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Author Topic: Placement of Lightening Arrestor  (Read 890 times)
KE3HO
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Posts: 235




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« Reply #15 on: November 08, 2007, 07:17:14 AM »

Tom,

<<Well, it's always a race between the front end and the protection device. Certainly I agree that a protection device can save an ESD sensitive (poorly designed) front end, but I strongly disagee that a device capable of passing high power transmitting signals will protect a receiver.

We only need to use common sense. If the device allows 500 volts peak out, it will let at least that amount in before it clamps if not more. 500 volts would smoke a sensitive front end.>>

I agree with you on this, and my comment was regarding protection devices with a DC ground path from the antenna to ground. For example, a high-pass filter with a series capacitor and a shunt inductor on the output with its cut-on frequency somewhere below the 160M band. For guys like me who disconnect antenna leads when not in use, the shunt inductor drains away charge that might otherwise build up in the feedline. Of course, a 10K resistor would probably work just as well, and I have used that before also. I am speaking only of built up static, not a lightning strike. The inductor would not make a very good ground path for the high frequency components of the lightning. I've wondered, too, if ringing and oscillations in the LC circuit make actually make the problem worse in the case of a lightning strike.

<<One interesting side comment. I've seen dozens of amplifier failures caused by using lightning arrestors on high power transmitters. What happens is the gas tube gets weak or the SWR gets high and the tube fires from RF peaks, and this dumps the amp into a short. This can do nasty things like blowing a bandswitch.

The peak voltage is 388 volts at 1500W with a 1:1 SWR into 50 ohms, so it is common with exciter transients and a little SWR to see several hundred volts. If that fires the gas tube you can easily lose the load and flash a component inside the amplifier over.>>

Agreed. When I started looking at protection devices and power ratings, I started calculating feedline voltages at various power and SWR levels. I quickly realized that a clamping device capable of protecting a receiver would only be useful at QRP and 1:1 SWR. A 2200V gas discharge tube (which is pretty common in 1500W rated protection devices) is not going to protect any receiver.

In retrospect, I think that you are right about the irrelevance of ground impedance and formation of step leaders. Step leaders come up from tree tops, wooden utility polls, etc., where the impedance to ground is relatively high. I should have thought that comment through further before writing.

73 - Jim
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KT8K
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Posts: 1490




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« Reply #16 on: November 08, 2007, 07:53:04 AM »

Lots of good info here.  I'm a bit shocked by the description of the innards of some commercial arrestors, as I thought I was buying gas discharge tubes, too.   I'm going to take a look!

Since lightning is initially a static electricity buildup, voltages go highest on insulators, and large conductive objects/areas spread the charge out and keep the voltage lower than it would be if it was building up on a small, insulated object.  Charles Steinmetz taught me that (a remarkable scientist - check him out).  That alone helps avoid a leader developing where you wouldn't want it, and, if it isn't there, or is of lower voltage than others nearby, it is less likely to be the one to draw the direct strike.  

Making as big a conductive area as possible (ground field) is best, and explains why ground system "halos" and bonding any nearby ground systems into one large one are smart things to do.  Of course, bonding things together limits the voltage differential between those objects, and limits current flow across the system to that which is produced by the traveling wave impulse generated by a strike.  If the strike is farther away, the current induced by the electromagnetic field from the strike will be smaller, which is good, too.  (Keep it away from me and my stuff!)

I always ground lightning arrestors and ground system components just outside the house, and place a choke coil in the feedlines running between that ground point and the house to further discourage a lightning induced surge.  The halo isn't in place yet at the new house, but I did make sure to run a ground wire from the main ground rod over to the electrical box ground right away.

BTW, thinking of the spark gap arrestor described inside the commercial unit, the handbook describes using automotive sparkplugs or pairs of sharp metal points attached between ladder line conductors and ground, providing lightning protection for that type of feedline, too.  And last, but not least, don't forget the insurance (thanks to WB2WIK for frequently reminding us of that)!
73 de kt8k - Tim
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KE3HO
Member

Posts: 235




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« Reply #17 on: November 08, 2007, 08:10:17 AM »

Sorry, I had my terminology mixed up. A "step leader" comes down from the clouds. A "streamer" comes up from ground-based objects.

73 - Jim
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KC9KEP
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Posts: 208


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« Reply #18 on: November 08, 2007, 01:24:05 PM »

Hmm ... This is a very interesting discussion!

I'm starting to understand that I know very little
about what makes a shack lightning-hardened!

I have a long-wire antenna .. a bit over 100' in
length, supported by 10-foot tripod atop my shed,
running to my house.

I use no lightning arrestor at all, and was thinking
about using one of those old fashioned 300 ohm
twin-lead types, thinking that something is better
than nothing.

Maybe not?

(The old ARRL books illustrate a home-made spark-gap
affair.)

--KC9KEP
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