Baluns: What They Do and How They Do It

Baluns:WhatTheyDo
And How TheyDo lt
By Roy lV. Lewallen,* IVTEL
* 5 4 7 0 S W 1 5 2 n dA v e . , B e a v e r t o n ,O R 9 7 0 0 7
I've always been a bit botheredby balunst
s i n c e I w a s n e v e r s u r e w h a t t h e y a r e s u p p o s e dt o d o t
let alone hotv they might go about doing it. The
majority of articles deal with various ways of
b u i l d i n g a n d t e s t i n g b a l u n s , o r t h e a d v a n t a g e so f
o n e t y p e o v e r a n o t h e r ,b u t a l m o s t n e v e r a w o r d a b o u t
w h e n o r w h y a b a l u n i s n e e e s s a r y ri f a t a l l . L i k e
m o s t a m a t e u r s , t h e r e h a v e b e e n f e w o c c a s i o n sw h e n I
have been able to tell if a balun has any effect on
an antennasystem, and when it has, the effect
hasn't alwaysbeengood!The turning point came when
I w a s t r y i n g t o m e a s u r et h e r e s o n a n t f r e q u e n c y o f a
f o l d e d d i p o l e t h r o u g h a o n e - v r a v e l e n g t hc o a x i a l l i n e .
The bridge null varied a great deal as I moved my
hand around the coaxial cable, or if the line or
bridge was moved. A hastily eonstructed balun installed at the center of the dipole eliminated the
p r o b l e m .B u t w h y ?
I found a brief, but clear explanation of one
p h e n o m e n o ni n v o l v e d i n a p a p e r b y l l l a x w e l l ' W 2 D U . U
H o w e v e r , m a n y q u e s t i o n sr e m a i n e d . T h i s l e d m e t o a n
i n v e s t i g a t i o n o f j u s t h o w b a l u n s a r e s u p p o s e dt o
w o r k , a n d w h a t p r o b l e m st h e y a r e s u p p o s e dt o c u r e .
O n e s u r p r i s i n g c o n c l u s i o nI f o u n d f r o m m y r e s e a r c h
is that one popular type of balun' when properly
d e s i g n e da n d u s e d i n a n a n t e n n a s y s t e m , m a y n o t
solve the probiems that baluns &re expected to
solve. Other results indicated that the type of feed
I i n e ( b a l a n c e do r u n b a l a n c e d )h a s l i t t l e t o d o w i t h
h o w w e l l a s y s t e m i s b a l a n c e d .l n o r d e r t o v e r i f y t
or refute, the theoretical results, several
experimentswere earefully set up and run, and the
data analyzed.The result is a much elearer view of
t h e o p e r a t i o n o f b a l u n s i n a n t e n n a s y s t e m s ,a n d s o m e
d e f i n i t e t ' d o s t ta n d t ' d o n t t s t r' e g a r d i n g t h e i r u s e .
What Prpblems ale Baluns fupposed to Solve?
Baluns usually solve problems caused by an
imbalance. An imbalance of what? To answer this
q u e s t i o n ,w e n e e d t o l o o k a t c u r r e n t f l o w i n
t r a n s m i s s i o nl i n e s .
In a coaxial cable, the curents on the inner
c o n d u c t o ra n d t h e i n s i d e o f t h e s h i e l d a r e e q u a l a n d
opposite. This is because the fields from the two
eurrentsare confined to the same spaee.\2 With the
presence of skin effeet, a different current flows
on the outside of the shield than on the inside.\]
T h e c u r r e n t o n t h e o u t s i d e , i f s i g n i f i c a n t ,c a u s e s
the feed Iine to act like an antenna, radiating a
f i e l d t h a t i s p r o p o r t i o n a lt o t h i s c u r r e n t .
A twin-iead feed line has similar properties,
d e s p i t ei t s d i f f e r e n t p h y s i c a l n a t u r e . S i n c e i t i s
physieally symmetrical, if the currents flowing
t h r o u g h t h e c o n d u c t o r sa r e e q u a l a n d o p p o s i t e r t h e
r a d i a t i o n f r o m t h e l i n e i s m i n i m a l ( a s s u m i n gt h a t
the conductor spacing is very small relative to a
w a v e l e n g t h ) .H o w e v e r , s e v e r a l f a c t o r s m a y c a u s e t h e
e u r r e n t s i n t h e t w o c o n d u c t o r st o b e i m b a l a n e e d ,
that is, other than equal and opposite. If this
h a p p e n s ,t h e b a l a n c e d f e e d l i n e w i l l r a d i a t e l i k e a
coaxial cable that has current on the outsideof the
s h i e l d . T h i s o c c u r s b e c a u s e t h e e o m p o n e n t so f t h e
currents on the two conductors that are equal and
opposite create fields whieh cancel. But the field
from any remaining eomponenton either eonductor
( c a i l e d a e o m m o n - m o d e ,s e e o n d a r y - m o d eo, r a n t e n n a
c u r r e n t ) w i l l c a u s e r a d i a t i o n . \ 4 . 5 . 6I n t h i s a r ticle, the eurfent on the outside of the coaxial
shield, or the antenna eurrent on the twin lead,
will be ealled the imbalanee current: They are
caused by the same things and produce the same
effeets.
