Transmission Line Operation
| Many articles reach conclusions about feeder balance based on antenna patterns alone. The thought is, a good pattern means good feeder balance. Unfortunately, a good pattern does not mean the system’s feed line is properly balanced. Even if imbalance does not distort pattern, imbalance can lead to RF in the shack, and can increase noise levels in the antenna system while receiving. Distant pattern does not confirm proper feed line balance, and is actually a relatively poor way to determine proper feeder operation. |
Related pages Skin Depth
Common Mode Current
Conventional Use of Transmission Lines
We all know a traditional transmission
line system appears like this:
Standard coaxial
transmission lines.
When this type of
line is working
without unwanted
radiation, all
currents are inside
the line. The
outside world is
isolated from the
inside of the cable
by the
skin depth of the
shield.
Voltage from the shield to “ground”, or the environment around the line, ideally
is zero.
Balanced
two-wire
transmission line.
In a balanced line, each conductor carries equal and opposite currents (just
like in coax) but each conductor has equal and opposite voltages to ground or
the environment around the line.
Twisted pair is
the same, or similar
to, a balanced line.
Fig 1.
In each of the
lines above, the
feed line will not radiate or
produce substantial
electric or magnetic fields external to the
immediate area of
the feed line. The lack of external
fields, even at a very small distances, is rooted in
three conditions:
1.) All outgoing
currents on one
conductor are
matched by equal
level and exactly
opposite phase
currents on a return
conductor at any
given point along
the line. This
causes an equal and opposite magnetic field along each conductor.
The opposing magnetic fields, caused by equal currents flowing opposite
directions at any
instant of time, cancel.
This prevents the
transmission line
currents from
creating magnetic fields outside the general area of the two conductors.
2.) All voltages from each conductor of the line to the outside environment
surrounding the line are either contained within
the coaxial cable shield, or
in a balanced line
conductor voltages are exactly
equal and opposite
to the
environment around the line.
3.) The vector product of differential current flowing
in conductors, and voltage between line conductors at any point along the line,
always equals the power transmitted in transmission line mode.
Number three above
is very important.
Two and three indicate a TEM,
or transverse
electromagnetic wave. To understand
this, think about how your
rig connects to your feed line. Everyone knows the alternating current coming
from our
transmitters have voltage across the output jack.
Our transmitters
also supply current. At any instant of time when
energy is being transferred to the load ((except when zero is being
crossed)) the voltage polarity of the two conductors is of opposite signs. Except
for zero crossing or when the transmitter is off, the potential difference is
always there. The vector product of voltage across the line and current flowing through the line always
equals applied power.
The conditions above are required to support energy flow through a
transmission line. That mode is called TEM mode, or transverse electromagnetic
mode. All two conductor transmission lines, either coaxial or balanced, transfer
energy through the line by TEM mode.
Here is what Edward Jordan and Keith Balmain said about TEM mode operation of
transmission lines in the classic Prentice-Hall Electrical Engineering Series
textbook “Electromagnetic Waves and Radiating Systems”:
To make a long story short, classic transmission line theory (called
“ordinary transmission line theory” in the text above) requires
a transverse
electromagnetic
wave to be launched from one end of the two parallel conductors forming our
transmission line. If we do not do that, we simply have two parallel conductors
magnetically and electrically coupled. Energy will not be confined to the
“transmission line”, and can radiate out into the surroundings
freely. This is a very important distinction when dealing with feed lines and
antennas! When a
line is operating
only in transmission line mode (transverse electromagnetic mode), we can sleeve the line with
ferrites and properties inside the line do not change. The
electrical length
inside the line does not change
and losses inside
the line do not change. This
occurs because energy in a two-conductor transmission line is
transferred via TEM mode; fields are confined to the general area between
or around the conductors.
Except for extremely
low leakage levels caused by slight flaws in the lines, signals don’t
leak in and signals don’t leak out when we have a transmission line operated in
transmission line mode.
Balance
Unbalanced lines
(coaxial cables) actually have equal and opposite currents
in the shield and center conductor at any place along the transmission line. So
do balanced lines. This leaves us with a question. What makes one line or
system balanced and the other unbalanced? Currents behave the same way and are
always balanced in a properly working transmission line, its source, or its load
regardless of line type, so what’s the difference?
