Tom Rauch w8ji at contesting.com
Fri Nov 4 09:01:53 EST 2005

> Balun is really a misnomer

Not really. Balance and UNbalanced in the context of
antennas describes voltages, not currents. All two terminal
feed systems, coaxial or parallel wires, have equal and
opposite phase currents in each conductor when properly
operating. The difference between balanced and unbalanced is
in the voltages from each conductor to the environment
around the conductor.

Antennas that have less than perfect grounds have voltage
from the shield terminal to "ground".  That means they are
not perfectly unbalanced.  They are also not perfectly
balanced. Most antennas are not perfectly balaunced or
unbalanced, and we have no problem with using the term
"balun" in those systems. The correct term and device is
actually a "current balun", because we want to be sure
currents are balanced (equal and opposite) at the two
antenna terminals.

> diameter.   However that said.. this may not be needed
depending upon the
> feed method used .. you would not gain a lot of advatage
in a shunt fed
> system  or a system where the vertical element is shunted
to ground through
> a coil  and will already will discharge static currents
and other noise
> producing effect to ground.

The problem has nothing to do with the feed system. The
problem stems from the ground system.

When the ground system is imperfect and has greater than
zero common mode impedance, there is a potential difference
between it and anything connected to that common point that
is "grounded". This is true even if it is a shunt fed

When we connect the coax, that potential difference between
the shield and the radial system causes a current to flow
over the outside of the shield. This is a particularly bad
problem with small radial systems, be they elevated or NOT

Consider a single radial and a single vertical. The common
mode impedance of the single radial just above dirt might be
50 ohms. If the vertical element impedance is 35 ohms and
one ampere is flowing, the radial end at the antenna must
have 50 volts potential to an earth neutral voltage
reference. The vertical element base voltage, with reference
to neutral earth potential, would be 35 volts.
These are the voltages required to maintain ALL current
flowing in the radial to equal current flowing in the

If we start adding radials, two may provide a common point
impedance of 25 ohms. Now we have 25 volts for every 35
volts from the antenna base to earth.  Four would be 12.5
ohms, and the voltage is lower.

This effect is why systems using a few elevated radials, say
a dozen or less, will  actually lose efficiency when a
current balun or common mode feedline choke (the terms are
interchangeable) is omitted.

I'm really very surprised so many people with small radial
systems (including elevated radials) omit the current balun
or common mode isolating device. I wonder what they are
thinking. The very first articles about elevated radials
indicated isolation of the feedline made an improvement in
performance. I measured actual system (and although I
initially though otherwise) measurements confirmed it was
true! After thinking about it, I realized it made perfect
sense. After all, I can't install a 1/4 wl groundplane with
four radials hundreds of feet in the air without some form
of isolation for the feedline shield or a judicious feedline
length selection. Omitting isolation causes feedline
radiation and a feedline interaction with SWR. As those
radials are moved closer to a lossy media like earth things
get even worse, not better.

The real bottom line is the advantage of using a current
balun on a Marconi antenna would depend on how many radials
you have, not what the particular feed system is. If you
have a small ground system with only a few radials, there is
the potential for significant current flowing over the
outside of the shield without a choke. This will add  loss
to the system, and can bring noise back to the antenna and
RFI to other things around the system. If you have a large
ground system, say twenty or more radials, a current balun
would be significantly less important.

It's the ground system that sets the requirement.

73 Tom

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