[RFI] Lightning Protection
k8ri at rogerhalstead.com
Mon Jul 2 10:48:15 PDT 2012
On 7/2/2012 9:44 AM, dalej wrote:
> I put large loops in my coax at the top of the tower. The idea being the lightning striking the antenna goes down the coax and won't make the bend so it just shoots out the coax and not down to the rig. I suppose it's not very effective, but I had some extra up there so I figured why not.
I call those drip loops to keep water/moisture out of the coax.
I ground the coax shield at both the top and bottom of the tower so I
don't worry about the lightning having to jump anywhere and my system
has taken many direct strikes. Most of those strikes do nothing, but I
have had a couple that removed the plating (and weatherproofing) from
every connector up there and with 6 antennas that is a lot of
connectors. 17 if I counted correctly with power dividers and rotator
loops. They were just bare brass (with a very rough finish). The
weatherproofing which was the self sealing tape covered with regular
Scotch 66 tape looked like sheets of expanded metal.
That was when I realized the fallacy in believing a good job of weather
proofing will always keep water out of the coax.
> Interesting discussion about lightning.
> Dale, k9vuj
> On 02, Jul 2012, at 8:08, Kim Elmore wrote:
>> No, not effective. Again, because *everything else* is in corona (tower legs, rivets, weld sputters, bolt threads, nut shoulders, joints of all kinds) and because lightning propagation isn't driven by small variations in the local electric field, which is all these devices can accomplish. Lightning begins well aloft in the cloud, when the e-field approaches 1 M V/m and propagates at the very high e-field at the tip of the stepped leader. The downward propagating stepped leader is typically met 100-200 m above the surface by an upward-propagating streamer, which is caused by the local e-field induced by the stepped leader. All of this happens faster (think relativistic speeds) than corona currents can diffuse away from the source.
>> Kim N5OP
>> On Jul 1, 2012, at 23:21, "KD7JYK DM09" <kd7jyk at earthlink.net> wrote:
>>> "I asked them about these corona brushes and was told that they are
>>> ineffective. Once the electric field exceeds about 50-100 kV per meter,
>>> everything -- grass, trees, fences, antennas -- are all in corona and the
>>> air is about as "saturated" with corona ionization as it can get. These
>>> corona brushes have no effect"
>>> Several of htese at a site won't lower the potential in the immediate area
>>> preventing charges in the 50-100 kV per meter range?
>>> I see the diasharge brushes on remote sites, radar, repeaters, surveillance,
>>> even airports surrounded by towers with brush arrays a few tens of feet
>>> Not effective at all? What about a row of air teminals on a house?
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