I have a T2FD antenna for 5-20 MHz next to a good ground point,
and I'd like to make the lead-in coax DC-insulated
to lessen the risk from a lightning strike.

I would line to put in a wideband RF transformer with
1) primary connected to the antenna coax and the ground
2) secondary connected to the lead-in coax, w/o any DC connection at
all.

This is a low power / RX installation, and a bit of loss should not be
a problem. There are already a few neon lamps in the T2FD balun, and
I'll add a spark gap at the primary.

I am very tempted to use either a toroid or a couple of ferrite rods
with 1:1 windings made of... spark plug wire?! The stuff takes 30kV
pulses without even blinking, If I use non-resistive type, an
appropriate container, and keep it all dry, breakdown voltage between
incoming and outgoing coax could exceed 50kV. A low-ohm, low-Z
grounding could help ensure that it's not easily exceeded.

Hopefully, very little pulse energy would come my way.

Any thoughts?
Any pointers to plans for high-insulation RF transformers?

SpamHog wrote:
I have a T2FD antenna for 5-20 MHz next to a good ground point,
and I'd like to make the lead-in coax DC-insulated
to lessen the risk from a lightning strike.

I would line to put in a wideband RF transformer with
1) primary connected to the antenna coax and the ground
2) secondary connected to the lead-in coax, w/o any DC connection at
all.

This is a low power / RX installation, and a bit of loss should not be
a problem. There are already a few neon lamps in the T2FD balun, and
I'll add a spark gap at the primary.

I am very tempted to use either a toroid or a couple of ferrite rods

A good RF ferrite toroid would out perform the rods, especially at the
low frequency end.

Perhaps something like the FT-240-43 on this page:
http://www.surplussales.com/FerToro/FerToro-1.html


with 1:1 windings made of... spark plug wire?! The stuff takes 30kV
pulses without even blinking, If I use non-resistive type, an
appropriate container, and keep it all dry, breakdown voltage between
incoming and outgoing coax could exceed 50kV. A low-ohm, low-Z
grounding could help ensure that it's not easily exceeded.

Hopefully, very little pulse energy would come my way.

Any thoughts?
Any pointers to plans for high-insulation RF transformers?

The silicone wire made for the high voltage connections to cathode ray
tubes is smaller per voltage rating than ignition wire, I think.
It is generally more flexible, also.

Something like:
http://www.surplussales.com/Wire-Cable/HVWire-1.html

If you have a fair ground at the antenna, I doubt you need more than
5kV rated wire, since the arc would have to pierce two wires, and the
instantaneous breakdown voltage is quite a bit more than the rating of
the wire.

Thank you John! I hadn't thought about CRT wire!
It surely is more flexible, which is necessary for threading
several turns through a toroid.

I happen to have the right toroid on hand, I'll get the wire.

Hmmm... I could also look at the curbs for discarded TV sets,
but I don't like the risk of getting zapped by residual charges.

On 7 Nov 2005 14:00:56 -0800, "SpamHog" wrote:

I am very tempted to use either a toroid or a couple of ferrite rods
with 1:1 windings made of... spark plug wire?! The stuff takes 30kV
pulses without even blinking, If I use non-resistive type, an
appropriate container, and keep it all dry, breakdown voltage between
incoming and outgoing coax could exceed 50kV. A low-ohm, low-Z
grounding could help ensure that it's not easily exceeded.

The real question is the grounding impedance.

You might even be able to measure the grounding resistance at DC or
50/60 Hz, but measuring the grounding impedance at a few hundred kHz
might be a bit problematic. Assuming the direct lightning hit is about
10-30 kA, a grounding impedance of only 1 ohm would create a potential
difference of 10-30 kV.

I do not know your mains wiring practises, but assuming that a
separate mains grounding electrode is used for each house, the
grounding impedance should be measured between the antenna ground and
house ground. If not, the grounding impedance should be measured
against some distant point.

Assuming that the house has a separate grounding electrode and your
antenna will get a hit, which is conducted to ground. Due to the
finite ground resistance, the house grounding electrode will move to
an elevated potential (several kV) compared to the surrounding (mains
neutral, telephone and CATV).

Thus, I think that the 50 kV isolation between your antenna and
receiver is more than enough and I would guess that more damage would
be caused due to the mains neutral, telephone and CATV connections to
your equipment.

Paul OH3LWR

Paul,

You make some interesting points.

The real question is the grounding impedance.
measuring the grounding impedance at a few hundred kHz
might be a bit problematic.

Most sources claim the energy is concentrated between DC and a few tens
of kHz. This means that impedance is an issue, but not a terrible one.
Typical faraday cages (few make lightning "rods" anymore...) are
anything but low impedance, yet thick enough to take 100kA without
melting.

Assuming the direct lightning hit is about 10-30 kA,
a grounding impedance of only 1 ohm would create
a potential difference of 10-30 kV.

Right. Perhaps even worse on the high end of the spectrum.

the grounding impedance should be measured
between the antenna ground and house ground.