Imbalanee eurrent, on either kind of line, is
the cause of a number of undesirable effects:
o pattern distortion (caused by the feed-line
r a d i a t i o n a d d i n g t o t h e a n t e n n a - r a d i a t e df i e l d ,
or by unequalcurrents in the antenna halves)
o TVI (radiation from a feed line coupling into
nearby television sets, house wiring, and so
on)
o R F i n t h e s h a c k ( e a u s e d b y a r r h o t t tr a d i a t o r the feed line - residingin the shack)
If you have read other articles on baluns, youtll
r e c o g n i z et h e s e a s t h e p r o b l e m s b a l u n s a r e s u p p o s e d
to solve. What isntt usually too clear is that they
are all caused by eurrent imbelanss, on either
eoaxial or twirrlead feed line. Of eourse, if the
imbalance current is suffieiently small to begin
w i t h , a b a l u n i s n o t n e c e s s a r ya t a l l . O r i t e a n b e
s a i d t h a t a p r o p e r l y d e s i g n e db a l u n w i l l n o t s o l v e
the problem being experienced.
What Causes S5ntem Imba-lance?
T h e f i r s t e a u s e o f i mb a l a n e e c u r r e n t s w a s
explained by I\4axwell. It will be repeated here for
c o m p l e t e n e s s .W h e n a b a l a n c e d a n t e n n a i s f e d w i t h
eoaxial eable (Fig.1), the outside of the shield
a p p e a r sa s a n e x t r a , s e p a r a t ee o n d u c t o rc o n n e e t e dt o
the right side of the antennaat the feed point. The
cunrent in the cablers center conductor flows into
the left half of the dipole. The equal and opposite
eurrent on the inside of the shield flows partly
into the right half of the dipole, and partly along
the outside of the shield. The proportionof current
w h i c h f l o w s e a e h w a y i s d e t e r m i n e db y t h e r e l a t i v e
i m p e d a n c e so f t h e t w o p a t h s . T h e c u r r e n t o n t h e
outside is the greatest when the total effective
length of the path along the outside of the eoaxial
eable from the antenna to ground is an integral
n u m b e r o f h a l f w a v e l e n g t h s ,s i n c e t h i s m a k e s t h e
i m p e d a n c ep r e s e n t e d b y t h e u n d e s i r a b l e p a t h
relatively low. If the rig is effectively an odd
n u m b e r o f q u a r t e r w a v e l e n g t h sf r o m a c t u a l g r o u n d ,i t
i s a t a v o l t a g e m a x i m u ma n d c a n b e h o t . O n t h e o t h e r
hand, there are other combinationsof lengths for
which the imbalance current will be negligible cases where a balun does not make any notieeable
difference.
The obvious solution to this problem is to feed
157
Current
--
r i - oq. .h f
to
ha'l
f
y' of dipole
Tnnan
fanndrrnl-nn
Current
to left
dipole
Rte
= CUrrenI
half of
Current on
outer surface
of shield
Current on
inner surface
ts^r1a
^ f
UI
AnnAcl
^ L i
^ t
JTTIgf,U
|
)nd
i
to
currents
Fig. 1 -- Inbalance causd by arrother path to
ground frorn only one side of the dipoJ-e.
the balanced antenna with a balanced feed line
you
problem
until
neatly,
twin lead. This solves the
encounter the problem which most of us have todayt
illustrated in Fig. 2. Suppose that we went ahead
and connected the line as shown schematically in
Fig.3. If the rig could be totaUy isolated front
ground, the feed-line conductor currents would be
-qual and opposite, just as they would be if coaxial
cable were used; the imbalance current would be
zero, and the feed line would not radiate.\1.
H o w e v e r ,w h e n w e c o n n e c t t h e r i g t o g r o u n d ,a s s h o w n
by the broken line, wetve again provided a third
conductor in parallel with the right side of the
feed line, and the same problem occurs as with the
coaxial cable (see Fig. 4). So either type of line
is unbalanced if a direct path to ground is provided
from one side, and both can be a balanced, nonradiating line if the imbalance current is eli minated.
Imbalance current can be eausedalso by
situations where the two sides of the antenna are
not precisely symmetricall Coupling to nearby objects, tne tilt relative to ground, or slight dilieren"es in lengths of the two antenna halves.\8
Another cause of imbalance currents is induction. lf
the feed line is not exactly placed at a right angle
to the antenna, a net current is induced into it by
the antenna fieid. This current appears as an imbalance current. At UHFr where the diameter of
c oaxial cable is a substantial fraction of the
length of the antenna elements, coaxial line is more
difficult to place symmetrically relative to the
antenna than twin lead is (ttris is sometimes given
as the only reBson for using a balun!). The problem
is negligible at VHF (except perhaps with very Iarge
diameter coaxial cable) or below.
