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 3:06 PM
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.
>
>
Yes, measuring a 10' long inductor was something new for me also! That's
why I gave the caveat in an earlier post that while the absolute values may
not be up to lab standards, the relative comparisons among the different
conductors should be accurate.
For the VNA I just did standard two port correction for the cables that
attached to the DUT / fixture. I played with several different "fixture"
configurations and wasn't real sure which was the best, but the variation
was only about 15% among three different schemes.
For a sanity check I took the 8924C and used it to measure the voltage drop
(and thus impedance) of the DUT that was terminated with a 50 ohm dummy load
to ground. To calibrate this I just removed the DUT and hooked the input
and output cables together, did a Normalize Save B and then used the
Normalize A-B to view the corrected measurement. The results agreed with
the VNA's Z measurement and seemed a little more repeatable with variations
in cabling placement.
If I had a large enough piece of metal, I would have preferred to make the
measurement over a perfect ground plane. But I wasn't that motivated and
had to return the rental VNA and get on with other projects.
>>
>>>
>> 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.
>
"Mechanical force" and "inductance" are just two sides of the same coin.
They are just two different manifestations of the magnetic field resulting
from current flowing through the conductor. I am confident that the
mechanical force (and inductance) at a right angle bend is much higher than
for a straight conductor. I've seen the mangled metal evidence first hand.
> 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.)
I wouldn't want a 10uH ground path from my entrance panel to ground, so each
bend would become a larger percentage of the overall. So based on your
theory, there is no reason to use anything larger than #6 wire, as long as
everthing 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).
>
I understand what you're saying, but personally prefer to follow best
practices that are field-proven, especially if it doesn't cost more. It is
hard and expensive to prove the real pro's are wrong when it comes to
lightning protection. I've seen several antennas on 300' towers get
vaporized with direct hits, with absolutely no damage to our repeaters.
That's the type of installation I want to emulate.
But it is always interesting to take the time to actually measure and model
something for a reality check of what we "believe" to be.
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|