A free-space inductor is first cousin to an isotropic antenna and a free-space
dipole -- useful theoretical models, but not much use in the ham shack.
The "characteristic impedance" of a stripline transmission line is different
than the inductance and "inductive reactance" of wire or strip above a ground
plane (the term I should have use instead of just impedance). The
characteristic impedance of a transmission media is relatively independent of
frequency. The inductive reactance of a straight wire inductor is definitely
very dependent upon frequency. Two different critters.
The formula and results used in the original stripline over ground plane
calculator I linked to agree with the three reference books I have access to.
At any rate, a wide flat copper strip beats 1/2" copper pipe no matter what
formula or parameter you want to use....
I'm bowing out of this debate so I can work on real antennas and towers!
Steve, W3AHL
----- Original Message -----
From: "Rick Karlquist" <richard@karlquist.com>
To: "Steve, W3AHL" <w3ahl@att.net>
Cc: "Richard Karlquist" <richard@karlquist.com>; "TowerTalk"
<towertalk@contesting.com>
Sent: Wednesday, August 19, 2009 1:42 PM
Subject: Re: [TowerTalk] inductance of tubing vs bar or strip
> 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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