[TowerTalk] grounding

[Gary Schafer]

see below.

Randy wrote:
> At 09:04 PM 10/8/2005, you wrote:
> 
>> Rob,
>>
>> Each installation has different circumstances that can be solved in
>> different ways. One of the biggest obstacles I see with a great number
>> of hams is they do not really understand what single point grounding
>> means. If you put some time into really understanding what is going on
>> with that concept you are way ahead of the game no matter what the
>> budget is. Just the way things are plugged in at the shack can make a
>> big difference.
>>
>> For example, it is not necessary to have the mains power coming in the
>> same place as the coax lines. Just run a power line from a convenient
>> place over the where you have your antenna entrance panel. Put your
>> power line protectors there. Now run ALL your shack equipment from that
>> one power point.
> 
> 
> If I interpret what you are saying correctly, you're going to need a 
> seriously
> large neutral conductor between the shack and the mains panel... if a 
> strike
> occurs to the antenna, and it's looking for "Earth".... if that 
> conductor can't
> momentarily handle about a bazillion amps, it's going to fail in a very 
> spec-
> tacular fashion. And then, lightning, being the fickle beast that it is, 
> is going
> off in search of something else betwixt it, and "Earth". Methinks that 
> "some"
> of the protection that "single point entry" offers is that, worst-case, 
> the whole
> damned thing might flash-over to, ideally, the ground rod right 
> underneath it.
> I have no knowledge or qualifications that lead me to that thought, but, 
> in my
> "mind's eye", that's what would occur.

No I don't think that you interpret correctly what I am saying.
The neutral conductor has nothing to do with anything here.

That "ground rod" under the single point panel is what the single point 
ground panel is connected to. And hopefully you will have more than just 
a ground rod at that point. The power protection needs to be at the 
single point ground panel along with the coax cables etc. so that 
everything is referenced to that one point. This is the whole point (no 
pun intended) about single point grounding.

> 
> 
>> The protectors for the power can be as little as some large mov's that
>> will give some protection. Add some gas tubes of the appropriate type.
>> If you want to go a little farther add some series inductors in the
>> power line ahead of the mov's and gas tubes.
> 
> 
> It sounds like an "old wive's tale", but, it doesn't cost anything to 
> tie knots
> between the equipment and the outlet...spikes *hate* inductance...
> BTW, it's worth noting that MOV's failure mode is "open".. yer never
> gonna know if it craps out, and they DO only take X number of whacks
> before they go...much like a guard dog you never knew that you had, and
> therefore never knew when it died.

Well, no again. Tying knots in cables may do something but may in fact 
do harm. A loop in a cable while providing some series inductance is 
also a great magnetic loop antenna. It can increase the pickup of 
induced fields from conductors carrying the strike current. Be careful 
where you use them.

The normal failure mode of an mov is "shorted". They do open when 
excessive energy is applied AFTER they have shorted. It is usually easy 
to tell when that has happened as they usually blow apart.

> 
> 
>> Look at what is in some of these protection devices. It isn't magic it
>> is just proper selection of components.
>>
>> As far as Polyphaser protection devices go, you really don't need them
>> if you disconnect the lines from the equipment and ground the shield and
>> center conductor of the coax when not in use.
> 
> 
> Assuming that you aren't using them when that first "bolt from the blue"
> occurs. That first strike from the cumulo-nimbus is going to hit 
> "somewhere"
> and... it's often very far ahead of the visible bottom of the cloud that 
> shows
> rainfall. Here in FL the tops of the thunderhead are often *miles* ahead of
> the rain. Hereabouts, that's what IS known as the "bolt from the blue"; the
> rule of thumb with me is: if you can hear thunder, you are currently in 
> danger,
> and have BEEN IN  danger...

The other alternative is not to install antennas and equipment in the 
first place.

> 
> 
>> A polyphaser device is a convenient way to ground the shield of the coax
>> line at your entrance point. You can do the same thing with a clamp
>> around the cable or connector, or plugging the connector into a mating
>> connector that is fastened to your ground.
> 
> 
> If that was true, then you'd just need to ground the tower...assuming a DC
> ground type antenna. Otherwise, you'd just have the polyphaser at the base
> of the tower, right by that teensy, single, galvanized steel rod.
> 
>> The only other thing that the protection device does for you is shunt
>> any lightning energy on the center conductor of the coax to ground when
>> you don't have it removed from the rig and connected to ground.
> 
> 
> Lightning doesn't really care what path it takes, and I suspect it's far 
> happier
> going down both the shield AND/OR center conductor than for me to tell 
> it which
> way to go..

Not sure what your point is here. A well grounded tower can have many 
thousands of volts difference in potential between its top and bottom 
during a strike due to the resistance and inductance the tower provides. 
The coax coming down the tower will have that same difference of 
potential between top and bottom of the tower. There will be a large 
current that flows in both the tower and coax as the coax will share 
some of the current.
In doing so there is what is called "transfer impedance" that is 
involved in inducing energy to the center conductor of the coax from the 
current flowing on the shield. So even with a grounded type antenna 
(coax shorted at top of tower) energy finds its way to the center 
conductor.

This is the reason the polyphaser type protectors are employed. If 
energy never made it to the center conductor you would only need ground 
the shield.

The base of the tower is the wrong place to put your polyphaser device. 
It should be at the entrance to your shack.
The coax shield should be grounded to the base of the tower. But putting 
the protection device at the tower base does little good as you still 
have a moderate length of coax between the tower and the shack. That 
length can still carry significant current and can pick up current 
induced from the currents in the nearby tower etc. So by the time it 
gets to the shack you may again have a large amount of energy back on 
the center conductor for the same reasons it got there while on the tower.


> 
> 
> 
>> It is more important to connect grounds in the right place than it is to
>> have elaborate ground systems.
> 
> 
> Ideally... I'd do both. Failing that, undoing all the wires from the tower
> to the rig and flinging them out the window is pretty good. But if you
> think that that will make lightning find Earth somewhere else, you're just
> kidding yourself. Do the best you can, and say your prayers.
> 
> 73 de
> KZ4RV
> 
> Randy
> 

I agree I would do both and as much of both as you can muster. But it is 
not always possible or the money may not be there to do it.

Lightning is not a nomad.  It does not just wander aimlessly trying to 
find earth. You can not stop a strike from happening but with the proper 
ground paths supplied you can control most of it to where you want it to 
go. But it does take some doing to provide a sufficient path.

By the way I do live in South Florida where there a few thunderstorms 
every year.  :>)

73
Gary  K4FMX