Better to run fat Copperweld AROUND the house and clamp to several ground
rods then rather than having lightning going THROUGH the house from one
ground point to the other. Everything you do should make Primary, secondary
strikes and surges want to drain outside and away from the house/shack/site.

A large coil or high value resistor is a good thing for bleeding off static
charges on a line so that when you throw the switch, nothing goes through
you to ground. It is common for antennas that don't have some DC grounding
feature to build static charges if open. Your antenna tuner or the input
transformer of your receiver has to drain that. Polyphaser makes good
coaxial type surge protectors, but I'm not sure about Ladder line. The gap
type arrestor does have limitations because you have to keep the gap wide
enough to avoid problems with the transmitter. Multiple series gaps only
raises the firing voltage, but the breakdown voltage will be set by the
largest gap in the series. A gap at .035 will fire at about 2-10kv
depending on humidity. You only want it wide enough so that it won't arc
when keying the transmitter. If there is a direct hit they will probably
explode or at least blow the electrodes away.

I would seriously read up on the situation in addition to asking questions
on a newsgroup. Polyphaser may still have a free publication you can order
that explains things in depth.


"Gary Pewitt" wrote in message
...

I guess I wasn't too clear. I wanted to use one switch each for two
ladder line lead ins. Since no one I can find makes arrestors for
ladder line I am building my own from spark plugs threaded into a big
copper plate which is grounded with copper strap to 5 ground rods in a
fairly small circle outside the shack window. It's about 70 ft to the
200 amp service box from the bedroom I am using for my shack.
I just won a bid on ech bay for a double throw make before break knife
switch which I'll be using also. I'm going to buy one of those MFJ
window lead through plates with multiple ladder line connectors or
maybe make my own version.
I am wondering, instead of using just two spark plugs, if it might be
more effective to use 3 pairs in line with the first pair having
really wide gaps, the second having a medium gap, and the third having
a small gap around .035"? I am going to enclose them to keep them dry
and they will be grounded to the same rods as everything else.
I'll also have a Drake R-8 receiver in my bedroom hooked to an
Evesdropper trap dipole with a Zap Trap arrestor on the co-ax. It's
too far to run a ground to the other end of the house so I plan to
ground it outside of my window. I know that's not the best way but I
don't want to run an 80 foot ground cable to the shack ground.
Thanks for all the good advice. 73 Gary N9ZSV




On Sat, 11 Oct 2008 22:12:20 -0500, Gary Pewitt
wrote:

Here's a simple question. I want to connect my transceiver to several
antennas using 450 ohm ladder line and a balanced tuner. I found a
couple of small double pole double throw knife switches for the ladder
line. There are only two ways to hook these up. I can hook the tuner
output to the center with the ground on the bottom and the antenna on
the top connectors. This will allow me to connect the transceiver to
the antenna or to ground. The second way is to connect the
transceiver to the top contacts, the antenna to the center, and ground
to the bottom contacts. This will let me connect the antenna to the
radio or to ground. Is it better to ground the radio and let the
antenna float? Or to ground the antenna and let the radio float?
Of course if I leave the switch handle sticking straight up nothing is
connected to anything.
I am inclined to think grounding the antenna is better but I have been
wrong before.

Thanks and 73 Gary N9ZSV

JB wrote:



enough to avoid problems with the transmitter. Multiple series gaps only
raises the firing voltage, but the breakdown voltage will be set by the
largest gap in the series. A gap at .035 will fire at about 2-10kv
depending on humidity. You only want it wide enough so that it won't arc

Humidity doesn't change the breakdown voltage very much. In fact,
increasing humidity increases the breakdown voltage. One correction
table for a "rod gap" with 1/2" square electrodes has a 10% correction
going from 15 torr to 30 torr water vapor pressure and a -16%
correction going from 15 to 2.5 torr, which is nowhere near the 5:1
variation cited above. Interestingly, on this basis water vapor is a
better insulator than nitrogen, since density of the humid air is
actually less. Typical breakdown voltage tolerance on a rod gap is +/- 8%.

