Baluns

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

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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.

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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

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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.

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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