Ground Systems
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Related
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| Damage-prone installations almost always include one or more of the following mistakes:
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Basic Ground System
Functions
A ground system
provides four
primary functions:
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To help disperse or divert energy from lightning strikes |
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To provide safety in case some problem or fault energizes the cabinet or equipment chassis with dangerous voltages |
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To provide a controlled RF return path for end-fed (single wire feed) antennas, or poorly configured or improperly designed transmission-line fed antennas |
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To provide a highly-conductive path for induced or directly-coupled radio-frequency currents, rather than having them flow in lossy soil |
All of the
functions above are
distinctly unique. There are different places in the system where different
functions are necessary. Some ground
systems can serve
two or more
functions quite
effectively.
If the antenna
system has bad
common mode current
problems, caused by
a faulty antenna or feed line design or
installation, a
ground can help
reduce common mode
noise reaching the
antenna. This is
really from an
antenna flaw, and
not from the
“reflection of
signals”. A proper antenna system is unaffected by anything done with
grounds on station equipment. The exception to this is a single-wire feed
antenna brought into the operating room.
A ground screen,
counterpoise, or
ground radial system
below the antenna
can reduce local
noise sensitivity by
reducing the
antenna’s response
to local noise. This
would apply to a
horizontally
polarized antenna,
because earth losses
can cause the wave
to tilt and have
vertical response.
As we all know,
vertically polarized
signal propagate
along the earth with
much less
attenuation than
horizontally
polarized signals.
Ground rods have no
effect on this, it
requires something
that actually covers
the lossy earth
under the
horizontally
polarized antenna.
A ground will
NOT…..
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A
ground normally
will not help
reception. The
exception is an
antenna system
design problem or
installation
problem causing
the antenna system
to be sensitive to
common mode
feed line currents. If adding a station ground helps reception or transmission,
there is an antenna system flaw.
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A
ground will not
reduce the chances
or number of lightning
strikes. A properly installed and bonded entrance ground can only reduce or
eliminate lightning
damage from hits.
Lightning
Ground
Lightning
is a high energy
stepped waveform
pulse. Rapidly
changing steps
in voltage contain
high frequency
energy. This energy
has a peak in the
dozens or hundreds
of kilohertz, with
the bulk of energy
ranging from low AC
frequencies to
perhaps 1MHz.
Damaging energy
extends to hundreds
of megahertz. Lightning should be
considered a dc to
VHF energy source
with the bulk of
energy at lower
frequencies.
Current is massive,
thousands of amperes
can flow in a
lightning strike.
A good ground must
have a
very low impedance
over a
very wide frequency range.
This rules out thin
wires, and loosely
woven braided
conductors should be
avoided. The very
best ground leads
are solid wide
smooth surfaces,
although braiding
sometimes must be
used in areas that
demand conductor flexibility.
Most
of the time damage
comes from lightning
strikes on power
lines. Lightning
most often follows
the utility lines to
the house, through
house wiring to
equipment, and to
ground in antenna
systems. The
real danger is
lightning flowing
through the
equipment and house
wiring to seek a
ground. Read this
page link
station ground
With
taller towers,
lightning can be a
frequent unwelcome
visitor. Tall
structures often
require a large-area
ground with low
impedance, wide,
smooth copper
flashing or heavy
gauge solid wire surrounding
critical areas, such
as a work area or
equipment area near
the tower base. Tall
towers need a
ground that
rapidly and evenly
spreads charges out
over a wide area.
The goal is to
prevent objects near
the structure from
rising significantly
faster in voltage
than other objects
located near the
tower. Very high
currents can flow
between things near
the tower, it
is important to
provide a low-impedance path for
these currents.
Lightning
grounds should
always provide a
common low impedance
path between
everything
conductive entering
a building. This
means power lines,
telephone lines, TV
antennas, and
metallic conduits or
pipes should all
share a common
ground connection
buss that has very
low impedance.
Normally the lowest
impedance connection
is provided by a
wide smooth surface
copper flashing,
although very heavy
round copper can be
used. Round copper
has lower RF
resistance per unit
length for a given
surface area, but
flat wide copper has
less reactance and
lower overall
impedance. This is
because fewer
magnetic flux lines
encircle any given
area of wide strip
than enclose the
surface area of a
compact conductor.
In effect the
magnetic field is
“spread
out”, reducing
inductance.
There
are many sites
detailing ground
connections,
Polyphaser
being the most
accurate
overall.
