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Re: [TowerTalk] inductance of tubing vs bar or strip

To: <richard@karlquist.com>
Subject: Re: [TowerTalk] inductance of tubing vs bar or strip
From: "Steve, W3AHL" <w3ahl@att.net>
Date: Wed, 19 Aug 2009 15:30:53 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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