5).\g This type is commonly used for providing
s i n g l e - e n d e dt o d i f f e r e n t i a l c o n v e r s i o n f o r d r i v i n g
balanced mixers, push-pull amplifiers, and so on. It
seems to be suitable for our purpose. An analysis of
its operation (see Appendix 1) shows that it does
i n d e e d p e r f o r m a n u n b a l a n c e d - t o - b a l a n c e dc o n version. The voltages at the balanced port are
caused to be equal, and opposite, in phase relative
to the cold side of the unbalanced port. Thus, the
use of this sort of balun will eliminate the problem
of current flow on the outside of a line only if tbe
There is nothing
antenna is perfectly ba-laaced.
gained by forcing the voltages of the two antenna
halves, whether balanced or not, to be egual and
opposite relative to the cold side of the balun
input (usuaUy connected to the shield of a coaxial
feed line), since the antenna field is proportional
to the cunents in the elements, not the voltages at
the feed point. I will call this type of balun a
voltage balun to emphasize that it balances the
output voltages regardlessof load impedances.
What Baluns Do
Letts recall what we want a balun to do: cause
the currents in the feed-Iine conductors to be equal
in magnitude and opposite in phase, resulting in a
zero imbalance current. IIow well do the popular
balun types do this?
One type of balun is known as a transformertype balun- or balun with a tertiary winding (Fig.
158
FLg. 2 - Ore solution to the inba-Lance probJ-en
is to feed the balanced antenna with a balanced
feed line - twirr 1ead.
I
I
I
cable, it ean be visualized as an RF choke aeting
only on the outside of the coaxial-eable shield,
reducing the current to a very small value. This is
t h e e x a c t f u n c t i o n a b a l u n n e e d s t o a e c o m p l i s hw h e n
u s e d i n a n B n t e n n as y s t e m .
A current balun can be constructedby winding
coaxial feed line into a coil, winding either type
of feed line onto a core, or by stringing ferrite
cores along either type of line.\I[ Even if the
balun is mediocre, there wilL be no effect on the
desiredproperties of the line itself (impedance,
eleetrical length, SIVR,and so on). A lessthan-perfect voltage balun ean have a profound
e f f e c t o n t h e i m p e d a n c es e e n a t i t s i n p u t b e e a u s eo f
t h e t e r t i a r y w i n d i n g . I m p e d a n c e - t r a n s f o r m i n( 4g: 1 )
b a l u n sa r e d i s c u s s e di n A p p e n d i x 3 .
H-7
Fig. 3 -- If the rig were isolated fronr gror.rnd,
the feed-line conductor currents would be equal
and opposite, the jnbalance current would be zero
and the feed ]i-ne would not radiate.
Fig. 5 - Voltage balun (see text). The bottom
winding is sometires referred to as a tertiary
winding. AlJ- windings are cl-osely coupled.
w
IN
OUT
Lr.lJ"A/ .
Fig. 6 -- Current bafun (see text).
windings are closely coupled.
Both
Erperiments
Fig. 4 -- Wlrenthe rig is connected to ground, _
a ihirO conductor in para1le1 with the right side
of the feed ]lne is introduced. This causes an
in both coaxial cable and twin lead
imbalance
feed thes.
Another type of balun that appears in the
literature has been called a choke-type balun (Fig.
6 ) . U - 0 I t r e s e m b l e st h e v o l t a g e b a l u n , e x e e p t t h a t
the tertiary winding is missing. The analysis of
both types of balun in Appendix 1 shows that the
effect of a tertiary winding is not a minor one. The
two types of baluns produce fundamentally different
results. The voltage balun causes equal and opposite
voltages to appear at the balanced port regardless
o f l o a d i m p e d a n e e s ,b u t t h e s e c o n d t y p e o f b a l u n
eausesequal and opposite currents on the conductors
a t b o t h p o r t s f o r a n y l o a d i m p e d a n e e s .F o r t h i s
reason, I will call this type of balun a curent
balun. Intuitively, the cument balun produces the
sort of effeet we would expect. When wound with
twisted pair or twin lead, it is nothing but &
b i f i l a r R F c h o k e t h a t i m p e d e sa n y n e t e u r r e n t w h i c h
tries to flow through it. When wound with coaxial
A s e r i e s o f e x p e r i m e n t s w a s d e s i g n e dt o t e s t
the validity of the results of the theoretical
investigation.A 10-meter dipole was set about 12
f e e t a b o v e t h e g r o u n d ,a n d a b o u t f i v e f e e t a b o v e t h e
edge of an elevated woodendeck (Fig. ?). One-half
w a v e l e n g t hf r o m t h e e e n t e r o f t h e d i p o l e , a 4 - f o o t
r o d w a s d r i v e n i n t o t h e g r o u n d ,w h i e h w a s e o m p l e t e l y
saturated with water at the time the experiments
w e r e r u n ( d u r i n g N o v e m b e r , i n O r e g o n ) .T o f u r t h e r
l o w e r g r o u n d - s y s t e mi m p e d a n c e , s i x r a d i a l s w e r e
placed on the ground around the ground rod. Two feed
l i n e s w e r e e u t t o a h a l f w a v e l e n g t h :o n e o f R G - S 9 / U
coaxial eable, and one of 72-ohm transmitting twin
lead. The velocity factors of the eables were not
t a k e n i n t o a c e o u n t ,s i n c e t h e i n t e n t w a s t o h a v e t h e
outside of the coaxial cable, or the two parallel
c o n d u e t o r so f t h e t w i n l e a d , b e a n e l e c t r i e a l h a l f w a v e l e n g t hl o n g . A l o w - p o w e r 1 0 - m e t e r t r a n s m i t t e r
l o e a t e d a t , a n d c o n n e e t e dt o , t h e g r o u n d s y s t e m w a s
used as a signal souree.