A
good lightning
ground is generally
a good equipment
fault safety ground,
but it might not be
a good RF ground!
RF
grounding sometimes
requires shielding
of the earth from
radio frequency
fields, or moving
the RF energy out of
the lossy soil.
Lightning generally
requires we connect
to the earth, and
freely move energy
into or out of lossy
soil. Certain RF
ground applications
require minimizing
RF current flowing
into lossy soil.
Tower and
Antenna Ground link (moved to new page from this page)
At
House
Despite
having two 300-ft
tall towers, four 130-ft
wire verticals in a four
square, a 200-ft
tower, a 200-foot
rotating tower, and
miles of receiving
feed lines covering
distances up to 3/4
mile… I don’t use
coaxial lightning surge
protectors. I have
bulkhead entrance
panels and use
common point grounds
and a few MOV’s on
power lines, but
none of my feed lines
have surge
suppression devices. All of
my feed and control
cables stay
connected during
lightning storms,
last count that was
about 50 cables. My
equipment stays
connected to power
mains through a main
disconnect switch.
The
sole source of my
transmitting antenna
disconnects is an
unmodified DX
Engineering RR-8 HD
antenna switch.
The
case is securely
grounded to the
hamshack entrance
ground.
I
fabricate my own
copper grounding
plate for use at
cable entrances:
Every
cable enters through
bulkhead connectors
attached to a plate
like those above.
This plate is tied
into the
common
ground at my
station entrance.
That ground is
common with power
line and utility
entrance ground to
the house.
Despite
multiple hits every
year on my tall
tower, I never
suffer equipment
damage. This is all
due to my use of
proper grounding
protocol. Everything
entering my
operating room,
including power line
safety grounds,
bonds to a single
point at the
entrance point of my
operating tables.
Safety
Grounds
Radios
that operate from
power mains stand a
chance of having the
power line
accidentally fault
to the chassis.
Worse yet, a linear
amplifier with high
voltage power
transformer might
develop a secondary
to primary short,
and that short might
cause the chassis to
rise to peak
secondary voltage
plus peak primary
voltage! An
amplifier with a
2400 volt RMS
transformer
operating on 120 or
240 volt USA power
mains might have a
chassis voltage as
high as 3600 volts
from a secondary to
primary failure
inside the
transformer.
Any
advice saying our
equipment doesn’t
require a safety
ground connection is very bad
advice. All power
mains operated
amateur radio gear,
especially devices
with HV inside,
requires a safety
ground.
The
safety ground does
not really need a
ground rod, it
actually only
requires a
connection back to
the power mains
service entrance
ground though a very
heavy conductor. It
may be desirable to augment
this safety ground
connection with a few extra
earthing ground rods. As with
lightning grounds,
any special
safety ground
must bond to
the utility entrance
ground. A good
lightning entrance
ground also makes a
good safety ground,
provided it is
brought from the
entrance point panel to
the operating area
where equipment is
bonded into the
ground. Do not go
outside beyond the
entrance panel
barrier with this
ground lead.
Equipment with
properly installed
safety ground plugs,
provided wiring and
plugs are up to
current codes, do
not
require a special
safety ground
connection. They are
grounded through
house wiring.
RF
Return-path Ground
An antenna system
using properly
installed and
connected
coaxial or balanced
lines would never
require a RF station
ground. All currents
flowing out to the
antenna would be
perfectly matched by
equal currents
flowing back on the
second conductor, be
it a shield or the
second identical
conductor of a
balanced line.
The problem is
many antenna
feedpoint or feed
systems are poorly
designed. It is the
abundance of poorly
designed systems
that cause problems.
These flawed systems
are behind notions
that good Ham shack
RF grounds are
required to reduce
TVI, prevent RF in
the shack, or
improve transmitting
or receiving
ability. The
troublesome currents
are called
common-mode
currents, because
they are not normal
push-pull
transmission line
currents found in
two-conductor
transmission lines,
like coaxial or
ladder lines.
