Antenna ground or rig ground?
 

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%.

Antenna ground or rig ground?
 

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.

Antenna ground or rig ground?
 

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?

Antenna ground or rig ground?
 

OK, thanks again for all the good advice. After concidering all the
suggestions and re-reading 3 different ARRL antenna manuals I have
decided to go with a buried cable (uninsulated of course) all around
the house and extending out to my vertical antenna base. I will run
ground straps from the electrical service box, my bedroom receiver,
and the entrypoint for my feed lines to the buried cable. I am going
to go with the spark plug (non resistor type) protection combined with
knife switches to disconnect the rig -and- ground the ballanced
antennas to the buried cable. All co-ax will have commercial gas type
protectors grounded to the cable. There will be a copper bus on the
back of the shack bench with -all- gear grounded through it to the
buried cable.
I will also put spark gaps on the base of the tower grounded to the
cable. There will be a large expensive surge protector in line with a
power distributing strip on the bench which will be unplugged at the
first indication of electrical storms.
The inspiration for all this is a large Oak tree out back with the top
15 or 20 feet blown off by a lightning strike.
If there is anything I have missed please call it to my attention.
Thanks 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