Current probes and baluns were built as deseribed in Appendix 2. Two of the eurrent probes
w e r e p e r m a n e n t l yw i r e d i n t o e a c h s i d e o f t h e d i p o l e
near the feed point, and a third was used for all
f e e d - l i n e m e a s u r e m e n t sA
. single detector was used
f o r a l l m e a s u r e m e n t sa, n d i t w a s c a l i b r a t e d o v e r t h e
range of encountered output Ievels by using a signal
source and precision attenuator. The results of the
experimenthave been corrected to aeeount for the
m e a s u r e dn o n l i n e a r i t y o f t h e d e t e e t o r .
159
No attempt was made to keep the power level or
impedance match constant from one test to another.
When running an experiment with no balunr a current
balun, and a voltage balun, the only -variation in
the system was to change the balun. Initially' the
inteni was to use the antenna current probe readings
as a measure of cument balance in the antenna
halves. However, a case was encounteredin which tbe
antenna halves showed equal currents, but a large
imbalance current was measured in the feed line at
the antenna feed point - a seemingly intpossible
combination! (Tne equal antenna currents were even
more suspiciousbecause no balun was being used, and
the antenna had intentionally been made nonsymmetrical for that test.) A bit of thought provided
the answer. The imbalance current is measured by
placing the feed Iine through the current-probe
ioroid-. In conjunction with the detector, it
m e a s u r e st h e m a g n i t u d e o f t h e v e c t o r s u m o f a l l
currents flowing through the toroid. Each antenna
c u r r e n t p r o b e , w i t h t h e d e t e e t o r , m e a s u r e st b e
Fig. 7 - Experirental setuP.
magnitude of the current in each half of the ant e n n a , a t t h e f e e d p o i n t . W h a t m u s t b e h a p p e n i n gi s
e"l.
5
that the currents in the dipole halves are equal in
! . 1
ino barun
p
h
a
s
e
.
A check
m a g n i t u d e ,b u t n o t 1 8 0 d e g r e e so u t o f
of the current-probe outputs with a good-quality
dual-channel oscilloscope confirmed the hypothesis:
The currents were 230 degrees, rather than 180
d e g r e e s ,a p a r t , a l t h o u g h e q u a l i n m a g n i t u d e .W h a t a n
interesting pettern that dipole would have! But this
illustrates how misleading the magnitudes of element
currents can be when judging balance. Measuringthe
i m b a l a n c ec u r r e n t i n t h e f e e d l i n e a t t h e f e e d p o i n t
does, however, provide a good indication of the
balance of the currents in the antenna halves. If
Fig. 8 -- Setup arrd results of experirnents 1
the imbalancecurrent is very small, the currents in
and 5. Nwbers are r€asured bal"ance in de.
in
equal
be
nearly
must
the
antenna
the sides of
magnitude and opposite ih phase. A significant imbalance current, on the other hand, indicates that
Discussion
one or both conditions have not been met.
I f t h e d i p o l e b a l a n c e ( s Y mm e t r y ) w e r e i n d e e d
M e a s u r e m e n to f t h e i m b a l a n c e c u r r e n t o n t h e
p
e
r
fect for experiment L, we would expect the
will
feed line also indicates how much the feed line
curents in the sides of the dipole to be unbalp
r
o
v
i
d
e
s
a
t
h
e
r
i
g
a
t
c
u
f
r
e
n
t
i
m
b
a
l
a
n
c
e
T
h
e
radiate.
anced, resulting in imbalance current on the feed
measure of RF in the shack. In the following testst
line. This is because the outside of the eoaxial
t h e m a g n i t u d e o f t h e e u r u e n t w & s m e a s u r e di n e a c h
appe&rs as a conductor in parallel with half
shield
i
m
b
a
l
a
n
c
e
t
h
e
o
f
conducior, then the magnitude
dipole. Also, either a current or voltage
o
f
t
h
e
current was neasured by placing the complete feed
- rnt to zero.