A few examples of
antennas producing
excessive RF
in-shack ground
currents:
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End-fed halfwaves, including end-fed “dipoles” of all types |
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Zepp antennas, especially those where the feed line is not an odd 1/4 wl and the antenna not a multiple of 1/2 wl) |
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Center fed dipole antennas without a balun or dipoles that do not use a feed line length that minimizes common-mode current |
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Verticals with poor grounds or somewhat sparse grounds, including “half-wave” verticals with small or no radial systems |
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End-fed longwires, off-center fed dipoles, or Windom antennas |
It can be
truthfully said if
we use a
two-conductor
feed line of any type
and have RF in the
shack problems, our
antenna feed system
is poorly designed
or constructed. An
RF ground in the
shack is absolutely
NOT required unless
something is wrong
with our antenna
system. The sole
exception to this is
a single wire
feeder, like a
longwire, brought
directly into the
shack.
An RF “return
path” ground or
antenna system
ground requires low
RF impedance. The
ground has to
spread current
around over a large
area. The best
system
uses many small
diameter conductors radiating
out at least 1/8th
wave, and preferably
further, from a central
connection point.
These conductors do
not need to contact
earth, they function
simply by providing
“electrical
mass”, or a low
RF impedance, for
the antenna system
to push against. It
is not a question of
surface area or
capacitance, it is a
question of
distributing charges
efficiently over a
large spatial area
(large in terms of the
operating
wavelength).
Induced
Ground Currents
All
efficient antennas,
including loops,
dipoles, verticals,
and beam antennas,
are surrounded by
very strong electric
and magnetic fields.
If the antenna is
close to earth (in
terms of operating
wavelength)
considerable current
can flow in the
lossy soil. Current
flowing in lossy
earth
causes power loss,
even when the
antenna and/or
ground system does
not have a
direct earth
connection. Earth
currents are
especially
problematic when
dipoles are placed
at small fractions
of a wavelength
above ground, or
when verticals are
mounted on or near the
earth’s surface.
Currents
like are minimized
by covering a large
area of earth
surrounding the
antenna with many
closely-spaced
conductors. The
conductors do not
need to be any
particular length,
they only
requirement is they
extend beyond the
area where field
density is high. If
the conductors are
less than .05
wavelength apart,
they can be
considered a large
single conductor
covering the entire
area. Much wider
spacing than .05wl,
and they allow the
lossy media between
the conductors to be
exposed to strong
fields.
The
diameter or gauge of
grounding system conductors
isn’t important, but
spacing of grounding
conductors and
overall length of
grounding conductors
are both very
important!
Isolating or
Disconnecting
feed lines
One
of the best ways to
protect receivers
and transmitters,
besides having a
good ground system
and cable entrance
feedthrough that is
bonded to the mains
ground, is to
disconnect the
feed line from the
equipment. This
disconnect has to be
done properly. The
best location is
normally away from
the operating desk,
right near the
entrance.
If
you use disconnect
relays or switches,
they should be a
double-make
double-break style.
A double-make
double-break looks
like this:
Lightning cannot
pass through from
side-to-side because
the shorting bar is
grounded when not
energized. Any arcs
across contacts
would go to ground.
As
an example of a
layout with proper
grounding, you
can see my station
entrance system
below:
This
is my house
entrance. It is
normally covered
with a box to
prevent weather and
sun exposure, to
visibly hide the
wires, and to
prevent physical
damage.
All
cables are shielded
and the shields are grounded outside
the house. The large
copper flashing
routes directly under the
house to the power
mains entrance
ground.
Control cables are
to the left,
transmitting
feed lines are in the
middle, and
receiving antennas
are to the right.
There are spare
cables also.
Inside House
All
transmitting
antennas enter
through a DXE RR-8
antenna switch. The switch case is
grounded through
large copper
flashing that ties
the station power
lines and receiving
antennas to a common
buss in the room
entrance box.
Cable shields are
bonded to that buss.
Receiving
antennas are to the
left, and are
grounded to the same
flashing.
My
station antenna
disconnect is
actually a DX
Engineering RR-8
antenna switch. The
RR-8 switch, like
the Ameritron
RCS-8V, provides
excellent center
conductor isolation
whether the switch
is configured to
ground the center
conductor or not.
This is because both
the DXE and RCS-8V
use a double-make
double-break
isolation system. I
like the DXE RR-8
better because it
has an all metal
cabinet that is
easier to ground.
This
box (I pieced
together two photos
to show it all)
looks like a window
seat when the cover
is on. Power and
telephone entrance
is on the left,
receiving antennas
are in the middle,
and transmitting
antennas are on the
right. All grounds
and shields are tied
together with wide
flashing.
ALL desk power comes from the sealed box. This box is the common
point ground for power, control wires, and antennas.
Go to
House ground layouts