Speaking of EMP Ive been bit in the past off my 1000' Beverage by a
discharge at least 5 miles away. Needless to say I terminated the f/b tuning
tests for that day.
Carl
KM1H
----- Original Message -----
From: "K4SAV" <RadioIR@charter.net>
To: <towertalk@contesting.com>
Sent: Monday, August 13, 2007 12:31 PM
Subject: Re: [TowerTalk] Ground System
> K8RI's assessment of the grounding problem is right on. I would like to
> add a few comments.
>
> Current waveforms for lightning simulation is not always the same.
> Different waveforms are used for different purposes, depending on the
> maximum waveforms expected in a particular application. Since towers
> are right out there in the open, they see a variety of possible
> waveforms. Cables coming off the tower have modified waveforms.
>
> Here is some data on some measured lightning waveforms.
> http://plaza.ufl.edu/rakov/ICLP2000/positive1.16.pdf
> As you can see the pulse width measured can vary a lot, and also there
> can exist a continuing current after the initial big pulse that may be
> hundreds to tens of thousands of amps for periods up to 10 ms. So,
> although you do have a lot of energy at 1 MHz and higher, there can also
> a lot of energy present even down in the audio frequency range. Not all
> lightning strikes have the same waveform.
>
> Inductance of long cables does very little for these long continuing
> currents. Hopefully most of this will go into the ground at the tower,
> but there is going to be some traveling down the cables, no matter how
> long the cables are. It is the low frequency components that are
> responsible for most of the burning associated with lightning. Coax
> shields won't take very much current before they melt. A large ground
> wire will lower the impedance between the tower and the common point
> ground, as well as reduce the voltage drop for these low frequency
> components.
>
> As most people know, but tend to forget at times, don't believe
> everything you read. In some cases it may even be written by someone
> who should know better. The Polyphaser book has several errors in the
> lightning area. The most glaring one is in their calculation of current
> distribution on guyed towers and in cables. By their calculations two
> runs of RG8 down the tower would have the same inductance as a 35
> inch-face tower. That leads to the conclusion that, to half the voltage
> drop down your tower all you have to do is run two coax lines down it.
> This should be obviously incorrect, even to a casual observer. You can
> end up with some large errors if you neglect mutual inductance.
>
> Also you can get some information about voltage drops by only using
> di/dt, as Polyphaser did, but that doesn't give enough information to
> draw real conclusions, because it considers only one part of a
> waveform. As a minimum, a SPICE simulation will produce closer results,
> but that too is only an approximation, because it also only considers
> one portion of what is happening. To calculate something closer to
> reality it has to be done by calculating the fields involved, probably
> using the method of moments. Here is one article using that approach.
> http://www.lightning.ece.ufl.edu/PDF/Kordietal2003.pdf
> Notice you will have to modify what was done in this article by
> substituting the appropriate waveform for the stimulus.
>
> Jerry, K4SAV
>
> Roger (K8RI) wrote:
>
>>
>>
>>
>>>According to literature from Polyphaser, if the ground systems are
>>>separated
>>>by more than 75 feet, they don't "see each other", so it doesn't make
>>>sense
>>>
>>>
>>
>>I think they are exagerating the 75 feet. Remember that key word
>>"business".
>>I think they make a good product and I use some, but I also think some of
>>their information is a bit misleading.
>>
>>It is true when looking at rapid rise times they (house and tower) will
>>see
>>less of each other the farther apart they are located, BUT they still do
>>see
>>each other. Typical rise times equate to a frequency containing the major
>>portion of the energy of roughly one MHz. Also if the cables tying the
>>tower
>>to the station ground consist of a series of ground rods and side branches
>>it will bleed off or "sink" a lot of the energy into the ground before
>>reaching the house/station. Therefore to me it makes good sense to tie
>>them
>>together.
>>
>>I've mentioned a number of times how often the big tower/array gets hit.
>>It
>>averages 3 verified hits a year. Tht is hits some one has seen. How many
>>times it's actually been hit I don't know. I do know there are no
>>unprotected coax connectors up there that have any plating left on them.
>>and
>>you can no loger get a PL-259 to screw onto the unused ports of the remote
>>antenna switch. Next time there will be protection over the connectors.
>>
>>
>>
>>>to bond them. I believe the reason is that the inductance of the
>>>connecting
>>>wire or strap is too great at that length to allow the systems to float
>>>to
>>>the same potential.
>>>
>>>
>>
>>It's not so much the inductance but rather the wave length. If they are
>>far
>>enough apart the rise and fall times are are such that even were the path
>>strictly resistive the delta T makes them appear as seperate entities for
>>all practical purposes. That distance is one whale of a lot farther than
>>75
>>feet. OTOH with a major strike, not one of the typical ones, the voltage
>>difference across a few feet can be substantial. So, depending on how you
>>view it, or the impression you wish to create, you could make the same
>>statement for 10 feet as you can certainly get enough voltage difference
>>to
>>wipe out solid state equipment.
>>
>>
>>
>>>I hadn't read that tidbit when I installed my first tower system, so I
>>>laid
>>>265 feet of 1/0 wire at the bottom of the trench to connect the tower
>>>ground
>>>
>>>
>>
>>"To me" I'd say 250 feet is approaching where I *might* consider not tying
>>them together, but more than likely I'd push to at least 300 feet. Even
>>then
>>I'd most likely make it a run consisting of a network to bleed off the
>>charge which insulated coax in PVC conduit will not. From the base of my
>>tower to the entrance groundging bulkhead is over 75 feet. I have two bare
>>copper cables (one on each side) paralleling the conduit run from the
>>tower
>>to the house. Those cables tie to the grounding plate which is the single
>>point ground for everything. There are runs perpendicular to those cables
>>that run out into the yard as well. Most of the back yard is one big
>>grounding network with 32 or 33 ground rods CadWelded(TM) to well over 600
>>feet of bare copper.
>>
>>
>>
>>>to the single point ground at the house. I'm pretty sure that was
>>>unnecessary (I like to think of it as a really long horizontal ground
>>>rod.)
>>>When I installed my new tower this year, which is about 225 feet from the
>>>shack, I omitted the bonding wire. I was convinced not only by
>>>Polyphaser's
>>>argument, but also by the fact that the outer conductors of the two
>>>1-5/8"
>>>
>>>
>>
>>Are those outter conductors bare?
>>
>>
>>
>>>runs of heliax between the tower and shack would undoubtedly present a
>>>much
>>>lower impedance path back to the shack than any wire or strap I could lay
>>>in
>>>
>>>
>>
>>Impedance is only a part of the equation. I think we rely far too much on
>>just the "impedance" of that connection when for longer, well designed
>>runs
>>the majority of the energy is disipated into the ground.
>>Do those outer conductors have the current carrying capacity of the wire
>>or
>>strap?
>>Do they provide a way to bleed off the charge?
>>
>>
>>
>>>the trench. I'm hoping the tower ground system is robust enough to
>>>dissipate
>>>any surges that may occur.
>>>
>>>
>>
>>It will (In most cases) if the system is well designed and covers a wide
>>enough area.
>>
>>Just remember a coax with an insulated jacket is a conductor straight into
>>your house while the bare copper is a path to ground that also goes to the
>>house.
>>
>>
>>
>>>73, Dick WC1M
>>>
>>>
>>
>>73
>>
>>Roger (K8RI)
>>
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>>
>>
>>
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>>
>>
>
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