Steve, W3AHL wrote:
> 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!
OK.. when you did your fixture cal, what did you use for standards (was
it the usual Short,Open, Load cal?). I'm curious, because I want to make
similar measurements, and I'm always interested in useful lab
technique.. measuring 10 foot wires is a challenge, because your fixture
has to be 10 feet long.
>
>>
> 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.
I've done those calculations, and that's why I assert that it's
mechanical forces, not inductance, that makes the difference.
Obviously, short is better than long, but there's this whole
applications literature about "avoid sharp corners" and aside from
mechanical forces or actual air breakdown, I don't think there's a good
justification for it.
And, of course, from an inductance thing, what you're really concerned
about is the di/dt, more than the actual current, because that's what
gives you the voltage rise. On the other hand, for a typical
20kA/microsecond sort of situation, adding a microhenry (20kV drop) to
a path that is already 10 uH (200kV drop) isn't going to be changing
things much. It's not going to change the actual discharge
characteristics much (whatever transient suppression you do isn't
depending on the low inductance of the lightning discharge path.. it's
depending on referencing everything to a common voltage, so it all goes
up and down together.)
A remarkably small wire will take the energy dissipated in the
resistance from the lightning discharge (lots o' current, but it only
lasts 50 microseconds, so the "action" (Amp^2*seconds) is low, and
action is what melts stuff.
There *is* a reason to keep the voltage low (which means keeping
inductances and resistances low) and that's to avoid flashovers from the
lightning energized component to something else. BUT, that doesn't take
extreme measures to reduce the voltage.. A 6" air gap will stand off a
couple hundred kV between relatively small wires. (hence the code
requirement for separation distances between lightning conductors and
others, and for antenna wiring, ditto), so whether the voltage is 200kV
or 220kV makes very little difference.
After all, the voltage at the top of the tower is pretty high when
struck, and nobody worries about trying to minimize that (unless you're
in the power industry, in which case secondary flashover from phase to
tower is a concern).
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|