This reminds me of a web page from your corner of Europe:
http://www.kolumbus.fi/oh5iy/back/Ham%20Radio.html

The take-home point is that one wants all sensitive equipment (and
people) to be on a high-insulation, high-impedance branch of a
multipath graph, while bolt energy is offered a low-insulation,
low-impedance branch to run through.

There are interesting implications:
Assuming that the house has a separate grounding electrode [...]
due to the finite ground resistance, the house grounding electrode
will move to an elevated potential (several kV) compared to the surrounding
mains neutral, telephone and CATV).

Not necessarily. If grounds are separate, inter-groundpoint resistance
will if anything decrease the risk of such an transient. If there was
a single grounding point, and ground resistance were high, there would
be a greater risk of the grounding point becoming a source of common
mode transient towards the land lines.

In my case, my shack is at the ground floor of a 9-story building, the
antenna is on top, and all grounds are pretty heavy duty and bonded -
but not low impedance. The risk for the land lines would be about the
same w/ or w/o my puny 20m-long T2FD. (The Other Guy has a 35ft self
supporting lattice with tribander, 4x15 @ 144, 3x32 @ 430, 2m dish @
1296, weather turnstyle, 40/80 dipoles, collinear @ 144, and a 6m
moonbounce dish @ 430. When I added the T2FD nobody noticed).

As for the DC block, I think I found the right container and location.
A 1 - meter PVC pipe that will hang horizontally under a roof edge less
than 2m from the flue lines. Pipe and coax can tiptoe away from the
chimney in a environment that stays dry when it rains.

I forgot to mention... the grounding is via two massive, parallel
steel flue lines, each a foot in diameter, that drop straight to a big
pig-iron furnace underground, all bonded to the common building ground.

On 9 Nov 2005 03:17:47 -0800, "SpamHog" wrote:

In my case, my shack is at the ground floor of a 9-story building, the
antenna is on top, and all grounds are pretty heavy duty and bonded -
but not low impedance.

I had assumed that you lived on the ground level and had the antenna
feed point somewhere further in the garden.

In your actual case, assuming that the separate grounding wires come
directly down from the roof and connected to the grounding of your
apartment at the ground level and assuming that the lightning bolt
rise time would be 1 kA/us. A thick wire has an inductance about 1
uH/m, thus, there would be a voltage gradient about 1 kV/m along the
grounding wire. If the antenna system grounding wire from the roof is
30 m long, the isolation transformer primary side potential would be
30 kV above the building neutral bar and also 30 kV above your
apartment potential as well as 30 kV above the isolation transformer
secondary. The building ground bar potential will be somewhere above
the average potential of the surrounding countryside.

With 50 kV isolation at the transformer, nearly 2 kA/us rise times
could be tolerated. To reduce the grounding wire inductance, several
grounding wires would be required well separated from each other.

Paul OH3LWR

On 9 Nov 2005 03:28:31 -0800, "SpamHog" wrote:

I forgot to mention... the grounding is via two massive, parallel
steel flue lines, each a foot in diameter, that drop straight to a big
pig-iron furnace underground, all bonded to the common building ground.

That should reduce the inductance considerably and thus the antenna
structure would be much closer to the building grounding bar potential
during a lightning hit.

Paul OH3LWR

A common lightning protection technique used in many commercial radio
stations is simply to wind maybe 50 turn of the coax around a 6 or 8"
carpet tube or PVC pipe, then ground the shield on the coax as it
leaves the bottom of the coil. Often but not always we would put a
spark gap to ground at the top of our so called "lightning choke". A
similar technique was employed to protect the "sampling loop" coax
feeders. The only thing that we used transformer lightning isolation
for was the tower lighting circuits (our tower lighting transformers
were large guys of conventional core construction, but with the
secondary winding isolated by about about 2" of air spacing in all
directions).

Each of our 4 225-ft towers sat on an insulator to isolate them from
ground, with a spark 1/4-inch spacing spark gap with two massive 2"
balls connected to the ground system. The only problem that we ever had
with this setup was that in the summer, occasionaly a large insect
would venture into one of the spark gaps and be instantly carbonized by
the r.f. being transmitted -- triggering automatic shutdown of the
transmitter in the process. When this happened, one of the operating
engineers would have to divine which of the four towers was shorted,
then fly like lightning itself with a flat file in hand to the tower
and clear the problem (while hoping that no one would take it upon
themselves to reset the transmitter while he was making repairs)!

Ahh, the fun days (NOT)!

Harry C.

Yesss! The lightning choke! An idea that even has intuitive appeal to
me, and I have already gotten some pipes to wind one on (and weatherize
it too).

Even w/o a dedicated grounding on the "cold" side, it has the effect of
slowing the rise and giving the spark gap more time to work on the bolt
energy.

Chokes are even available commercially, and are used a lot in animal
concentration camps (well, make that on electrified cattle fences).

See
http://www.kencove.com/ShopDetail.php?recordID=MWLA
and
http://www.kencove.com/ShopDetail.php?recordID=MLC.

Now off to buy some CRT high voltage wire for the DC grounding / DC
block wideband transformer.