reduce the imbalance
balun
should
A
s
i
n
g
l
e
t
o
r
o
i
d
.
line through the current-probe
placed
relative
symmetrically
is
Iine
Since
the
feed
figure of merit, balance, was calculated as:
to the antenna, no additional current should be
induced into the feed line, so the imbalance should
end of the line when
also be quite small at the
of
nagnit
Gverage
is
used.
balun
type
of
either
in
each
of currents
W i t h t h e n o n s y m m e t r i c a -d i p o l e ( e x p e r i m e n t 5 ) '
balance (dB) = 2o rog
we would expect the voltage balun to do worse than
in experiment f. iVe would also expect the current
balun to do about the same, and the no-balun case to
be considerably worse.
a
u
s
i
n
g
d
o
n
e
w
e
r
e
4
Experiments 1 through
nominally symmetrical dipole, although results
Results
indicate that some asymmetry was present' For exIn experiment 1r the voltage balun did not
intentionally
feriments 5 through i, the dipole was
perform as well as the current balun, indicating
b
y
f
i
v
e
s
i
d
e
o
n
e
b
y
l
e
n
g
t
h
e
n
i
n
g
t"O" nontymmetrical
s o m e a s y mm e t r y i n t h e d i p o l e . A t t h e f r e q u e n c y
inches, anb shorteningthe other side by the same
chosen, the small differences in connections and a
amount.
slight tilt of the antenna could easily account for
Bperiments I and 5:
what happened. When no balun is usedr a curious
is the much better balance at the rig end
result
f
o
r
s
y
m
m
e
t
r
i
c
a
l
See Fig. 8. The dipole was
than at the antenna end of the feed line. This may
5
'
e
x
p
e
r
i
m
e
n
t
f
o
r
n
o
n
s
y
m
m
e
t
r
i
c
a
l
1
,
experiment
be because the feed lines werenrt exactly an effective half wavelength longr because there was a
7,-7-
{1
H
ltl
l*j
tti
' .q
. _i
il
k{
tu4
H
hd
w1
jr rll
l',.'!
ii: ':1
ffi
ffi
ffi
F'i
fi1
Hrj
$,i
* i
ffi
I
t;.
160
wire of about six inehes in length conneetingthe
rig to the ground system' or because the feed line
was doubled baek on itself for a short distance near
the rig to provide strain relief. Perhaps the
doubling baek generated enough induetance to cause a
eurrent balun, or RF choking effect. The better
balance at the rig end can be seen in the results of
all experiments.
The no-balun result was worse with the nonsymmetrical dipole than the symmetrieal oner 8s
expected, and the current balun did about the same
in both cases. The voltage balun, although slightly
w o r s e w i t h t h e n o n s y m m e t r i c a la n t e n n a , w a s b e t t e r
than expected, but still definitely inferior to the
cument balun.
Bperiments
2 and 6:
See Fig. 9. The dipole was symmetrical fon
e x p e r i m e n t 2 , n o n s y m m e t r i e a lf o r e x p e r i m e n t 6 .
Discnssion
The results of these experiments should duplicate those of the previous pair, sinee the feed line
is placed symmetricaily relative to the antenna to
avoid indueed current. The only difference is that
the balun is placed farther down in a symmetricalor
n o n s y m m e t . r i e asl y s t e m .
Results
The trend is clearly the same as in experiments
1 and 5; the current balun provides the best balance, the voltage balun is seeond best, and a feed
line with no balun is the worst case. The balance
with no balun was better in this experimentt
however, (except at the rig end with the nons y m m e t r i c a l a n t e n n a ,w h i e h w a s a b o u t t h e s a m e ) , a n d
the balanee at the rig end was substantially better
w h e n u s i n gt h e c u r r e n t b a l u n . T i m e d i d n o t p e r m i t m e
to run additional experiments to explain these differences, but the ability of the current balun to
achieve superior balance was again illustrated.
exD.
io balun
<J-----__.l
2
-
ex^
9,1
2,7
I9. 5
voltate
balun
current
bal,un
2 1. 3
17.3
9,8
balun
41,5
33.4
balun
34.6
32.6
balun
l-no
volra8e
Lcurrent
t
ba lun
voltage
erp. 3
4.8
3.J
l9.l
.xp.
7
0.3
14.5
1 3, 4
12.5
t0.4
l . 3. 4
10.4
L2,O
33.7
Fig. 10 - Setup and results of experinents 3
and 7. Nmrbers are neasured balance in dB.
t h e s e e x p e r i m e n t ss h o u l d b e s i m i l a r t o t h o s e o f t h e
previous pair.
