Comments below.
Steve, W3AHL
----- Original Message -----
From: "jimlux" <jimlux@earthlink.net>
To: "Steve, W3AHL" <w3ahl@att.net>
Cc: "TowerTalk" <towertalk@contesting.com>
Sent: Thursday, August 20, 2009 9:40 AM
Subject: Re: [TowerTalk] inductance of tubing vs bar or strip
<snip...>
>>
>> I did a quick measurement with a 4' piece of #6 copper wire. Adding a
>> 1" radius 90 degree bend increased the impedance about 5 ohms at 10 MHz,
>> which would equate to an inductance of about 0.08 uH for the bend.
>> This was not a precision test, but gives an order-of-magnitude estimate
>> at least.
>
>
> But how did you do the measurement? Fixturing effects at 10MHz will be
> significant. It's not like you can just hook up a inductance meter,
> because the leads are there too.
>
>
As with any measurements you need to calibrate out the fixture effects
somehow. I have an HP8753C VNA (rented for a consulting project) and an
HP8924C "ham service monitor" that both allow you to calibrate offsets for
the fixturing. Measuring inductance at 10 MHz is easy compared to 1 GHz!
>>
>> My experience with this relates to PCB trace layout modeling and the
>> effect of sharp corners on trace inductance on fast edge rate signals.
>> I'm retired now and don't have access to the EM modeling software we
>> used.
>>
>> The mechanical stress is only high because of the interaction of the
>> magnetic fields with surrounding objects. The interaction of magnetic
>> fields in a right angle bend of the current-carrying conductor not only
>> cause high stresses at the bend, but an increase in self-inductance at
>> the bend also.
>
> I agree, but I think the increase in L is small, if not negligible, over
> the same length of wire in a straight line (that is, a 90 degree bend
> with radius 10 cm will have pretty much the same inductance as a
> straight piece of the same wire that is 17 cm (10*pi/2) long. I think
> you have to start getting up to 180 degrees of bend before the field of
> one piece of the turn starts really interacting with the field of
> another piece.
>
>
Nothing is neglible when you are dealing with lightning protection grounding
where peak currents routinely exceed 20,000 amps. For the right angle bend
I measured, at 1MHz (typical frequency used for lighting edge rates) just
calculate the additional voltage drop across the 0.5 ohm reactance added by
the 1" radius bend. Then multiply that by 3or 4 more bends.... That's why
they recommend keeping the bend radius in primary grounds to greater than
8". It's very enlightening to visit large commercial tower sites and see
the great lengths they go to minimizing inductance, loops, etc.
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