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QST articles outline a logicalapproach for you to foliow when designing
or improving an exrsting system, The QST articles also give informatron
on the required length of ground stakes and many other dimenstons not
covered in this article. The problem of designing a safe system is similar
to plugging holes in a leaking boai: lf you plug all of the holes except
one, the boat sinks.
After reading the 0ST articles, you should then look at some profes-
sional resources. One good source of information can be found at the
Polyphaser TCA Hotlinks in this article,
The Problem Defined Through Examples
An Ungrounded System Example
A very simple poftable system is shown in Figure 1 . This is a typical
system that you might take out camping. The small transceiver is bat-
tery powered and is connected directly to a good dipole that is RF iso-
lated by the use of a Balun, There is no connection to ground in this
example, Since a Balun is used there will be very little RF current on the
outside of the transceive; battery or coaxial cable. All of the RF radiat-
ing current is confined to the dipole Hence, the RF pedormance is
excellent and the ground is not used for any connections.
However, there is a serious problem with the system. lf lightning
strikes the antenna, it will find its way to ground and in this case the
Radio Operator provides the conductron path. Also note that if lightning
strikes the ground near the operating site Ground Spreading Current
flows out in all directions
A Grounded System Example
The same portable system is shown in Figure 2 where a grounding
stake has been added to the system. The ground stake is connected
between the radio chassis and physicalground. Since the RF system is
welldesigned, the safety grounding stake does not intedere with the
performance of the radio This simple grounding system does not pro-
vide much if any protection to the radio but there is a path to ground
that will give some protection to the operator,
Your Basic Needs
The two simplified examples above show us that:
. A g00d RF system can be designed that does not depend on a
physical ground, However, a physical ground can be an integral paft of
ihe antenna, An example of this is the vertical antenna mounted directly
on ground with the aid of ground radials
o A safety ground must be used since lightning will always find iis
way to ground except for cloud-to-cloud strikes that do not reach us,
. lf a Balun 0r some other RF isolator is not used,there can be large
interactions between the RF system and the Safety ground system,ln
the above examples, the grounding stake can make the antenna per-
formance very poor if a Balun is not used.
Safety Grounding
lntroduction
Lightning can strike power lines and enter into the house wiring as
many of know from experience. Your house is grounded at the terminal
box where most (not all) of the unwanted current will flow into the
ground, Lightning can also strike the antenna/tower and enter into the
house via coaxial cabling. Also lightning can strike near-by. This pro-
duces large ground currents that can couple to the coaxial cables that
feed the antenna sysiem. Hence, we have to attend to three dangers:
Figure 1
Ungrounded A,ntenna System
Physical Ground
Figure 1. Block diagram of a simple ungrounded antenna system.
Notice that the radio operator is not protected. RF isolation is provided
by the Balun.
power line strikes, antenna/tower strikes and near-by ground strikes.
After you have assembled an isolated antenna that does not produce
unwanted RF currenis on the outside of coaxial cables and in the
house, ii is now possible to design a good safety ground for your radio
station.
The following discussion starts at the tower or antenna end of the
system and works its way back to the transmitter/receiver to a Single
Point Ground Panel, Here we are assuming that lightning strikes at the
antenna/tower,
The Antenna/Tower Problem
The best way to prevent unwanted coaxial currents fron reaching your
equipment is to keep the current from entering the house by divefting it
into the ground.
During a strike, the tower and coaxial cables on the tower will share the
current. This means that coaxial lines should be grounded at the top
and bottom of the tower t0 prevent current from continuing towards
your house on the cables,
A block diagram of the recommended system is shown in Figure 3.
Here, the antenna RF currents are isolated from the cables with a Balun
connected as close as possible to the antenna, The coaxial cable ai the
top of the tower, just below the Balun, is grounded with the aid of a
Bulkhead Feedthru connector that is connected directly to the tower
with a metallic mounting flange. Note that the outside if the Bulkhead
connector is grounded, not the center conductor.
The cable is also grounded at the bottom of the tower and the tower
is grounded via a series of ground radials as described in TCA Hotlink,
5 Most of the unwanted current from the lightning strike will now be
diverted into ground. The system is not perfect due to the inductance
and resistance of the connections and the finite resistance of the
ground radials. Some of the current will stiil travel towards the house.
A nearby strike will produce ground currents that can be coupled to
!l[ ) QCWA Journa . Winter 2Oa7 . w\\w,qcwa org