Results:
With no balun, the results were those of experiments 1 and 5 (the test with no balun was not
rerun). With the current balun, the results were
s i m i l a r t o t h o s e o f e x p e r i m e n t s2 a n d 6 , i n d i e a t i n g
that coaxial cable can be used as a balanced feed
line (in the sense diseussed earlier) with a balanced or somewhat-unbalancedload. This data also
points to the possibility that a eurrent balun could
be added to an existing antenna system at the rig
end of the line, with results similar to those
obtained by plaeing it at the antenn&, in some cases
at least. This would certainly be worth a try in
s y s t e m s w h e r e t h e s y m p t o m si n d i c a t e t f e n e e d f o r a
balun, but the antenna itself is diffieult to get
t o . W i t h t h e s y m m e t r i e a la n t e n n a ,t h e v o l t a g e b a l u n
made balanee worse at both ends of the feed line
than no balun at all. The balanced port of the
voltage balun sees two unequal impedancesto ground:
the eoaxial center conduetor, ending in one dipole
half, and the eoaxial shield terminating in the
other. The shield is eapabie of radiating but the
inner conduetor isnrt, and the two are of different
diameters, aceountingfor the different impedanees.
The voltage balun predictably generates unequal
c u r r e n t s i n t h e d i f f e r e n t i m p e d a n c e s ,e a u s i n g
additional current imbalanee. A voltage balun was
not evaluated in this applieation with a nonsymmetrical dipole, having shown distinctly inferior
results even with a svmmetrical one.
Eperiment
4:
S e e F i g . 1 1 . T h e d i p o l e w a s s y mm e t r i e a l . T h e
voltage balun was conneeted with the balaneed port
toward the antenna.
Discrlssion:
Fig. 9 - Setup and resuLts of experinrents 2
and 5. Nunbers are measured baLanced in dB.
Bryeriments
3 and 7:
See Fig. 10. The dipole was symmetrieal for
e x p e r i m e n t 3 , n o n s y m m e t r i e a lf o r e x p e r i m e n t 7 . T h e
voltage balun was connected with the balanced port
toward the antenna.
Diserrqsion:
These experiments, and experiment 4t were
eonducted to test the idea that coaxial cable and
twin-lead feed lines would behave in the same
fashion, as theorized earlier. If so, the results of
Like experiments 3 and 7, this was intended to
t e s t t h e s i m i l a r i t y b e t w e e n p e r f o r m a n c eo f t h e t w o
kinds of feed line, provided that all other conditions and connectionsare the same. If the two
feed lines aet the same, the results should duplicate those of experiment 1.
Results:
The current balun again causes the predieted
results, except it shows improved performance at
the rig over experiment 1. In eontrast, the voltage
balun gave strikingly poorer balance at the antenna,
and markedly poorer performanee at the antenna end
of the feed line, eompared to no balun at all. If
t h e a n t e n n a w e r e e o m p l e t e l y s y mm e t r i c a l , t h e r e
should be no current imbalance at the input end of
161
e x p' 4
L!aBe
balun
b al u n
0, I
28.8
o 1
balun
balun
3.5
26.4
17.3
n.l.h^6i
D^-+
'rColdi'Side
4 1. 6
The Yoltage Balun
Fig. U - SetuP and resufts of experi:rent 4.
AEr
llr-rrbers €rre lreasurd ba].ance i n
the voltage balun, but with the moderate (unintentional) irnbalance presented by the actual antenna, the curent balance on the feed Une was
seriously degraded. This configuration isnrt likely
to be used in actual practice, but helps illustrate
the operation of the baluns and feed lines.
Conclusiors
Although some aspects of the experimental
results remain to be explained (as they always will
be unless performed under extremely controlled
eonditions)r they certainly support the theoretical
analysis. The current balun gave superior balance at
every measuredpoint in each experiment. The voltage
balun improved balance in most cases, explaining its
acceptance in spite of the theoretieally and experimentally demonstrated superiority of the current
balun to cure the problems we have discussed.
As always, finding the answers to questions
generates yet more questions. Lack of time did not
permit experiments with the feed line placed nonsymmetrically with respect to the antenna, to induce
imbalance current into the feed line. The results of
such an experiment should be interesting and enIightening.
Is there an optimum point in the feed line to
place a balun? Supposethe effective distance along
the feed line/ground wire from the antenna to ground
is an integral number of half wavelengths, and the
balun is placed a quarter wavelength below the ant e n n & , a s s o m e t i m e s r e c o m m e n d e d .W o u l d n r tt h e i m balance current be conducted as before? lVould induced cunent, if present, be reduced? Whatts the
effect of poor coaxial shield coverage?
More work needs to be done in evaluating the
various styles of current baluns (such &s coaxial
cable wound into a choke, coadal cable wound on a
ferrite or powdered iron core, insertion of the feed
tine through one or more ferrite coresr and so on)
for their primary characteristie: causing currents
to be equal in magnitude and out of phase. The
method Irve used is briefly described in Appendix 2'
but how good is good enough?
The basic investigation reported here does
answer some of the major questions regarding baluns.
