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[TowerTalk] Force12 C-3 and Omni-VI+ Static Build-up

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Subject: [TowerTalk] Force12 C-3 and Omni-VI+ Static Build-up
From: (Tom Rauch)
Date: Tue, 22 Jun 1999 13:04:29 -0400
Hi Eric,

Thanks for your reply. It appears we are mostly in agreement.

I have a hard time beleiving that the majority of damage caused by 
lightning is "dc damage" and that a small dc blocking cap would 
make an notable difference. 

I asked:
> >Please tell me what "not all that fast" is in terms of seconds.
> Not all that fast (for a "typical" stroke) is a time to peak
> current of 1.5 to 2 microseconds.  If the fall time was similarly
> fast (it isn't), and the total rise plus fall time represented
> the period of one complete cycle (it doesn't), then the peak of
> the energy distribution could be expected to be found in the 280
> kHz range.

OK, I follow that.

Now, assume we have a capacitor that has a negligible impedance 
at 1.8 MHz in series with the transmission line. To not affect SWR 
appreciably, it must have an impedance less than one or two ohms.

Does anyone actually believe an additional ten ohms of series 
reactance in a system that already has dozens or thousands of 
ohms of series reactance would change receiver fault current a 
detectable amount?!

I sure don't, especially since a large proportion of energy is above 
280 kilohertz.

The capacitor, if the transmission line appears inductive at low 
frequencies, could actually increase fault current. 

> >Since no one seems to know what size the capacitor is, we might 
> >deduce a minimum value that is used by the lowest operating 
> >frequency rating.
> We might.  Or we could ask the manufacturer.

Well, we know the capacitor has to be a certain worse case value, 
or it will screw up the feed system. If it has more than a few ohms 
on the lowest frequency used, it would cause SWR problems. 
> >Another point we should consider, if rise time is SLOW and the 
> >signal is mostly DC, a shunt inductance would help protect the 
> >receiver greatly. 
> First, I never said that "the signal is mostly DC".  I said that
> the risetime isn't all that fast.  Now I've defined the meaning
> of "that fast".  It is about an order of magnitude slower than
> the response time of the gas discharge tubes under discussion.

I believe advertisements claim most damage is caused by the dc 
components of lightning, hence the capacitor helps. Obviously that 
is not true if the rise time is in the order of a millisecond, let alone 
a microsecond. Now it might be true if the rise time is in the order 
of several seconds, but I don't wanna be anywhere near that hit.

As a matter of fact if the series path is inductive, any series 
capacitive reactance could INCREASE current, couldn't it?
> Secondly, many protection devices are based on exactly this shunt
> inductance principle.

True, when the operating frequency is very very high. But certainly 
not at short wave frequencies and lower. 
> >Since damage commonly occurs through a T network tuner with a
> >few hundred pF in series and a few uH to ground, I find the
> >claim slew rate is "dc-like" hard to rationalize.
> First, have you ever had such damage happen when the tuner was
> behind a lightning suppression device like the ones under
> discussion?

No, because the lightning devices should never be placed at a point 
of high SWR. Normal RF might damage them. 

My point was a hi-pass filter with infinite series impedance and 
nearly zero shunt reactance for dc doesn't cure the problem. I 
doubt a capacitor with less voltage rating, much less series  
reactance, and no shunt reactance would work anywhere near as 
well in the same application.

I simply don't believe the claim most damage is caused by dc, 
unless they have some very special type of lightning in mind that I 
don't know about. 

> Second, once the unprotected center conductor voltage gets to a
> high enough value to jump the first capacitor, the risetime seen
> by the next part of the chain is indeed quite fast.  And even
> though this represents a small fraction of the total energy to be
> delivered rather slowly by the stroke, it is entirely sufficient
> to damage subsequent sensitive RF components.  And it's risetime
> is short enough to pass easily through the T net's other cap.

As is much of the energy in the strike. 
> >The single worse and ONLY case of major lightning damage to a 
> >rig I've had was when lightning struck a shunt fed tower (VERY well
> >grounded, with 120 radials of number 9 bare wire in swamp). Enough energy
> >made it through a 300 pF 15 kV vacuum cap to destroy the loading cap and
> >relay board in a T4XC.
> There really isn't enough information in this statement to permit
> fault analysis.  But... 
> >
> >If the stroke was "dc", you can bet there wouldn't have been all
> >that voltage in that path.
> Really?  Well, I didn't really say there was no time variance to
> the current waveform.  

If there is time-variance, it is not dc by definition. If there is time-
variance, it will pass.

The experiment suggested ignores the dc path from the tower to 
ground. In order to elevate the tap point sufficiently to arc the 
capacitor, the slew rate would have had to been fast enough and 
current high enough to elevate the tap-point forty feet above ground 
to far more than 15 kV (15 kV vacuum caps typically hold off at 
least 20 kV or more) for a long enough time to charge the capacitor 
to the point of failure.

No  way would that happen with dc. The tower would have melted.

I think the claim lightning damage is from dc, and that adding a 
small capacitor in series with the path will make a great 
improvement, is based on some odd lightning you and I aren't 
familiar with. Perhaps lightning caused when Thor drops his Yugo's 
jumper cables on your tower, instead of throwing a lightning bolt.

73, Tom W8JI

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