enough to avoid problems with the transmitter. Multiple series gaps
only
raises the firing voltage, but the breakdown voltage will be set by the
largest gap in the series. A gap at .035 will fire at about 2-10kv
depending on humidity. You only want it wide enough so that it won't
arc

Humidity doesn't change the breakdown voltage very much. In fact,
increasing humidity increases the breakdown voltage. One correction
table for a "rod gap" with 1/2" square electrodes has a 10% correction
going from 15 torr to 30 torr water vapor pressure and a -16%
correction going from 15 to 2.5 torr, which is nowhere near the 5:1
variation cited above. Interestingly, on this basis water vapor is a
better insulator than nitrogen, since density of the humid air is
actually less. Typical breakdown voltage tolerance on a rod gap is +/-
8%.

That was off the top of my head so I stand corrected but there ARE
variations based on other real world stuff like air pressure and dirt on the
insulators. 5-20kv for inside an engine is real world but that depends on
other stuff too, like what kind of spark-plug, compression, resistors,
mixture, timing, but I digress. The point is the gap may have to be
adjusted for best results and might be too wide for protection. I invited
him to read up on the subject because I have other projects I have my head
into

So what are your experiences with spark gap transmission line arresters?
Perhaps you have the precise gap he should be using? My dim recollection is
of a chart for the transmitter power in use.

JB wrote:
enough to avoid problems with the transmitter. Multiple series gaps
only
raises the firing voltage, but the breakdown voltage will be set by the
largest gap in the series. A gap at .035 will fire at about 2-10kv
depending on humidity. You only want it wide enough so that it won't
arc
Humidity doesn't change the breakdown voltage very much. In fact,
increasing humidity increases the breakdown voltage. One correction
table for a "rod gap" with 1/2" square electrodes has a 10% correction
going from 15 torr to 30 torr water vapor pressure and a -16%
correction going from 15 to 2.5 torr, which is nowhere near the 5:1
variation cited above. Interestingly, on this basis water vapor is a
better insulator than nitrogen, since density of the humid air is
actually less. Typical breakdown voltage tolerance on a rod gap is +/-
8%.
That was off the top of my head so I stand corrected but there ARE
variations based on other real world stuff like air pressure and dirt on the
insulators. 5-20kv for inside an engine is real world but that depends on
other stuff too, like what kind of spark-plug, compression, resistors,
mixture, timing, but I digress. The point is the gap may have to be
adjusted for best results and might be too wide for protection. I invited
him to read up on the subject because I have other projects I have my head
into

So what are your experiences with spark gap transmission line arresters?
Perhaps you have the precise gap he should be using? My dim recollection is
of a chart for the transmitter power in use.

A good rule of thumb for small gaps with large radius of curvature
electrodes is 30kV/cm (e.g. a 1cm gap between 10cm diameter spheres).

In strongly non uniform field gaps (e.g. needle gaps), the breakdown is
typically 1/3 or less.. that is, 10kV/cm


70kV/inch or 25 kV/inch, respectively.

A sparkplug with a 0.035inch gap would breakdown around 1-2kV in normal
air. In an engine, where the density at firing is probably 5-10 times
higher, the voltage is 10-20kV as you've given.

The usual way to set spark gaps for this kind of thing would be to make
it adjustable, fire up the transmitter at worst case max power and
reflection, and set the gap to be somewhat bigger than causes the gap to
break down. A good worst case would be 2x peak RF voltage (i.e. 2.8
Vrms), since that's what you'd get with a 100% reflection at just the
wrong phase. (unless the source can supply reactive power.. then it's a
lot more complex, because of the potential for resonant rise) You could
use a current limited DC power supply or something like a neon sign
transformer to set the spacing.

But making things practical..

say 2 kW and a 450 ohm transmission line. Erms = sqrt(2000*450) = about
950Vrms.. call it 1350V peak. With sharp ended electrodes (like the
sheet metal V shapes shown in old ARRL handbooks) one might want a
spacing of 0.050 inches?