At 11:50 AM 12/5/2006, K4SAV wrote:
>It's not immediately obvious as to what is going on with these coils.
>
>The additional loss is from the dissipation factor of the dielectric
>which is a function of the electric field strength. Also the dielectric
>increases the capacitance across the coil which lowers its self
>resonance frequency. The low frequency value of inductance is a constant
>and independent of the dielectric. The shunt capacitance forms a
>parallel circuit with inductance and its effective impedance goes up, so
>the effective inductance goes up. You get just the opposite effect for a
>series capacitor. In the previous post, I just manually adjusted the
>number of turns to keep the desired value of inductance.
Which is why I assumed the parasitic capacitor got about twice as big
(frequency ratio of 0.70 is about doubling the LC product, and the
L's just not changing much). To get twice the C with essentially the
same dimensions, the dielectric constant would need to be at least 2,
IF the coil were totally immersed. If it's just wound on the
surface, some of the E flux isn't in the dielectric so the effect
would be smaller (or, correspondingly, the dielectric constant needs
to be much bigger, say, 5 or 6). I would imagine most of the E field
flux is between the turns, as opposed to "across the diameter" of the
coil, so whether it's a solid rod or a pipe wouldn't make a heck of a
lot of difference.
>Here is what happens when holding the turns at 30 for both, and at 1.85
>MHz. With an air dielectric, L=83.6 uh, C=5.96 pf, Rs= 1.2, self
>resonant at 7.4 MHz. Low frequency inductance of this coil is 78.4 uH.
>With PVC, L=89.3 uH, C=11.6 pf, Rs= 2.5, self resonant at 5.3 MHz. Low
>frequency inductance of this coil is the same as for the air coil, 78.4 uH.
>
>I'm not sure what the program assumes for the location of the
>dielectric, but since this program is intended to be used for
>calculating the inductance of a coil would on a form, I would hope that
>the calculation is not for being immersed in the dielectric. Also I do
>not know what the program assumes for the dielectric properties of PVC.
>I guess you could derive them based on some of the calculation results.
Which program is it? Some of the programs make a lot of the
assumptions we've talked about, or use tabular values or equations
from historical sources (NBS circular 74, Terman, Grover, Wheeler,
etc.), but material properties and construction practices have
changed a lot over the past 50-100 years, so some of the assumptions
might not be valid. Using the approximation is fine, as long as you
know what it is (which is where some of the programs fall down.. they
don't document what they're doing internally).
(this comes up all the time in Tesla coils.. Tesla was using
materials very different from modern materials, so his design
equations aren't necessarily valid)
Jim, W6RMK
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