Bal uns: What They Do And How They Do lt

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2 Current to ri -o.. qhf ha'l f -- y' of dipole the balanced antenna with a balanced feed line - twin lead. This solves the problem neatly, until you 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. However, when we connect the rig to ground, as shown 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 m- inated. Imbalance current can be eaused also 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. 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 Rte Tnnan fanndrrnl-nn Current = CUrrenI to left half of dipole Current on inner surface ^f ^Li ^t i UI JTTIgf,U Fig Inbalance causd by arrother path to ground frorn only one side of the dipoj-e. ts^r1a AnnAcl Current on outer surface of shield 5).\g This type is commonly used for providing single-ended to differential conversion for driving 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 indeed perform an unbalanced-to-balanced conversion. 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 antenna is perfectly ba-laaced. There is nothing 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 regardless of load impedances. )nd to currents FLg. 2 - Ore solution to the inba-lance probj-en is to feed the balanced antenna with a balanced feed line - twirr 1ead.

3 I I I H-7 Fig 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. 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 the exact function a balun needs to aecomplish when used in an Bntenna system. A current balun can be constructed by 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 desired properties of the line itself (impedance, eleetrical length, SIVR, and so on). A lessthan-perfect voltage balun ean have a profound effect on the impedance seen at its input beeause of the tertiary winding. Impedance-transforming (4:1) baluns are discussed in Appendix 3. Fig. 5 - Voltage balun (see text). The bottom winding is sometires referred to as a tertiary winding. AlJ- windings are cl-osely coupled. IN w Lr.lJ"A/. Fig Current bafun (see text). windings are closely coupled. Both OUT Fig Wlren the rig is connected to ground, _ a ihiro conductor in para1le1 with the right side of the feed ]lne is introduced. This causes an imbalance in both coaxial cable and twin lead feed thes. Another type of balun that appears in the literature has been called a choke-type balun (Fig. 6).U-0 It resembles the voltage balun, exeept that 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 of load impedanees, but the second type of balun eauses equal and opposite currents on the conductors at both ports for any load impedanees. For this 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 & bifilar RF choke that impedes any net eurrent which tries to flow through it. When wound with coaxial Erperiments A series of experiments was designed to test the validity of the results of the theoretical investigation. A 10-meter dipole was set about 12 feet above the ground, and about five feet above the edge of an elevated wooden deck (Fig.?). One-half wavelength from the eenter of the dipole, a 4-foot rod was driven into the ground, whieh was eompletely saturated with water at the time the experiments were run (during November, in Oregon). To further lower ground-system impedance, six radials were placed on the ground around the ground rod. Two feed lines were eut to a half wavelength: one of RG-S9/U coaxial eable, and one of 72-ohm transmitting twin lead. The velocity factors of the eables were not taken into aceount, since the intent was to have the outside of the coaxial cable, or the two parallel conduetors of the twin lead, be an electrieal halfwavelength long. A low-power 10-meter transmitter loeated at, and conneeted to, the ground system was used as a signal souree. Current probes and baluns were built as deseribed in Appendix 2. Two of the eurrent probes were permanently wired into each side of the dipole near the feed point, and a third was used for all feed-line measurements. A single detector was used for all measurements, and it was calibrated over the range of encountered output Ievels by using a signal source and precision attenuator. The results of the experiment have been corrected to aeeount for the measured nonlinearity of the deteetor. 159

4 {1 Hltl l*j ' tti.q il._ i k{ tu4 H hd w1 jr rll l',.'! ii: ':1 ffi ffi ffi F'i fi1 Hrj \$,i * i ffi t;. I 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 encountered in 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 suspicious because 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 measures the magnitude of the vector sum of all currents flowing through the toroid. Each antenna current probe, with the deteetor, measures tbe magnitude of the current in each half of the antenna, at the feed point. What must be happening is that the currents in the dipole halves are equal in magnitude, but not 180 degrees out of phase. A check of the current-probe outputs with a good-quality dual-channel oscilloscope confirmed the hypothesis: The currents were 230 degrees, rather than 180 degrees, apart, although equal in magnitude. What an interesting pettern that dipole would have! But this illustrates how misleading the magnitudes of element currents can be when judging balance. Measuring the imbalance current in the feed line at the feed point does, however, provide a good indication of the balance of the currents in the antenna halves. If the imbalance current is very small, the currents in the sides of the antenna must be nearly equal in magnitude and opposite ih phase. A significant imbalance current, on the other hand, indicates that one or both conditions have not been met. Measurement of the imbalance current on the feed line also indicates how much the feed line will radiate. The imbalance cufrent at the rig provides a measure of RF in the shack. In the following testst the magnitude of the euruent w&s measured in each conducior, then the magnitude of the imbalance current was neasured by placing the complete feed line through the current-probe toroid. A single figure of merit, balance, was calculated as: balance (db) = 2o rog Bperiments I and 5: See Fig. 8. The dipole was symmetrical for experiment 1, nonsymmetrical for experiment 5' 160 Gverage of nagnit of currents in each Fig. 7 - Experirental setup. ino barun Fig Setup arrd results of experirnents 1 and 5. Nwbers are r asured bal"ance in de. Discussion If the dipole balance (sym metry) were indeed perfect for experiment L, we would expect the curents in the sides of the dipole to be unbalanced, resulting in imbalance current on the feed line. This is because the outside of the eoaxial shield appe&rs as a conductor in parallel with half of the dipole. Also, either a current or voltage balun should reduce the imbalance - rnt to zero. Since the feed Iine is placed symmetrically relative to the antenna, no additional current should be induced into the feed line, so the imbalance should also be quite small at the end of the line when either type of balun is used. With the nonsymmetrica- dipole (experiment 5)' 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. Results Experiments 1 through 4 were- done using a nominally symmetrical dipole, although results indicate that some asymmetry was present' For exferiments 5 through i, the dipole was intentionally t"o" nontymmetrical by lengthening one side by five inches, anb shortening the other side by the same amount. 7,-7- e"l. 5 In experiment 1r the voltage balun did not perform as well as the current balun, indicating some asym metry in the dipole. At the frequency chosen, the small differences in connections and a slight tilt of the antenna could easily account for what happened. When no balun is usedr a curious result is the much better balance at the rig end than at the antenna end of the feed line. This may be because the feed lines werenrt exactly an effective half wavelength longr because there was a!.1

