On Fri, 12 Aug 2011 02:23:13 -0700 (PDT), 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!

So I'm considering some options. Which make sense to you seasoned
hams? Do any of these leave me open to my homeowner's insurance being
voided if I get a direct hit by lightning?

Hi Rees,

For the Home:

If you implement an intention to control lightning hazard, I would
suspect a jury would see it your way if you sued the insurance company
for failing to uphold their policy with you. However, they might
grieve at yielding to the insurance's arguments against you.

It wouldn't get to trial if you could demonstrate that you met the
electrical code.

The elephant in the room is: have you properly installed that ground
rod? Driving a long one deep into the ground does not qualify for a
"yes" to that question.

Ground is a very complex and mysterious thing for many, many people.
The electrical code doesn't go any great distance to inform you of its
mysteries, but it does give you solutions that work.

For the Receiver:

As for protecting your receiver from the lightning risk presented by
putting wire up into the air. All of the lighting protectors you have
links to guarantee you will see a couple of hundred volts across the
terminals. The receiver is not going to survive. The distinction
between near strike and direct strike are not really material to the
problem of frying the receiver. The difference is merely one of
personal drama.

You may encounter this potential even on a seemingly cloudless day.

Receiver solutions:

Short the potential. This may be accomplish by using the "Folded
Dipole" design of horizontal antenna. A "Folded Monopole" design for
a vertical antenna. Other designs that employ loop coupling would
also reduce the risk. All such options offer a DC short, but with an
RF impedance. This insures static electricity drain, but allows an RF
voltage (the short wave signal) to develop. A large inductor across
the feedline or conventional dipole feed point provides the same
protection. The coil acts as a DC short, but as an RF open.

Some SWL lightning protectors employ paired diodes to reduce the
voltage risk to less than 10V - trivial indeed. Unfortunately, these
same diodes will conduct in the presence of large RF fields (recall my
discussion about nearby AM transmitters, and "nearby" is relatively
near for RF but seemingly distant if you had to walk or even drive to
it). This conduction will ruin listening opportunities with a jungle
of mysterious signals "that shouldn't be there." Example, hearing an
FM rock station feed on the 60M band.

Having a Tuner between your radio and antenna shifts the dynamics such
that the gap arrestors are useful - until you throw the bypass switch.

73's
Richard Clark, KB7QHC

Thanks, Richard!

But help me understand this:

All of the lighting protectors you have
links to guarantee you will see a couple of hundred volts across the
terminals.

http://www.universal-radio.com/catal...tect/5611.html says

* 230V DC discharge voltage ±15%
* Max 1000V surge (1x 40 µS duration)
* Max 6000A surge (1x 40 µS duration)

Which tells me that if the incoming current exceeds 230V, it dumps up
to 1000V and 6000A to ground. Right?

Rees

On Sat, 13 Aug 2011 05:52:34 -0700 (PDT), DrYattz
wrote:

Which tells me that if the incoming current exceeds 230V,
.... if the incoming voltage exceeds...
it dumps up to 1000V and 6000A to ground. Right?
Sort of (voltage does not dump, so to speak).

Hi Rees,

The voltage in excess of 230V sustains the dumped current. The 230V
is still there, all the same.

That is to say that the resistance of the gap is (for all practical
purposes) infinite at voltages below 230V. For voltages applied that
exceed 230V, the resistance of the gap is 230V/2300A = 0.1 Ohm.

I chose 2300A both for ease of math, and as being representative.

Hence the gap undergoes a astronomical change in resistance in a very
short time (about a microsecond or less).

This is great fortune to the house's protection. This is poor fortune
to the radio's first transistor which resides in an environment of, at
most, 9V to 12V as a supply for gain, and 0.5V as typically the
highest voltage seen at the input. Transistors are rated to sustain
higher voltages, typically in the 10s of volts, sometimes higher.

230V to 1000V surge (as guaranteed) offer a speedy death. Of course,
of the original source being orders of magnitude greater, lightning
boosts speedy death to instantaneous (within nanoseconds).

Let's take a scenario closer to experience, and one that could easily
be likened to a near death experience at that. Open the hood of your
car, grab an unconnected spark plug lead with the end just a tenth of
an inch from the engine block (a common test of the ignition system).
You observe a spark. Dare you hold the metal clip where that spark is
jumping from it to the block? Dare you even hold the lead on its
insulation? The gap guarantees a certain voltage, and the system
guarantees a certain current (otherwise you would be forever stalled
in the driveway).

Why the hesitation in holding this lead, IF ALL the voltage and
current is dumped into the block? Experience will inform you of why
you hesitate, and why the transistor fears elevated potential.

An antenna invites access to elevated potentials from many sources
other than lightning. Even on a clear day, a dipole can accumulate
enough charge to make the spark gap sizzle. This would be
extraordinary circumstances in some parts of the US, and typical in
other parts. Yet and all as this may be commonplace, radios still
play and life goes on. There are many other factors to consider
insofar as what the input transistor has to suffer or enjoy.

73's
Richard Clark, KB7QHC