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1994-11-27
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Article 22263 in rec.ham-radio:
From: gary@ke4zv.UUCP (Gary Coffman)
Subject: A primer on grounding
Summary: Broadcast quality ground systems
Keywords: Lightning, RF, Grounds
Message-ID: <827@ke4zv.UUCP>
Date: 3 Jul 90 18:18:38 GMT
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Followup-To: Rec.ham-radio
Organization: Gannett Technologies Group
Lines: 123
Grounding Techniques for Radio Installations
by Gary Coffman KE4ZV
There are three main reasons to ground radio equipment.
1. Safety grounding to protect operators from accidental
electrical shock.
2. RF grounding to prevent spurious and harmonic radiation
and to enhance antenna efficency.
3. Lightning protection.
Each of these require different grounding technique. A careful analysis
of the ground methods used is required to determine if all three
objectives are met.
Safety grounding is in many ways the easiest criteria to meet. Simply
bonding all equipment cabinets to the power company ground with conductors
of low resistance and adequate current carrying capacity to blow the
circuit breakers will meet safety requirements.
Effective RF grounding is often much harder to achieve. The ground
path must not offer any significant impedance at the frequencies of
interest. Since the frequencies of interest are often octaves apart,
this is challenging. The frequencies of interest are the fundamental
frequency of the transmitter, the harmonic frequencies of the transmitter,
and any spurious frequencies the transmitter may generate. The latter
is usually the toughest.
Grounding for lightning protection is difficult due both to the
magnitudes of the voltages and currents involved and to the fact
that the lightning waveform is a step function and has considerable
RF energy.
Probably the WORST problem one faces in designing an effective
grounding system is the prevention of GROUND LOOPS. Ground loops
will cause unintended currents to flow in circuitry, often with
disasterous effects. Either damage or degraded operation will
inevitably be the result of a ground loop.
The best method of securing a good ground for a radio installation
is to use a GROUND WINDOW. The ground window technique requires
that every cable that enters or leaves the radio room pass through
one small area where all ground connections are made.
The power company ground must be bonded to the ground window and
surge suppressors such as those marketed by Lightning Protection
Associates should be installed in series with the hot wires. Note
that simple shunt protectors will not be sufficient to protect the
equipment in the event of a direct lightning strike. Series protectors
are designed to open the circuit under severe overload.
All coaxial cables must have their shields attached to the ground
window and have their inner conductors clamped with an arc cartridge
designed to fail shorted. In addition the inner conductor should
be fused in a manner that will open the line when the arc cartridge
fails.
+
Telephone cables must enter through the ground window and have their
leads clamped with MOVs and arc cartridges that are designed to fail
shorted. In addition all leads must be fused in a manner that will
open the line when the arc cartridge fails.
Each piece of equipment in the radio room must be attached to the
ground window by a wide heavy strap installed so as to be as short
and straight as possible. Neatness definitely does not count here.
Don't "dress" the ground cable, make it short and direct. Do not
"daisy chain" grounds. Make sure that the only path from one
piece of equipment to another is via the ground window. This means
that interconnecting shielded cables should go from the equipment
out to the ground window, have the shield bonded to the ground
window, then return to the next piece of equipment. This is the
only sure way to prevent circulating ground currents.
Now that every piece of equipment in the radio room is at the same
potential as the ground window, the ground window must be brought
to true earth ground. This is fairly easy for DC and low frequency
AC, just make sure the cable is heavy enough to have the smallest
possible voltage drop across it. For RF, inductance and resonance
effects must be considered as well as skin effect. A wide flat copper
strap that is routed as straight as possible to earth ground is
preferred. For maximum lightning protection, the ground strap must
never travel upward because the space charge will resist the current
flow. Sharp bends will act as single turn inductances and should be
avoided. Since a single ground cable will exhibit resonance at certain
frequencies due to it's length, several ground cables should be used
with each a different length. The lengths should be chosen such that
a cable that is near a quarter wavelength at a given frequency will be
paralleled by a cable that is near a half wavelength. In practice,
several cables varying from the shortest possible length to
twice the shortest length should be paralleled so that at least
one will present a low impedance at any frequency. Do not coil
the longer cables, instead fan them at the center point.
The method used to route cables down the tower will effect
the degree of lightning protection achieved. If possible use
a plumber's delight type of antenna that is dc grounded to the
tower. Route the coax down the inside of the tower, and ground
the shield of the coax to the tower atHs/_.x6 points. This last
will short out the single turn transformer formed by the tower
and the shield of the cable. Unless this transformer is shorted,
currents flowing in the tower to ground will induce currents in
the shield of the coax.
The true ground must be more than a single stake driven into the
earth. If the tower is ground mounted, use the base of the tower
as the center of the ground field, otherwise drive an eight foot
ground rod to form the center point of the ground field. Run radials
out from the center of the ground field to a buried loop connecting
a series of ground rods separated from each other by no less than
eight feet. A minimum of eight rods should be used. If possible,
the radials should continue outward for one quarter wavelength at
the lowest frequency of operation. If your tower is not ground
mounted, bring it's base ground back to the ground window using
multiple lengths of cable as discussed above. Do not run a ground
cable directly from the isolated tower to the true ground or a
ground loop will surely be created that can allow damaging
circulating currents to develop.
This all sounds like a lot of work and expense, but field experience
has proven that a system like the one described will withstand
direct lightning hits without loss of equipment, air time, or
lives.
End of article 22263
\.