Index |
Balun (published in
Electron #4, 2007) Introduction Baluns have been around from the beginning of
wired and wireless communication. Baluns in radio antenna systems in
particular have been discussed from the early radio-days and are may still
being found in current radio magazines. Although the balun is a simple and
straight forward circuit component, the design and application has often been
surrounded by some sort of 'magic', with magistrates riding hobby-horses.
Despite the long balun history, or may be just because of this, the subject
of baluns is still popping-up regularly 'on the waves' and therefore it seems
a good idea to put various aspects around these components a little bit into
perspective in a short series of articles. Let's first take a short look into some balun
applications in radio-communication. The word 'balun' is an assembly of balance
to unbalance transformer, a
transformer to connect a balanced system to an unbalanced system without
influencing parameters in either system. Balanced to unbalanced transitions
are part of our everyday radio-life as in balanced microphones for more
dynamic range, at inputs and outputs of balanced amplifiers either base band
audio or at HF, feeding balanced antenna systems etc. Baluns therefore may be
discovered at various places and it is clear the same functionality to look
somewhat different at different parts of the systems. - Around a balanced microphone a low-power
balancing voltage-transformer may be an optimal component. - At the input of a balanced HF FET
amplifier, both branches should be fed with a symmetrical voltage. - At the output of the same amplifier
currents should be balanced for symmetrical loading and canceling of even
harmonics. - At a symmetrical antenna, both halves
should be fed with an equal and opposite phase current to obtains a symmetrical radiation pattern. These are only a few of many situations
requiring a balun at the transition point. It is clear each situation in the
above examples to require a somewhat different approach and before we start
thinking about baluns, it is obvious we should first recognize the particular
'problem' we are trying to solve and the voltages, currents and impedances
around the application. In almost all modern commercial transceivers
we find a balanced, semi-conductor amplifier at the transmitter output stage,
that is coupled to an a-symmetrical output terminal by means of a wide-band
balun. Various options are open to connect this a-symmetric output to the
antenna, of which one is to apply an a-symmetrical feed-line to a balanced
dipole antenna, again through a wide band balun. This we recognize as a
double 'transformation' of which the last part has to be constructed by the
owner of the system. This double transformation did become popular
after the last world war, when much ex-army material became available to
destitute European radio-hams. Part of this army dump was consisting of loads
of coaxial transmission- lines since this had shown to be a reliable means
for quickly rigging up transportable transmitter stations in the field.
Before this period almost all transmitter stations where connected to the
antenna through symmetrical feed-lines, that not only exhibited very low loss
but were also easy to mach to the high(er) impedance of the tube transmitters at that time. Pro's and con's of
various antenna feeding methods have been discussed more extensively in the
chapter "Where does the power
go", and appear to be closely related to the impedance (range) of
the antenna system. In this article-series baluns will be
discussed mainly for connecting a-symmetrical feed-line to balanced
antenna's, to ensure equal but opposite phase currents in each dipole half and
no parasitic currents outside this circuit. When designing symmetrical antenna's in an
antenna design environment (e.g. Mmana or EZNEC), we ensure the design to be perfectly
symmetrical and find radiation patterns to match this symmetry, so why should
the practical antenna currents deviate from this perfect symmetry? In practice dipole antenna's rarely are
perfectly symmetrical because of all sorts of obstacles around or near this
antenna, (a-symmetric) vicinity of trees, wires to couple to metal parts
(roofing, drainage), different soil-types or soil humidity etc. A second important factor is the feed-line
itself. When an a-symmetric feed-line is connected to a symmetrical antenna, both
antenna halves 'see' the impedance of this feed-line, with one halve to also
'see' the outside of the a-symmetric feed-line. If this additional wire
exhibits a high impedance as compared to the antenna at the feed point,
relatively little current is 'leaking' to this additional antenna. This
parasitic antenna however may also exhibits a low impedance depending on the
length of the feed-line. In this latter situation the out-side feed-line may
become the main antenna with a completely different radiating pattern than
designed for. This will not only be noticed at your distant communication
party since the parasitic antenna is 'stealing away' communication power, but
also at your neighbors when they unwillingly are listening to your
radio-contacts through the stereo equipment. It may even be apparent at your
position at the controls when HF feedback creeps into your equipment or make
your transceiver 'hot' to the tough. When receiving reciprocal problems arise with
the unwanted additional antenna to tap into the 'electro-smog' around your
house and your neighborhood, since this type of noise usually is vertically
polarized. All in all it may be clear that balanced
(antenna) systems should remain balanced and should be isolated from
unbalanced feed-lines by means a proper balancing device. It is also clear
this device should be of the current balancing type, also known as a 1 : 1
current transformer or sleeve choke. In a current transformer, conductors are
tightly coupled; a current in one conductor will have a current flow in the
other of equal magnitude but opposite phase, a schematic example may be found
in figure 1.
This 1 : 1 current transformer consists of
windings of equal turns that are tightly coupled. Current 'i1'
is entering the transformer at
terminal A and is leaving at terminal C. The current flowing in winding A-C, will induce a current in winding D-B of
equal magnitude, that is leaving as 'i2' at terminal C. This current will introduce a voltage across
load resistor Rb of magnitude u2
= i1 ( = -i2) x Rb.
Since this transformer has equal number of turns for both windings, and
therefore equal impedance, an equal voltage will appear across terminals A-B,
making u1 = u2. This current transformer also forces
currents i1 and i2 to be equal and equal to the current
through the generator, provided no other currents than i1 and i2
can flow anywhere in the circuit. This description is similar to what is
happening inside a transmission-line. Again the conductors making up for this
line will be carrying currents of equal magnitude and opposite phase because
all of the electromagnetic filed is forced to stay 'inside' the
transmission-line. Let's look at figure 2 of the transmission-line as a 1 : 1
current transformer.
