Fig. 1 An
inexpensive arc gap protector
Photos by Mark Persons
|Fig. 1 shows a hand-built
arc gap, sometimes referred to as a ball gap,
for use in AM broadcast transmission systems.
Gaps like this are proven technology to arc
over from RF to ground and help protect AM
broadcast equipment against high voltages from
static electricity and lightning strikes.
My preference is to put one at the 50 ohm
input and another at the antenna of all antenna coupling
networks. They also should be installed in AM phasors
where transmission lines enter from each tower. Another
good place for one is at the input side of the phasor
where transmitter feed lines come in. The idea is to
limit the maximum voltage to a safe non-destructive
value by diverting excess energy to ground. The project
I am describing is on the cheap.
EASY TO DO
Two arc gaps of
different sizes are shown in Fig. 2, but the theme is
the same. The larger L piece is a standard plated
corner brace, sometimes called corner iron or angle
bracket, available from local hardware stores. Brass
acorn nuts are used on each end of the gap, which is
good practice for this kind of device. They have a
smooth rounded face where the arc should occur.
I used a 1/4-20 machine screw, with lock
washer and nut, allows for adjusting the gap dimension
without laying hands on RF at the other side of the gap.
The smaller L bracket is made of aluminum and is also
threaded to accommodate plated brass hardware. I use
non-ferrous metals in RF circuits because they will not
vibrate at the radio frequency, heat up and sometimes
melt. Don't laugh, it has happened in high-power
systems. You gotta think about these things! If in
doubt, use an ordinary bar magnet as a test tool. You
should not use any hardware for RF if it is attracted to
a magnet. Yes, the larger angle bracket is steel, but it
is not on the RF side; it is on ground and not
Fig. 2: Arc gaps that anyone can build.
A 2x3/4-inch square porcelain insulator
is bolted to the two L brackets. The large arc gap has a
3x1-inch diameter insulator. The bottom bolt for each
insulator has a flat head and is recessed to be flush
with the bottom surface of the bracket. I do this by
using a much larger drill bit to provide a bevel where
the mounting hole is. Yes, there are fiber or nylon
washers at each end of the insulator to help prevent
breakage, especially during temperature changes.
Porcelain is brittle and it cracks instead of giving.
Also, I like to round off corners, if for no other
reason than to prevent injury as my hands work on the
Construction of these devices assumes you
are handy with tools and like to build things. Parts for
this project came out of my junk box but would cost less
than $10 new. Machining and assembly time for each one
was about 30 minutes. (Maybe I should have been a
machinist instead of a broadcast engineer!)
Nice, new arc gaps are available from several sources,
including Kintronic Labs (kintronic.com).
One of their most popular models is the AG-3-1.5B, which
sells for $185.
INSTALLATION & PLACEMENT
Two steel bolts with
lock washers and nuts will hold this arc gap assembly
down, usually to a metal surface. That surface needs to
be at ground potential or a wire must be connected to
that end of the arc gap to put it at ground potential.
Fig. 3: An arc gap protecting a
Delta toroid transformer.
It is especially critical to
install an arc gap wherever the system has a
Delta brand antenna ammeter or similar toroid
sample transformer. See Fig. 3.
The best location is on the
antenna side of the sample coil where lightning
is most likely to come in. Run the RF conductor
as close to the center of the transformer as
possible. Remember, lightning will take the
shortest path. That conductor to
transformer bushing distance should be larger
than the arc gap spacing. If the transformer has
a black mark across its white bushing, chances
are it has been hit by lightning. The usual
symptom is the sample output voltage will
double, causing major metering problems.
Arc gaps of this type should be used
inside, not out in the elements. In addition, you want
to mount them in such a way that the acorn nuts are
horizontal from each other. That will allow an arc to
clear quickly. Mounting an arc gap vertically can lead
to a continuous arc that does not quench easily.
One important attribute of this design is the arc
distance is held constant over time because of its rigid
mountings. You don't want it to be flimsy and cause
DON'T TRY THIS AT HOME!
A Jacob's Ladder, not
to be confused with the movie of the same name, is shown
in Fig. 4.
Fig. 4: A Jacob's Ladder arc gap
Jacob's Ladders are sometimes
used as arc gaps in high-voltage situations to
naturally and dramatically quench arcs after
they start. This one has #6 solid copper wire
where the arc occurs. Not having any
lightning available at the moment, I connected a
neon sign lighting transformer and carefully
turned on the power. The arc you see started at
the lower narrow part, then progressed up the
wider space until it popped off the end and went
SETTING THE ARC GAP
How much spacing
between points is enough or too much? Nautel's transmitter people researched
the subject and published a paper; you can access the
PDF at tinyurl.com/rw-nautel-gap.
The company is big on lightning protection to enhance
the reliability of their transmitters. Here is
more from another Nautel paper:
All of that assumes 50 ohms impedance
with no reactance at sea level. Wider spacing is
required for higher altitudes. Diameter of the ball
(acorn nut) plays into this as well. High positive or
negative reactance can be a real wildcard in the
equation. AM modulation, as you know, adds 50 percent or
more RF power on peaks and of course more voltage.
For me, it is easier to set the gap wide,
then slowly reduce the spacing while the station is
fully modulated. It can be an exciting moment when the
arc occurs! I then back off the gap to twice the
distance and tighten it down.
Storms with lightning
can even happen during winter months in Minnesota. It is
surprising but true that we get thunder snow. That's
right, snowstorm conditions in dry air can bring high
static charges and lightning at times.
Make it part of your routine maintenance
to check all arc gaps for cleanliness. You might be
surprised to find gaps badly pitted from arcing. Filing
the edges smooth might be required. Be glad they helped
save the equipment while sacrificing themselves in the
process. It is cheap insurance and makes perfect sense.