The thing separating balanced from unbalanced lines or systems, including antenna feedpoints, is
voltage from each conductor or terminal with respect to a physical
or imaginary reference point representing the world around the source, feed line, or
load. In the case where the feed line does not radiate both systems
have equal and opposite currents at every point. These points include the source,
the entire length of transmission line, and the load. It is the voltage that actually makes
sources or loads “balanced” or “unbalanced”, and the
containment of fields inside a shield that causes a coaxial transmission line to
be considered “unbalanced”.’
We often hear a balanced feeder
is properly balanced when the feeder has equal
currents in each
leg. Many articles, test methods, and even some test
equipment measures feed line current with independent RF current metering of each
conductor, not accounting for phase. Two-conductor feed lines must have equal currents in each conductor to be
balanced, but having equal currents, conversely, does not mean the feed line is balanced.
As a matter of fact a transmission line with exactly equal currents in
each conductor can be perfectly unbalanced, rather than balanced! Equal currents
only mean a feed line might be balanced, equal currents
do not mean the feed line is balanced. The complete rules
for a balanced
feeder, referring to the drawing below, are:
- Conductors 1 and 2 must
have equal
currents at either end entering and leaving that particular end, and current
levels must be equal between conductors at any point along the line - Each
conductor, all through the system, must
have currents 180
degrees
out-of-phase with
the opposing conductor at any given point all along the line - Each
conductor must
have equal and
opposite voltages
in reference to
ground (A or C), or to the
spatial area
around the
conductors (B). As
with current, voltages from each conductor to A, B, or C must be
180 degrees out-of-phase with each other.
Current not only has to be equal in both conductors at all points in the
system, current
also has to be exactly out-of-phase in opposing conductors. We must measure phase, or at least the
vector difference in currents, or we cannot be sure current is balanced. Even if
we measure phase and level, we can still have a large voltage imbalance, and
voltage imbalance
can cause strong external electric fields. Unless all conditions above are
met, the feed line
could be a source of
unwanted local or distant energy. Even if it does not distort pattern, imbalance
can
lead to RF in the
shack, and can increase noise levels in the
antenna system while receiving. Distant pattern
Common Mode Excitation
We can make a transmission line become a conventional radiating conductor if
we apply energy in a non-TEM mode. This can be useful when we wish to use a
feed line as an antenna or as a conventional conductor. When we excite a cable
like this, the cable
freely radiates:
or like this:
All of the situations immediately above cause common-mode problems. This is
because even if the equal current rule is met, the line violates equal and
opposite voltage rules.
When imperfectly excited, things outside the line influence transmission
through the feed line. Adding
a ferrite core will add loss and modify the differential impedance of the line,
changing SWR. This is true even when currents are equal in the two
conductors, and can even be true when currents are equal and opposite, as long as
the line was excited from end-to-end!
The key to having a line behave like a transmission line is feeding it
differentially at one end, and not applying voltage across the length of one or
both conductors.
This is a transmission line as we generally know it, and as dozens of
reputable engineering textbooks define it:
The above configuration shows a direct wire connection from source to load,
and cannot transform voltage, current, or impedance based on turns ratio. This
fits the definition of classic transmission line, which requires TEM
mode in coaxial or parallel wire lines. Jordan and Balmain cover this
extensively in “Electromagnetic Waves and Radiating Systems” (Guided
Waves, p215, 2nd edition). Kraus also covers this in “Antennas” in various
sections dealing with transmission lines and wave propagation, as does Terman in
his “Radio Engineers Handbook” Circuit Theory chapter under the
subheading “Transmission Lines”.
Most engineering text I have clearly state parallel conductor transmission
lines employ TEM mode of energy transfer. If not, they are not considered
transmission lines.
Uses of Parallel or Concentric Conductors in Non-transmission Line Mode
Parallel or concentric conductors (coaxial conductors) in non-TEM mode have
two major applications, antennas and transformers. Antennas include (but are not
limited to):
- Coaxial dipoles
- Coaxial or
“shielded” loop antennas - Folded dipoles
- Folded unipoles or monopoles
- Broadband Transformers
See Lenz’s
Law
since 7/1/2005