Technical
Hands-On Radio
by Mark Spencer, WA8SME
H. Ward Silver, NØAX, n0ax@arrl.org
The Myth of the RF Ground
Sooner or later, just about every ham who
builds an HF station — whether it’s at
home, in the car, at Field Day, or for portable operation — experiences the rite of passage called an RF burn. Although painful, it
rarely creates a physical mark, just a certain
wariness on the part of the burn-ee. What’s
happening here?
You just had an exciting encounter with a
high-impedance point on your antenna system. Impedance being the ratio of voltage to
current, when power is applied, a high RF
voltage will be present at these points. Who
says you can’t feel RF? “But wait,” you exclaim, “the antenna is up in the air and connected to the antenna tuner! I’m not
touching my antenna system!”
Oh yes, you are! Unless your station is built
inside an RF-tight metal enclosure or is otherwise isolated from the antenna and feed
line, every coax shield, every enclosure,
every unshielded wire…anything connected to the transmitter directly or indirectly should be treated as part of the
antenna system. That includes you when
touching any of those conductors! Take a
look at Figure 1, which shows a typical
home station. Everything in that figure is
part of the antenna system of that station.
If the station is operating on 10 meters, at
least one potential hot spot is never more
than about 8 feet away. Why? Consider the
wavelength at 28 MHz. It’s 33.4 feet and
¼ wavelength is approximately 8.3 feet.
When a conductor is excited by RF, either
directly from a signal source or by picking
up radiated energy, a pattern of peaks and
nulls for both voltage and current is created.
Peaks are ½-wavelength apart and so are the
nulls, with peaks and nulls offset ¼-wavelength apart. Whether the conductor happens to be a wire, the outer surface of a coax
shield, an equipment enclosure, or a
“ground” wire makes no difference. It’s all
a conducting surface as far as the RF is concerned, regardless of what we call it.
While less dramatic than “getting bit,” one
Experiment #144
Dipole
Feed Line
Tuner
Ground
Wire
Ground
Rod
Earth
Figure 1 — The complete antenna system for a typical shack. Everything conductive, including the
operator holding a microphone, is part of the antenna system.
Power
Supply
has to watch out for RF currents, too. Any
time the voltage “over here” is different than
the voltage “over there,” current will flow.
When the current flows on the outside of an
enclosure or coax shield, it’s generally not a
big problem. The fun begins when it finds a
way into the electronics via an unshielded
connection (like a power cord) or an improperly connected shield that conducts the current inside an enclosure instead of keeping it
outside. RF that’s where it shouldn’t be can
wreak havoc with a circuit’s operation: audio
gets garbled, keyboards stop working, control interfaces stop controlling.
The situation gets particularly interesting
when instead of a coax-fed dipole, the antenna wire itself is connected directly to the
antenna tuner. This type of connection is
often used for portable operating as an
“end-fed” or “random wire” antenna with a
“counterpoise” (a piece of wire laid on the
Coax Jumper
Radio
Interface
Cables
Shielded
Mic
Cable
Operator
AC Safety Ground
ground or floor) replacing the ground rod.
In this case, the antenna itself consists of
everything from the end of the counterpoise
to the end of the wire in the air. The equipment and operator are thus all connected to
the feed point of the antenna. Imagine the
feed point of the dipole in Figure 1 being
connected right at the output of the antenna
tuner and you get the idea. This also explains why the results of using these directly
fed antennas can be inconsistent, because
there is so much variation in what the antenna system actually consists of.
1
Obviously, we would like to control the RF
voltages and currents so they don’t cause
our equipment to malfunction or burn our
fingers. The natural tendency is to think,
“I’ll just ground everything and it will be at
zero volts — problem solved!” Not so fast!
You’re partly right, but we have a failure to
communicate, as they say.
Grounding and Bonding
1
Poorly shielded connections and equipment
will radiate RF from internal electronics, too.
This creates on-the-air interference.
There are grounds and then there are
grounds. Consider the actual ground, the
QS1501-HoR01
PC
Reprinted with permission; copyright ARRL.