In figure transfered power with no reflection of
energy back to the generator. This also requires a voltage at the output of
the line to be equal to the voltage at the input (for an ideal
transmission-line). Again transmission-line currents are equal and of
opposite phase and equal to the current through the generator. As in a
current transformer this is true as long as no other currents will flow. Since the output of the transmission-line is
'free floating' this may be connected to any other position in the circuit.
If, for some reason the bottom part of the load resistor Rb
is at a voltage U with respect to ground (e.g. when connected to the top of
the generator), an additional current will flow through the outside of the
transmission-line, that is no longer coupled to the currents at the inside.
Since this current also has to be delivered by the generator, this leakage
current is lost from the load. In terms of a transceiver circuit, power as
delivered to the antenna system by the transceiver (generator) will no longer
be only absorbed by load Rb, the
radiation resistance, but also by a (dummy?) load Zsleeve.
In the receiving position energy as received by the antenna will no longer be
delivered to the receiver only. The amount of additional current will depend
on the impedance of the outside of the transmission-line, depicted in figure
2 as the sleeve impedance Zsleeve. At
very long transmission-lines, this current will be very small, but so will
the currents through the transmission-line by means of internal loss. At
short lines we may enhance the ratio of
transmission-line current to sleeve current by enhancing the sleeve
impedance. This will make no difference to the transmission-line currents,
since these are 'locked-up' inside the line and are running as a
differential-mode current. So either in a straight line, curled up or made
into an inductor makes no difference to the transmission-line current,
because no differential current will be generated as the outside current is
common-mode only. This common-mode current may be further diminished by
winding around a high permeability ferrite core (the dashed line above the
transmission-line). With this construction we created a common-mode choke to
block off any outside current, again approaching the ideal current
transformer of figure 1. Current transformer as a balun Let's look at an antenna system with the
above transmission-line current transformer as a balun to connect a
symmetrical dipole antenna to an a-symmetrical feed-line, For this model the
antenna radiation resistance has been split-up in two over each dipole half.
To close the loop, the ground return current has been depicted by the ground
symbol at each end of the antenna and the ground symbol at the transceiver.
This model may be found in figure 3.
The generator of figure 2 is the transceiver
in figure 3 with the load depicted as the two halve radiation resistances.
Current delivered to the antenna will create voltages across each half of the
dipole, creating a voltage gradient at the outside of the feed-line from the
transceiver to the antenna. This voltage will create an additional feed-line
current that will be 'subtracted' from the antenna current and will therefore
not contribute to the dipole radiation. This parasitic current may be
minimized if the sleeve impedance is maximized, presented in figure 3 as the
impedance Zsleeve. The effectiveness of the sleeve impedance in
minimizing this parasitic current depend on the ratio of Zsleeve
to Rant; the higher Rant, the higher Zsleeve should be. This is especially
important when the (dipole) antenna is driven outside resonance and Rant
is relatively high. To still be effective at minimizing parasitic
currents, Zsleeve should be higher
still, a lesson that is often forgotten when applying a balun to an antenna. When testing balun effectiveness it should be
checked at the highest operational load impedance and in a maximum unbalance
situation, with the Zsleeve to carry the
same voltage as the load. At these test conditions insertion-loss and SWR
should still satisfy the specifications to qualify. In all test and
measurement conditions in the following chapters, baluns will always be
tested under these maximum unbalanced conditions to ensure the antenna system
to operate in all practical situations.
So sleeve impedance should be high, but how
high is high enough? A rule of practice is the parasitic current to be lower
than 1/4 of the load current to be insignificant enough. Since power is
scaling with current squared, the amount of power lost under this parasitic
current regime will result in an insignificant deviation of the S-meter at
the receiving side as compared to an ideal no-loss condition. To reach this current condition, the
impedance of Zsleeve in practice should
be in the 150 - 300 Ohm range when the balun is applied with a resonating
dipole antenna. This impedance value is not hard to obtain with various types
of coils (turns of transmission-line), either with or without a ferrite core. Analyze your antenna system Baluns will be applied at various antenna
systems and frequency range . At designing or acquiring these components
however, one should be very much aware of the impedance range the balun is
specified for related to the range of impedances that will occur in your
specific situation. Many such baluns on the market are designed for 50 Ohm
system impedance and will operate according to specifications (power,
frequency range) only at that impedance. Under practical conditions this
specified impedance range may easily be violated, especially when an antenna
is driven outside resonance with voltages to easily become much higher than
the component has been designed for. As an example: A balun that is specified for
250 W. in a 50 Ohm environment will already by overstressed by a factor of
two when driven at 100 W. in a 500 Ohm environment, which is still a low
value for a dipole out of resonance. Since an antenna tuner usually will
translate this 500 Ohm impedance nicely to 50 Ohm again, this condition may
easily go un-noticed. Burned baluns and / or damaged transceivers may be the
result. A good analysis of the tuner / balun
situation at various impedance levels has been published by Kevin Schmidt,
W9CF, in a paper called: 'Putting a
balun and a tuner together'. In this analysis, system impedances are
discussed under various load conditions, as are requirements to the balun in
a balanced feed-line situation. Also a method is being presented to calculate
and measure these requirements. The next articles in this series will discuss
various types of (antenna) baluns with and without core material. I would
appreciate your comments, questions and remarks to this and any of the other
balun articles. Bob J. van Donselaar, on9cvd@veron.nl
|
|