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Author Topic: Antenna GROUNDING...solid or stranded?  (Read 11905 times)
W6RMK
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Posts: 656




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« Reply #15 on: April 03, 2012, 05:15:58 AM »

Quote: "Even 180 degree turns don't have that much inductance, beyond the length of the wire involved.  You need to start getting towards 360 degrees before the inductance starts to rise.  It's because inductance is all about magnetic field coupling, and the field from a wire doesn't couple well to a wire that's not parallel (why we put antennas at right angles to avoid interaction)."

Anyone who talks 180 degree turns while talking about lightning protection proves how little they know about lightning protection!

Same with the "Sharp bends" etc.

Real life lightning protection at high voltage and high power is different from "paper" calculations at low power levels.......


Sigh...

I've done a fair amount of work with high energy pulsed power and high voltage. While I can't say I've spent my entire professional career of 30 years doing it, I've probably broken, fused, vaporized, or blown up more copper and aluminum wire with high energy pulses than most of the people on this forum.

I've also got a decent background in physics and am familiar with most of the literature in the lightning effects and damage field including a wide variety of industrial and government specifications for lightning protection, which are NOT always entirely laws of physics based. As pointed out earlier, economic considerations come in to formulating recommendations: do you want to load 5 different kinds of cable on the truck or one? What's the relative price of labor and copper wire? What is easiest/fastest to connect to on a construction site? Are there government subsidies or taxes or regulatory compliance aspects? 

I can substantiate all of my statements with experimental evidence and/or analysis.  I would point you, again, to the work of E.B. Rosa at the NBS, who spent quite a while working out all the inductance formulas and then testing them against real life.  I think you'll have to agree that the inductance formulas aren't going to have changed in the last 100 years.

If you want to provide advice that is contrary to what's in the code, you need to be able to provide a basis for that recommendation.
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W9IQ
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Posts: 102




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« Reply #16 on: April 03, 2012, 02:13:11 PM »

What I find is often lacking in the discussion of grounding systems for lightning is a realization that a lightning strike presents a pulse with a very rapid rise in current (high di/dt). So attempts to analyze the system from a DC or resistance perspective will lead to incorrect conclusions.

The pulse nature and fast rise time of the lightning strike means that the current flowing in the grounding conductor will exhibit skin effects similar to that of continuous RF current with which we as Amateur Radio operators are more familiar. This means that heavy, round conductors are improving lightning grounding only due to the increased surface area of the copper - the inner part of the conductor is largely "wasted" copper since it hardly participates in the current flowing to/from the ground.

A lightning strike will often be less than 1/10 the skin depth of standard 50/60 Hz household current. Considering the potential current of a lightning strike can be in the 30,000 Amp range, having adequate surface area to carry the current is absolutely critical.

In order to maximize the surface area and minimize the amount of "wasted" copper, most professional tower grounding systems use wide copper straps or bars. For the same cross sectional area of copper, as the frequency rises, a "flatter" shape will carry AC current more efficiently. From a code inspection standpoint, these are selected and rated to exceed the minimum code requirement.

Professionals that deal with designing lighting protection systems often reference GPR - Ground Potential Rise. Simply put, this is a calculation of the inductance of the Earth compared with the inductance of the grounding system. By then applying estimated pulse rise times and assuming a particular strike current (in kA), the voltage rise of the grounding system during a strike can then be calculated. Notice that the calculations are based on inductance, which can be correlated with the surface areas of the conductors involved and other factors.

There is much more to implementing a proper lightning protection system. There are nice resources on the Internet and easy reading, practical application booklets available from some of the equipment manufacturers. If you would like a more technical treatment, look for the wealth of information on the topic from IEEE and classic reference texts.

- Glenn DJ0IQ and W9IQ
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N3ZC
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Posts: 1282




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« Reply #17 on: April 04, 2012, 07:12:43 AM »

"In order to maximize the surface area and minimize the amount of "wasted" copper, most professional tower grounding systems use wide copper straps or bars. For the same cross sectional area of copper, as the frequency rises, a "flatter" shape will carry AC current more efficiently. From a code inspection standpoint, these are selected and rated to exceed the minimum code requirement"

Yes indeed. Great point. I used copper strap as a ring around the base of my tower. that ring is interconnected with the service ground with solid #4 copper wire (& ground rods).

http://www.gacopper.com/022-CopperStrap.html

  Good Luck,
                 73'..Tom N3ZC



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K9KJM
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Posts: 2415




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« Reply #18 on: April 04, 2012, 10:28:11 PM »

Flat copper strap can usually be had for lots less money from a local upscale roofing company that installs copper roof flashing on upscale homes, And even at local discount home supply stores in the roofing department.  (You  might have to have them order it)   A few months ago it was still selling for 39 bucks for a 10 foot long roll, 6 inches wide.   Which you can cut with a hand held tin snips into three   2 inch wide straps for a total of 30 lineal feet at a reasonable cost.

Georgia copper may (Or may not) have quality products.  At Dayton I asked the crabby guy at their booth to see his sample and he refused to let me even touch it!

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NO9E
Member

Posts: 417




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« Reply #19 on: April 05, 2012, 05:49:36 AM »

Most people care much less about the truth than about feeling good and using the kind of information (or misinformation) they acquired over the lifetime.  So all one can do is state the case (yours was excellent IMHO) and let the other complain. Those willing will appreciate and those extremly critical probably will never change.
Ignacy, NO9E

Quote
I've done a fair amount of work with high energy pulsed power and high voltage. While I can't say I've spent my entire professional career of 30 years doing it, I've probably broken, fused, vaporized, or blown up more copper and aluminum wire with high energy pulses than most of the people on this forum.
/quote]
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N0YXB
Member

Posts: 320




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« Reply #20 on: April 06, 2012, 10:35:26 AM »

Thanks W6RMK, very interesting.  I love factual discussions and learned something new.
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