QST® – Devoted entirely to Amateur Radio www.arrl.org January 2015 63
Safety Ground: Heavy Wire or Strap as Short as Practical
Clamp
Securely
Three-wire Grounded Power Outlets Connected
to Grounded House Electrical System
Bond to
AC
Service
Entrance
Ground
Rod
Figure 2 — The ground bus shown in this figure provides effective bonding between the various pieces of equipment at RF. The ac safety ground is also
connected to the ground bus, but is of limited effectiveness at RF.
soil itself. The Earth acts as a “zero voltage
reference” for ac power and low-frequency
systems. The ac safety ground in your home
consists of the power wiring’s ground wire
(bare or with green insulation), which is
connected to the Earth through a ground rod
that is connected to the main circuit breaker
panel. The ac neutral of a typical two-phase
home is also connected to this same ground.
(See the National Electrical Code and your
local building codes for a complete description of what is required in your particular
circumstances.)
Any exposed conductive enclosure of an
appliance or machine — including your
radio equipment — should be connected to
the ac safety ground to conduct fault or
leakage current away from you and back to
the Earth. It is this current flow that trips
Ground Fault Circuit Interrupter (GFCI)
circuit breakers.
The purpose of the ac safety ground has
nothing to do with RF and it should never be
expected to act as any kind of voltage reference above a few hundred kilohertz. That
means even if all of your equipment is properly grounded for ac safety, you still have no
control over RF voltages and currents. In
fact, as Figure 1 illustrates, the ac safety
wiring is a part of your antenna system, too.
What if you install a ground rod outside the
station and run wide copper strap to it like
all the literature tells you? There is another
word for ground connections and that is
“antenna!” Any ground connection longer
1
than about
64 January 2015 ARRL, the national association for Amateur Radio
⁄10 of a wavelength begins to act
8 ft
Ground
Rod
Ground
Transceiver
like an antenna, including transmission
line-like effects. If the electrical length is
close to ¼ wavelength (or any odd number
of ¼ wavelengths) the impedance of the
wire becomes very high, effectively becoming a resonant open circuit.
Back in the days when most amateurs operated below 15 MHz, a few feet of wire was
electrically short enough to serve as a common connection. As operation at shorter
wavelengths became more common, the
connection to a ground rod got electrically
longer and less effective. Hams with shacks
on an upper floor had (and have) the same
problem at any frequency.
The solution is to stop looking for the elusive
“zero voltage connection” at RF. The Earth
is not a magic drain into which all of our
unwanted RF can be poured via a wire. An
electrically long connection to the Earth is
useless at RF and often causes RF-related
problems. Let’s go back to what the problem
really is: we have places in the shack where
high RF voltage exists and RF voltage differences that cause RF current to flow. These
problems can be addressed by bonding.
Bonding sounds heavy-duty (and expensive) but all it really consists of is connecting
equipment enclosures together with short
conductors so they have the same voltage.
This is partially taken care of by the lowimpedance connection provided by shields
of coaxial cables between equipment.
However, accessories, computers, and
power supplies generally aren’t connected
together with coax, so we have to provide
Computer
Operating Position
®
www.arrl.org
Keyer
Antenna
Switch
another path. A common solution (shown in
Figure 2) is to provide a wide, flat common
ground bus at the back of or even under the
shack equipment. Each piece of equipment,
including computers and other non-radio
electronics, is then connected to the bus
with a short wire or strap.
the purpose of RF bonding, no other connections are required.
It is a good idea to add a connection to the ac
safety ground from the ground bus or to use
it as part of a lightning protection system,
but the effect of that connection at RF will
be unpredictable due to the configuration of
the entire antenna system, as discussed earlier. From an RF perspective, keeping all of
the equipment as close as possible to the
same voltage is the important thing.
Bonding will not result in there being zero
RF voltage on the equipment. Bonding does
keep all of the equipment at about the same
voltage, so RF current flow between pieces
of equipment is greatly reduced with the
added benefit of reducing the effect of
lightning-caused voltage surges that affect
the station ground connection.
Next month we will talk about shielding,
both of devices and of cables, and how this
affects resistance to RFI.
2
Avoid using coax braid as an RF connection.
Once removed from the protection of the
confining jacket, the braid’s individual strands
begin to loosen and corrode, increasing the
impedance at RF. Use heavy wire or solid
strap, such as copper flashing, for best results.
3
W. Ronald Block, KB2UYT, “Lightning
Protection for the Amateur Radio Stations,
Parts 1-3,” Jun, Jul, and Aug 2002, QST.
Reprinted with permission; copyright ARRL.
RF Bonding Bus
(1/2" Copper Pipe)
Hose Clamp
Wire to Equipment
Enclosure
2
To accomplish
3
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