Steve, W3AHL wrote:
>>
> In my original example I used a 12" long 1" D. conductor which had a
> calculated free-space inductance of 0.148 uH, which would scale to 1.48 uH
> for ten feet, versus a calculated inductance of 2.884 uH. This is caused
I don't know how "free space" inductance is defined, but these
numbers correspond to a characteristic impedance of 148 to 288 ohms,
hardly free space. A 1 inch wire 10 feet high over ground
would have considerably less inductance, and it still isn't in free space.
>> The "strip over groundplane" calculators and my measurements seem to both
> indicate inductance increases when a conductor approaches a ground
I think the source of this confusion is erroneous web sites
such as:
http://www.daycounter.com/Calculators/Microstrip-Inductor-Calculator.phtml
which indeed predicts that inductance increases when the spacing
to ground decreases. This calculator is flat out wrong. Unfortunately,
this is the first result on Google for "microstrip inductance".
The correct formula was published by the highly respected RF
engineer, Harold Wheeler, and is republished here:
http://en.wikipedia.org/wiki/Microstrip
(Wikipedia cites Wheeler's original IEEE papers, in case you
don't want to take Wikipedia's word of it. Wikipedia of course
is far from infallible).
Notice that "h" (height above ground) appears in the numerator,
indicating that impedance increases with height, and therefore
inductance increases with height.
Rick N6RK
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