On 8/12/2011 2:23 AM, DrYattz wrote:
Having but recently returned to the world of shortwave radio, I'm
trying to be better informed about technical and safety issues than I
was as a kid. Now a homeowner with a mortgage and a homeowner's
insurance policy, I'm worried about lightning protection. I'm
receiving only, not transmitting. I have a ground rod just outside the
window where the antenna wire enters, and I intend to disconnect the
antenna outside the house when I'm not using the shortwave. But I
want to be extra careful!


Separate two issues:
1) having the house not burn down
2) protecting the front end of your radio

#1 is fairly straightforward and what your homeowner's insurance company
might care about

#2 is pretty darn tricky, but the homeowner's insurance company doesn't
care much about your radio.


As others will point out, a single rod pounded in does not generally a
code compliant ground make. And a single rod pounded in might actually
make things worse, as far as damage and destruction goes.

The goal is to get the lightning energy to go "somewhere else" than in
your house. A secondary goal, since sending ALL lightning energy
somewhere else is very difficult, is to not have the remainder destroy
the radio.


First off.. almost all amateur antenna installations do not fully comply
with the electrical code (and before I get flamed.. let me ask all you
would be commenters: do you have a listed antenna discharge unit? are
all your conductors no less than AWG 14 hard drawn copper or copper clad
steel? Is your antenna lead in entirely contained within a metallic
raceway? I thought not.. pace...we're going for practical here)

That said, I think that a "good faith" effort to do the right thing will
help, and besides, unless you're in a disaster area, insurance companies
generally don't work to try and deny claims on flimsy basis. there's
the whole "reasonable person" aspect and your looking for decent
information is a good start of what a "reasonable person" would do.

You might take a look at the safety chapter in the ARRL handbook as a
start (the library has it, almost certainly, or you can find someone who
will loan it to you).


A antenna discharge unit (open spark gap with a spacing of a few tenths
of an inch) to a ground rod or rods will go a long way towards keeping
most of the lightning current out of the house. It takes several kV to
jump the gap, but once the gap is conducting, the voltage drops to a few
tens of volts, and that's nothing in the overall scheme of things where
there are many kilovolts along your wire.

The real problem comes in with the fact that no matter how good a job
you do in connecting that gap to the rod, there's going to some
distance, and some voltage drop between the voltage at that arrestor
(during the strike) and "the rest of the house ground potential". If
your radio is sitting in the middle, it WILL conduct, and it WILL be
exciting.

The idea, then, is to make sure that no big voltages occur across two
terminals of the radio (that could either be antenna and ground
terminal, e.g. coax, or antenna and power supply).

The big deal is the AC power.. your wall socket is close to ground
potential (in terms of kilovolts, anyway). So if your radio's antenna
is at 2 or 3 kV, and the power cord is at ground, your radio will fry.

Run the radio off batteries? As long as you're not touching it, and
it's not hooked up with an audio cable to your PC or something, then it
just floats up to a few kV, and then, floats back down.


Most UL listed wall wart power supplies these days can hold off several
thousand volts between inside and outside (that's the so called "hi pot"
test rating).


Beyond that, it starts getting into gas tubes, and similar stuff to try
and protect the innards of the radio. How much does your radio cost?
Do you want to spend hundreds of dollars on surge suppressors to protect
a $200 radio?

it's also remarkably hard to build a very sensitive receiver that will
tolerate even a few tens of volts on the input.

Jim Lux wrote in
:

....
Most UL listed wall wart power supplies these days can hold off
several thousand volts between inside and outside (that's the so
called "hi pot" test rating).

That may be so at DC or 60 Hz, but it may be relatively transparent to
the spectral components of lightning discharge current, components that
may me significant to 100MHz or more.

There is no simple broadband equivalent circuit of the power
transformer, but at 100MHz, it might look more like some series
capacitance of the order of 100pF from primary to secondary for common
mode excitation... and that may well allow damaging currents to flow
(without insulation breakdown or permanent damage to the transformer),
whether driven from the coax shield, the power line, or more likely,
both.

Just another factor that makes design of bullet proof solutions so
challenging.

Owen

On 8/15/2011 4:46 PM, Owen Duffy wrote:
Jim wrote in
:

...
Most UL listed wall wart power supplies these days can hold off
several thousand volts between inside and outside (that's the so
called "hi pot" test rating).

That may be so at DC or 60 Hz, but it may be relatively transparent to
the spectral components of lightning discharge current, components that
may me significant to 100MHz or more.


yes.. although for the most part, if you're looking at the transient
from a "spark gap arrested lightning impulse" you probably don't have
quite that spectrum.


There is no simple broadband equivalent circuit of the power
transformer, but at 100MHz, it might look more like some series
capacitance of the order of 100pF from primary to secondary for common
mode excitation... and that may well allow damaging currents to flow
(without insulation breakdown or permanent damage to the transformer),
whether driven from the coax shield, the power line, or more likely,
both.

Could be.

Standler's book on transient protection comments that several studies
have shown that most modern consumer electronics can tolerate standard
transient impulses (both the 1 us and 6 us rise time variety) of several
kV. The not so halcyon ESD days of TTL gates connected directly to the
device connector pins are long gone.




Just another factor that makes design of bullet proof solutions so
challenging.

Owen