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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