5 wire of about six inehes in length conneeting the 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 worse with the nonsymmetrical antenna, was better than expected, but still definitely inferior to the cument balun. Bperiments 2 and 6: See Fig. 9. The dipole was symmetrical fon experiment 2, nonsymmetrieal for experiment 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 symmetrical or nonsymmet.rieal system. 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 nonsymmetrical antenna, whieh was about the same), and the balanee at the rig end was substantially better when using the current balun. Time did not permit me to run additional experiments to explain these differences, but the ability of the current balun to achieve superior balance was again illustrated. exd. 2 ex^ t - io balun 9,1 2,7 voltate current balun bal,un 21.3 I9. 5 balun 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 experiment 3, nonsymmetrieal for experiment 7. The voltage balun was connected with the balanced port toward the antenna. Diserrqsion: ,8 l-no balun <J volra8e.l Lcurrent balun 41, ba lun voltage 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 erp. 3.xp J l9.l ,4 t l L2,O 33.7 Fig Setup and results of experinents 3 and 7. Nmrbers are neasured balance in db. these experiments should be similar to those of the 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 similar to those of experiments 2 and 6, indieating that coaxial cable can be used as a balanced feed line (in the sense diseussed earlier) with a balanced or somewhat-unbalanced load. 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 systems where the symptoms indicate tfe need for a balun, but the antenna itself is diffieult to get to. With the symmetrieal antenna, the voltage balun 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 impedances to 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, aceounting for the different impedanees. The voltage balun predictably generates unequal currents in the different impedances, eausing 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: See Fig. 11. The dipole was sym metrieal. The voltage balun was conneeted with the balaneed port toward the antenna. Discrlssion: Like experiments 3 and 7, this was intended to test the similarity between performance of the two kinds of feed line, provided that all other conditions and connections are 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 the antenna were eompletely sym metrical, there should be no current imbalance at the input end of 161

6 exp ' 4 L!aBe balun 0, I ba lun 28.8 o 1 balun 3.5 balun 26.4 n.l.h^6i D^ 'rcoldi'side Fig. U - SetuP and resufts llr-rrbers rre lreasurd ba].ance of in 41.6 experi:rent 4. AEr 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 measured point 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 en- Iightening. Is there an optimum point in the feed line to place a balun? Suppose the 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 antenn&, as sometimes recommended. Wouldnrt the imbalance 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 Analysis of both balun types assumes I'idealx operation: All flux is linked to all windings (coefficient of coupling is one), and each winding has sufficient self-impedance to make the magnegligible. netizing current The Yoltage Balun Because of transformer sction, V1 - V3 = Y2 - \,tb = Yfi - Vl. The third term comes about because of the ntertiary'r winding shown at the bottom. 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 - tt), s6 {Ii- + I0-12) = (Ic - I1). Because of the property of coaxial cable discussed in the body of ihe- arficle, Ii = -Ier so Ic - I = Ic - 11; thus 16 = 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 Y fit the only way f or I1 and 12 to f ulf ill this requirement is for the impedances from each side of 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 magnitude and opposite in Phase. 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 outside. The currents on the inside - equal and opposite happen regardless of the outside environment, 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 is the same and for the same reasons, but operation is more dif ficuit to visualize. The current balun L62

7 Appendix 2z Construetion and Test of Baluns, Curent Probes and Deteetor Yoltage Belun The voltage balun was constructed using the method deseribed in Ref.8. A piece of no.26 wire 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 shield of the seeond being connected as the tertiary 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 value, the other ends of whieh were connected to the 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, depending on which resistor was connected to which lead of the balaneed outout. Current Balun 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, regardless of whieh lead is eonneeted to eaeh resistor. The results were within 0.2 db of theoretical, with either lead connected to either resistor. Cument Probe The current probes were constructed as shown in Fig.12. The output voltage equals ten times the current, in amperes, being measured. Insertion resistance is one ohm. 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. 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 because of transformer imperfections. 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 conductors being 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. Low-Z Unbalanced Fig. A3-1 - # ui-l? LLL6LL-L Unbal-anced The 4:1-vo:.tage balun. FT82-43 ferrite core H{ -L t Low-Z curtent-carryinb lo oe neasured conductor bifilar lurns on FT37-72 feraire core Fig. A3-2 - The 4:1 vottage balun used with a 1;1 current balun. Fig detec!or Current probe. DNC nale!o curlent probe I -Cemaniw diode r.t-tt--f-- Y =.ool, af P ro Dvlt (10 Hetolh inpu! resistance) No other to this connectlons polnt. Fig Detector. Fig. A3-3 - A 4:1 current bal"un. 163

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