I now know what symptoms I can expect a balun to
cure, why it will (or wontt!) cure them, how to
predict and measure the balunrs success in doing so,
and what type of balun to use. I hope you dor too!
Appendix 1: A Brief Analysis of Balun Operation
A n a l y s i s o f both balun types assumes I'idealx
operation: All f l u x i s l i n k e d t o a l l w i n d i n g s
(coefficient of coupling is one), and each winding
has sufficient s e l f - i m p e d a n c et o m a k e t h e m a g netizing current negligible.
L62
Because of transformer sction, V1 - V3 = Y2
\,tb = Yfi - Vl. The third term comes about because
o f t h e n t e r t i a r y ' r w i n d i n g s h o w n a t t h e b o t t o m-.
iearranging the last two- terms, v2 - v0 = {vl
V0). So-relative to V! (the voltage at the cold
side of the unbalanced port), the voltages at the
balanced port are equal and opposite.
The current It flowing in the tertiary winding
is, by inspection, -([ + lb - I.2) and also (Ie
=
(Ic
of
the
I1). Because
12)
tt), s6 {Ii- + I0
property of coaxial cable discussed in the body of
- 11;
ihe- arficle, Ii = -Ier so Ic - I9 + 12 = Ic
thus 16 = 11 + 12. So for the cument on the outside of the shield, I!, to be zerot load currents
I1 and 12 must be equal and opposite. Since Vl and
V2 are forced to be equal and opposite relative to
Yf i t t h e o n l y w a y f o r I 1 a n d 1 2 t o f u l f i l l t h i s
r e q u i r e m e n ti s f o r t h e i m p e d a n c e sf r o m e a c h s i d e o f
the balanced port to the cold side of the unbalanced
port to be equal. Thus, onIV a perfectly balanced
ioao will cause no current on the outside of the
coaxial cable. Ironieally, if this does occurt
current It = B, and the tertiary winding accomplishes no function.
The Curent
Balun
ln an ideal transformer of two windings having
an equal number of turns, the currents in the
windings are forced to be equal and opposite. So Ic
= -(Ii + Io). Again, Ii = -Ier so Ic = Ic - Ior
resulting in Io = !. This result is independent of
the load impedances. And, since the load currents
are the winding currents, they are also equal in
m a g n i t u d ea n d o P P o s i t ei n P h a s e .
If the balun is constructed by winding coaxial
cable on a core or into an air-core coil' or by
stringing ferrite beads on the outside' the operation can be understood by observing that the inside
of the coaxiel cable t'canrt tellt whatrs going on
equal and
outside. The currents on the inside happen regardless of the outside enopposite
vironment, but the construction eauses a high
impedance to current flow on the outsider acting
like a choke to the imbalance current (hence the
appropriate name choke balun). When constructed of
twisted-pair line, the effect on imbalance current
i s t h e s a m e a n d f o r t h e s a m e r e a s o n s ,b u t o p e r a t i o n
is more dif ficuit to visualize.
The current
balun
Appendix 2z Construetion and Test of
Curent Probes and Deteetor
Baluns,
Yoltage Belun
The voltage balun was constructed using the
m e t h o d d e s e r i b e di n R e f . 8 . A p i e c e o f n o . 2 6 w i r e
was laid along a length of RG-178/U cable (smal]diameter Teflon-insulated eoaxial cable), and
heat-shrinkable tubing was applied over the assembly. The modified cable was wound on an FT82-61 core
using ten turns. This construetion method was
decided on after trying to wind a balun with two
pieces of coaxial cable in bifilar fashion, the
s h i e l d o f t h e s e e o n db e i n g c o n n e c t e da s t h e t e r t i a r y
winding. The latter construction method was much
poorer in providing good voltage balanee.
Voltage balance was evaluated by eonnecting the
cold side of the unbalanced port to a ground plane
and the balanced port to two resistors of unequal
v a l u e , t h e o t h e r e n d s o f w h i e h w e r e c o n n e c t e dt o t h e
same point on the ground plane. Using resistors of
27 and 54 ohms, the ratios of voltages appearing at
the two resistors were measured as about 3/4 and
L-1/2 dB, dependingon which resistor was connected
t o w h i c h l e a d o f t h e b a l a n e e do u t o u t .
Balun
Current
The current balun consisted of 15 turns of
RG-1?8/U coaxial eable on an FT82-61 core. Performanee was evaluated by connecting the output end
to 27- and 54-ohm resistors to ground, and measuring
the voltages across them. A properly working current
balun should generate twice the voitage aeross the
54-ohm resistor than aeross the 2?-ohm resistor,
r e g a r d l e s so f w h i e h l e a d i s e o n n e e t e d t o e a e h
resistor. The results were within 0.2 dB of theoretical, with either lead connected to either
resistor.
