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ANTENNA TOPICS
BUYING, BUILDING AND UNDERSTANDING ANTENNAS
Dan Farber, AC0LW
danfarber@monitoringtimes.com
Antenna, Feedline and Ground
Linking Our Radios to the Cosmos
Welcome back, my friends. This
month, I want to look at the three actors
in the title in a more interactive
way – a way that recognizes the trio as an organic
whole rather than separate entities. It seems that
for many of us the interconnection between our
radios and the ether is one of the trickiest parts
of the hobby to understand and to get right.
Grouping the antenna, feedline and ground into
a single entity may help us to see more clearly
how they interact and help us assemble better
and more efficient antenna systems. That means
more quality time on the air – and I’m sure we
could all get behind that!
❖ Grounding
Let’s start by trying to clarify some the
terminology. For example, ground. Turns out
that “ground” can mean RF ground, AC ground,
DC ground, safety ground … Which one are we
concerned with here?
Well, it turns out that antenna systems have
two completely different issues called ground.
They must be protected from lightning by a
safety ground, often involving lightning arrestors;
and to work efficiently they may need to
have RF grounding ... depending. Is it a “balanced”
antenna?
One of the common ways to “balun” from coax
to a dipole.
64 MONITORING TIMES August 2012
anced and unbalanced
from the perspective
of AC circuits. Think
of the 240 Volt power
that comes into your
home’s electrical panel.
There are two “hot” conductors
and a “ground”
(neutral) conductor.
By connecting only to
the two “hots,” we can
power 240V items like
dryers and stoves. By
connecting to a “hot”
and the “ground,” we
obtain 120V for everything
else. “Balanced,”
then, equals “two hots,”
independent of ground.
“Unbalanced” is “a hot
and a ground.” We can
think of antennas and
feedlines in this same
way. Balanced means
“two hots.” Unbalanced
is “a hot and a ground.”
Another popular
balun configuration
for the coax-fed dipole.
It’s easy to see this concept among antennas
and feedlines. Dipoles – and all their descendants,
like beams – clearly have two equally
sized elements that are independent of ground:
a “balanced” arrangement. Conversely, vertical
antennas have a single active element – the “hot”
– and, ideally, an extensive system of radials: the
“ground.”
❖ Feedlines
Similarly, transmission lines are pretty
clear-cut on this issue: Coaxial cable has that
well-insulated center conductor, the “hot,”
surrounded by a braided jacket, the “ground”
– while ladder line or twin-lead or open-wire
feeders have two equal and unshielded conductors:
“two hots.” As long as we keep in mind
throughout that “ground” in this context means
RF ground, we can understand the basic concept
that a balanced antenna, for the purpose of this
discussion, really means largely independent of
RF ground, while unbalanced antenna means
highly dependent on RF ground.
What do I mean by independent of RF
ground? I’m thinking of the hundreds of contacts
I made from a third-floor station, using a 90 foot
dipole fed with ladder line. A true wire run to
an earth or water-pipe ground would have been
over 25 feet long, so I didn’t have an RF ground.
However, the dipole, undeterred, proceeded to
❖ Balanced vs Unbalanced
That brings up our next semantic bugbear,
balanced. What do we mean by “balanced”?
The term is used for antennas and for feedlines,
but as we’ll see it’s quite common to connect
an unbalanced feedline to a balanced antenna.
Confused yet?
It’s always helped me to visualize balwork
the nation and the world on all the HF
bands, with absolutely no RFI or other interference
issues of any kind.
However, when I would feed the rain gutter
with a single wire as a random antenna – which
is about as “unbalanced” a load as a tuner ever
has to look at – stray RF sprayed everywhere,
burning my fingers on the key paddles, garbling
TV reception on every floor of the house,
sometimes scrambling PCs and the telephone
service, and once even re-programming the
kitchen microwave, two floors below. I hadn’t
gotten smart enough yet to tumble to the radial
counterpoise wire or artificial RF ground ideas,
or I could have tamed this beast.
One lesson sunk in very clearly, though: RF
ground is a much less crucial topic for a balanced
antenna; but an unbalanced one absolutely must
have robust RF grounding.
We begin to see what I had intimated at the
beginning, that antenna, feedline and ground
cannot be meaningfully separated, but are an organic
entity, and each case is potentially unique.
❖ The Balanced Dipole
Let’s try looking at it another way: consider
the ubiquitous dipole. Is it some specific
resonant length? If so, do you wish to limit its
usefulness to the one band where it’s resonant?
If you can answer yes to both questions, then
coaxial feed for this dipole becomes meaningful.
In any other case, though, you’ll probably do
much better to feed the dipole with a balanced
line like window line or Twin-lead from your
tuner’s balanced output and enjoy the single
dipole’s use on any number of bands.
Even with coaxial feed, the now singleband
dipole still properly needs a balun of
some sort at the antenna end to work efficiently.
Notice, though, that it is the balanced aspect
of the antenna that rules the day here – a good
RF ground is not nearly as important, in either
case, as is the notion of balanced feed. The real
secret of coax’s success is that its characteristic
impedance is so close to that of the dipole at
resonance.
❖ The Unbalanced Vertical
Now turn aside to a completely different
world – the vertical antenna. Dude! Somebody
stuck one end of a dipole straight into the
ground! Well, that’s basically it, but for some
reason the solid earth isn’t so good as a radiator.
Oh yeah, that’s right: It’s busy being ground.
Here’s the ultimate exposition of “a hot
and a ground” – the vertical. As any of you who