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

To: "Richard \(Rick\) Karlquist" <richard@karlquist.com>
Subject: Re: [TowerTalk] inductance of tubing vs bar or strip
From: "Steve, W3AHL" <w3ahl@att.net>
Date: Wed, 19 Aug 2009 13:06:07 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Comments embedded below.

Steve, W3AHL
----- Original Message ----- 
From: "Richard (Rick) Karlquist" <richard@karlquist.com>
To: "Steve, W3AHL" <w3ahl@att.net>
Cc: "TowerTalk" <towertalk@contesting.com>
Sent: Wednesday, August 19, 2009 12:36 PM
Subject: Re: [TowerTalk] inductance of tubing vs bar or strip


> Steve, W3AHL wrote:
>
>> No, you can't just multiply the inductance of one foot of wire by ten to
>> find the inductance of 10'.  Use the calculator.
>
> Can you give us more details on what you mean here or a citation
> from a text book?  AFAIK, inductance is proportional to length,
> assuming the whole length is the same distance from ground, as long
> as the length is small compared to a wavelength.  (~10% error
> at 1/10 wavelength).
>

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 by 
the "end-effect", where the ends of the conductor have a lower inductance 
than the middle segments.   This same end-effect affects antenna element 
scaling somewhat when the target frequency is much different than the 
original.

>>
>> The inductance of both round and strip conductors increase as they 
>> approach
>> a ground plane or other conductors.  So the free space inductance given 
>> by
>
> No, it's the other way around.  Inductance decreases as the conductor
> approaches the ground plane.  This is because the enclosed area
> of flux is smaller.
>

The "strip over groundplane" calculators and my measurements seem to both 
indicate inductance increases when a conductor approaches a ground plane.  I 
would be interested in any authoritative evidence to the contrary, since I 
have been to known to be wrong occasionally!

> Rick N6RK
>
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