Appendix 3: Impedanee-Transforming (4:1) Baluns
The common 4:1 balun, shown sehematically in
Fig. A3-1, is a voltage balun. If used with a eurrent balun as in Fig. A3-2, the combination acts
like a 4:1 current balun. Or it can be converted to
a 4:1 transforming eurrent balun by adding a third
winding, as shown in Fig. A3-3. A 1:1 voltage balun
could be eonverted to a 4:1 current balun by reeonnecting the existing windings. The difficulty
with using this eonfiguration is that, Iike the 1:l
voltage balun, all windings must be elosely coupled,
and rather severe impedance changes ean occur bec a u s eo f t r a n s f o r m e r i m p e r f e c t i o n s .
A better approaeh is shown in Fig. A3-4. Oldtimers will recognize this as the eonfiguration used
by the balun eoils commonly used some years ago.
This balun does foree equal and opposite cuments at
the input and output, so it is a true current balun,
and it performs a 4:1 impedanee transformation. Although it does require two cores whieh must not be
coupledr* it has several advantages:Itrs much
easier to tightly couple two eonductors than three,
itrs mueh more forgiving than the other configurations, and it lends itself to easy construetion.
One method is simply to wind eoaxial eable on two
cores, with the center conductorsbeing the conduetors shown on the outsides in the figure. This
balun can also be used in all-eoaxial-cable systems.
Besides effecting a 4:1 impedanee transformation, it
will greatly reduce any current flowing on the outsides of the lines.
* If ferrite rods or air-core coils are used,
donrt plaee them end to end. Place them side by side
and spaced a fair distance, or, better yet, at right
angles. Less eare needs to be taken with toroidal
eoils.
Cument Probe
The current probes were constructed as shown in
Fig.12. The output voltage equals ten times the
c u r r e n t , i n a m p e r e s , b e i n g m e a s u r e d .I n s e r t i o n
r e s i s t a n c ei s o n e o h m .
ui-L
?
LLL6LL-L
Low-Z
Unbalanced
Unbal-anced
Deteetor
The detector is shown in Fig. 13. It was
ealibrated using a signal source and precision
attenuators, at the operating frequency. Calibration
using a dc source was found to be inaceurate.
FT82-43
ferrite
#
Fig. A3-1 -
The 4:1-vo:.tage balun.
core
H{
-L
t
Low-Z
curtent-carryinB
lo
conductor
oe neasured
bifilar
lurns
on
FT37-72
feraire
core
Fig. A3-2 - The 4:1 vottage baLun used with a
1;1 current balun.
detec!or
Fig. 12 D N Cn a l e
I
r.t-tT--f-Y
Current probe.
!o curlent
-Cemaniw
=
Fig. 13 -
probe
diode
.ool aF
, P
No other
ro Dvlt (10 Hetolh
inpu!
resistance)
to
Fig. A3-3 Detector.
this
connectlons
polnt.
A 4:1 current bal"un.
163
\1 A v e r y c l e a r d e v e l o p m e n to f t h e p h e n o m e n o no f
Hi oh-7
Fig. A3-4 -
A superior
4:1 current ba-l_un.
References
U
\2
M a x w e l l , W a l t e r , W 2 D U ' t ' S o m eA s p e c t s o f t h e
B a l u n P r o b l e m r t tQ S T , M a r c h 1 9 8 3 , p . 3 8 .
If a perfect shield is assumed(a reasonable
approximation for this analysis),the result
follows directly from Amperers Law. For a more
detailed explanation, see Electromagnetic Energy
Tlansmission and Radiation, by Richard B. Adler,
Lan Jen Chu, and Robert M. Fano(Wiley, 1960).
skin effect may be found in Chapter 7 of Elee
tric Tlansmission Lines by Hugh H. Skilling
(McGraw-Hill,1951).
"NanosecondPulse
\4 Winningstad,C. Norman,
Transformers,rr IRE tlansactions on Nuclear
Scierrce,March 1959.
\1 Matick, Richard E., I'TransmissionLine Pulse
Transformers - Theory and Applicationrr Ptoeeedingsof the IEBE' Vol. 56, No. 1' Jan.
1968.
Hall, Gerald L.' KITD, ed., The ARRL Anteruu
Book, 14th ed., Chapter 5, (ARRL' 1982).
practice, there is always an RF path from the
In
V
rig to ground, and its impedance should be made
as low as possible. The rig should alwal's be de
grounded for safetY.
See Ref. 6.
\8
\a
Nagle, John J., K4KJr "High-Performance
BroadbandBalun," IIam Radio, Feb. 1980, p. 28.
u-0 S e e R e f . 1 .
Reisert, Joe, WlJR, "Simpleand Efficient
BroadbandBalun,r' Ham Radio, Sept. 1978' p.
\s
\u
1 t
The article of Reference 1 above is available at http://www.w2du/r2ch21.pdf
Thisarticlewas publishedin the ARRLAntennaCompendium,
Vol. 1, copyright
O 1985
by theAmericanRadioRelayLeague,Inc.Reproduced
by permission.
164

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