 50 Type 77 beads on a length of 1/4" teflon coax. I'm
>not trying to suggest that these designs are optimum -- in fact, a friend
>opines that the solenoid, in particular, has too many turns for the
>job. The bead choke numbers don't seem that much different from Cal-Av's
>though, at least at the lower frequencies.
>
>The numbers were.... drumroll:
>
>Solenoid
>F (mhz) Z (ohms) Phase Angle (deg)
>
>2 384 90
><snip>
>Bead choke
>F Z Phase
>1.69-14 H
>14.8 650 60
>15 644 60
><SNIP>
>24 417 66
>I suspect that the measurements above 20 MHz or so should probably be
>discredited, because my test rig had a pair of 16-inch clipleads in it,
>but the lower-frequency readings are interesting. A friend who is much
>more knowledgeable about the theory of such things says that the Phase
>Angle readings are a clear signal of much larger losses in the beads,
>hence the heating talked about in discussions of bead chokes encapsulated
>in foam material.
Indeed.. the solenoid is almost purely inductive (except for small losses
due to skin effect and finite resistivity of the shield material), so the
phase should be 90..
The beads show a phase angle of 60 odd degrees, implying that the XL is
about 1.7 times the R.. .. from that you could calculate the heat
dissipated, for a given amount of power incident in the choke. I wonder
why the Z is so high below 14 MHz.. Do you happen to know what material it
is? Maybe the mu starts to drop off above 14 MHz, so the inductance starts
to drop, making the resistive losses more significant?
>(well, maybe 1000 on 10) for several years. My homebrew chokes are just
>wrapped in electrical tape, though.
Which is not a bad thermal path.. it's thin, and in close contact.
Jim, W6RMK
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