A rather more complete treatment than the last sent:

On 13 Aug 2004 02:16:01 -0700, (SpamHog) wrote:
On the TOP of my building there's a 50 ft steel lattice tower
(HF/VHF/UHF beams) and a tornado-proof 20 ft dish (1/2 kW UHF
moonbounce). They are some 80ft apart, both grounded.

Now how did I suspect that?

Right now, I still have no real grounding in my flat - I just have a
virtual ground connecting all the conductive material in the shack,
which does help reduce noise and hazards. I intend to at least bring
the wiring up to regulation. That won't help with lightning safety,
though.

Well, let's put some closure to this thing called ground. For much of
discussion, ground is actually a large pool of excess charge that is
highly mobile. That is, you can draw on its charge without
significantly polarizing it. When you can't, you get electrical
discharges (ground under a threatening sky becomes polarized and
lightning occurs). In some sense, the bulk of this metal satisfies
this criteria, in other senses it does not.

Insofar as RF ground, anything, even earth, which is a significant
distance (in terms of wavelength) away presents a situation in which
it is isolated and a problematic reference. In this situation, you
can conspire through tuning mechanisms to optimize that path to be a
good conductor (series resonate the lead going there); this works for
RF but for lightning that is another matter (and at different
frequencies simultaneously).

I intend to string the T2FD between the fixed part of the tower
(~40ft) and the base of the dish. I could use either for grounding,
but I'd opt for grounding at the dish, which is right atop my place.

Your need for ground when you are using a dipole is unwarranted in
terms of RF. Where would you connect it anyway? A dipole, by
definition is self complimentary. However, the surrounding metal is
certainly a disturbance to the self. Make sure the ends stand off
from the supports by a good amount - I can only guess, and that would
be 10 feet.

Now, down in the shack you still have the same problem of ground being
far away in terms of wavelength for RF. However, you are still using
a dipole and the dipole is going to force its own relationships with
the nearby structures. Presumably the T2FD will smooth out those
wrinkles. Insofar as safety ground goes, it is hard to believe that
is not supplied in your apartment - somewhere. Further, with regard
to lightning, you are probably already living inside a faraday shield
(the building's steel, skeletal structure), but make sure you ground
the coax coming into the apartment at the window or where it
penetrates a wall. To answer my earlier question, connect the coax
shield to the building frame before it begins its descent to your
apartment.

Anticipating what follows, perhaps you should simply loop couple all
of that steel at the top of the building. No ground issue there. No
direct connections. No HV problems.

Please elaborate. Do you mean turning the warship atop my bldg into a
ground reference loop?

Think of a Gamma or Delta match. Although these connect to the metal
of the structure they drive, they are inductive loops. You could as
easily build the loop and place it in close proximity to the structure
to accomplish the same thing. This gives you isolation except for the
frequency the loop is tuned to (and would limit you to a narrow band
of operation unless you have some means to remotely tune it).

... for RF purposes, most of the
sink effect comes from strapping together everything in sight,
including cabinets, metal furniture, building structures, drains,
floor support etc etc.

Suffice it to say it is adequate.

I would like to maximize the chance that any burst of energy will take
the ground path rather than the coax path. Why should putting, say,
3kV insulation between the antenna and the coax not matter?

It is based on hope.

Incidentally, you do find kV-range insulation in commercial products
as well.

Much of that is simply opportunistic and chance given the insulation
is suited to other purposes, like holding things together as a
framework made out of plastic.

Lead dressing is adequate and mounting
does the rest.

What do you mean?

Keeping leads apart and away from conductive structures replaces all
the exotic insulation you could imagine. Besides, if we are talking
about your T2FD and coax, I cannot imagine this is a problem in the
first place. You are going to want to drive the coax shield to ground
as soon as possible, not isolate it with insulation. You are going to
want to isolate the T2FD ends, but any induced potentials will have to
reckon with the built in resistor snubbing them. Such potential does
not merit heroic insulation efforts.

So it snubs any EMP but not an RF signal. What is the spectrum of an
EMP? Is it more like DC or more like all across the RF spectrum?

It is mostly Pulsed DC with very little RF over 1MHz. The pulse shape
is characteristically described as several µS rise and about 20 µS
fall. If you consider the strike contains 100,000A and integrate this
pulse over one second, then the duty cycle reduces this to 10 - 100 A.

This is not trivial, but neither is it catastrophic UNLESS that same
current is trying to push its way through Z or R! Remember,
lightning's source is an infinite constant current generator capable
of supporting any potential necessary to maintain that strike level.

Consider this: You have a huge resistor of 1000 Ohms. You are within
reach of one of its leads tied to ground. It is supporting 100,000A
through it - hence there is 100 Million volts across it. You reach
out and grab that lead going to ground. Are you going to be hit with
that same gazillion volts? The ground lead presents all of 0.001 Ohm
and with that same current, for 10's of µS you feel the tingle of 100V
- Maybe. Your body resistance is easily 1-10 KOhms and the current of
that 100V is enough to kill you if sustained (10's of µS hardly
qualifies) but probably a burning pin-prick sensation. If we convert
this burn into power, it renders you sinking (or simmering) 100
mW-seconds. In equivalent heat experience, this is like grabbing a
Christmas tree bulb (the big ones) for a hundredth of a second.

Now I understand. FYI, what gets you in a lightning near-hit _is_ an
EMP.

I once had an audible arc in an antenna due to a 1-lightning
thunderstorm. That single hit happened in the midst of a city, and
took out several power stations and countless other equipment. My
antenna was 8 miles away.

You heard the arc. That is an expression of power. This power
conversion is easily measured in the microwatts. It may sound loud,
but hearing is deceptive because it is subjective.

Still and all, microwatts for the front end of a receiver can be
overpowering. You can rest assured that this power was generated due
to the current encountering either Z or R such that it was absorbed by
the heat conversion.

Great! But what about losses?

I NEVER saw baluns with windings atop each other
- they were always directly wound on the core.
And I don't know why.

To maintain a wide bandwidth and to preserve balance.

All of yesteryear's professional toob radios and
many consumer ones included a tiny neon lamp in their front end.

I have repaired many such sets, professional and consumer alike. Yes,
some included the NE-2. That practice was discarded just as many
years ago too. There are probably a billion TVs out there, and not
one of them with this kind of protection.

Returning to the issue of lightning, I would suggest you mine the
archives of rec.radio.amateur.antenna for Richard Harrison, KB5WZI
for thousands of commercial and amateur tower installations over a
career spanning 50 years. He also discusses feedlines and lightning.

73's
Richard Clark, KB7QHC