A Better Antenna Balun

A Better Antenna-Tuner Balun Which balun to use? The hybrid balun promises advantages over both voltage and current baluns. By Andrew Roos, ZS1AN aluns that are situated be- is effective only if the load impedance Fig 1. Although ZW is shown on only tween an antenna tuning unit is well balanced with respect to one of the transformer windings, its Band a non-resonant antenna ground. effects are felt equally on both wind- may be subjected to high-impedance, I then introduce a new design: the ings due to the coupling action of the highly reactive or unbalanced loads “hybrid” balun, which overcomes these “ideal transformer,” which can be that can prevent the balun from func- limitations of the voltage and current expressed by two rules: tioning effectively. baluns. It can operate with much 1. The currents in the two windings In this article, I apply the analyti- higher load impedances than can cur- of the “ideal transformer” are equal cal model of the transmission-line rent baluns and with unbalanced load and opposite. transformer developed by Roy impedances that voltage baluns could 2. VAC = VBD = (I1 – I2) ZW Lewallen1 to analyze the performance not drive effectively. The article con- The model assumes that the length of the 1:1 current balun and the 4:1 cludes by describing a simple test I of the transmission line used to con- voltage balun in this application. The conducted to confirm that the hybrid struct the TLT is short in terms of analysis shows that the current balun balun operates as predicted. wavelength, so transmission line ef- operates effectively only for small load fects can be ignored. Although this as- impedances, while the voltage balun Lewallen’s Model of a 1:1 Trans- sumption holds for the lower part of mission-Line Transformer 1 the HF region, some transmission- Notes appear on page 34. Lewallen models the 1:1 transmis- line effects do come into play at higher sion-line transformer (TLT) as an frequencies. PO Box 350, Newlands ideal 1:1 transformer with a “wind- The use of this model does not im- 7725, South Africa ing impedance,” ZW, in parallel with ply that TLTs operate like conven- [email protected] one of the windings,2 as shown in tional transformers. The ideal Sep/Oct 2005 29 1:1 transformer is simply a modeling The 4:1 Voltage Balun I2 = VS / Z2 convenience, and the resulting model We can use the same model to ana- We can calculate Witt’s measure of applies to any device that exhibits lyze the performance of the Ruthroff’s imbalance: common-mode impedance, irrespec- 4:1 voltage balun shown in Fig 3. Once tive of its operating principle. IMB = 2 |I – I | / |I + I | again, Z1 and Z2 represent the load. 1 2 1 2 According to Lewallen’s model, the = 2 |Z – Z | / |Z + Z | (Eq 4) The 1:1 Current Balun 2 1 2 1 voltages across the two windings of the Fig 2 shows the schematic of transmission-line transformer (TLT) The degree to which the currents Guanella’s 1:1 “current” balun. Z and 1 are equal, and both equal VS, so are balanced depends only on the bal- Z represent the load. The junction ance of the load impedances with re- 2 I = V / Z between Z1 and Z2 is grounded to rep- 1 S 1 spect to ground. In order to keep resent the (typically capacitive) cou- and IMB ≤ 0.1, the impedances must be pling of the antenna system to ground. The load is balanced with respect to ground when Z1 = Z2. The principle limitation of this balun is that it can only maintain balance between the currents flowing into I 1 C the load, I1 and I2, for relatively low A load impedances. The ability to maintain the correct current balance in the I 2 I 1 load is of course the reason for using B D a balun in the first place, so the extent to which a balun maintains this balance is the primary measure of its I -I Z W effectiveness. In the case of the 1:1 cur- 1 2 rent balun, Lewallen shows that qx0509-roos01 I / I = (Z + Z ) / Z (Eq 1) 1 2 2 W W Fig 1—Lewallen’s model of the 1:1 TLT. Where ZW is the “winding impedance” (common-mode impedance) of the balun. We can use this to calculate the measure of imbalance pro- posed by Witt3 as: I 1 V S IMB = 2 |(I1 – I2) / (I1 + I2)| = 2 |(I1 – I1 (Z2 + ZW)/ ZW) / (I1 + I1 (Z2 Z 1 + ZW)/ ZW)| = 2 |Z2| / |(Z2 + 2 ZW)| (Eq 2) (Note that |X| means the magnitude of complex variable X.) If IMB is Z 2 small, then |ZW| >> Z2. So as a good approximation, I 2 IMB = |Z2| / |ZW| (Eq 3) Although Witt does not suggest an qx0509-roos02 acceptable maximum figure for IMB, 0.1 seems a reasonable value since Fig 2—A schematic of Guanella’s 1:1 current balun. this means that the magnitude of the common-mode (unbalanced) current flowing in the antenna is one-tenth the magnitude of the differential-mode I 1 (balanced) current. To prevent IMB from exceeding 0.1 we require that |Z | ≤ 0.1 |Z |. This limits the use- 2 W Z 1 fulness of the current balun. Charles V S Rauch4 measured the common-mode impedance of several commercially V manufactured current baluns using a S network analyzer and found that most V had a common-mode impedance of less S Z 2 Ω than 1 k at 15 MHz, giving a maxi- I 2 mum acceptable value of |Z2| of less than 100 Ω. For balanced loads, this means that the baluns tested by qx0509-roos03 Rauch would be effective only for load impedances under 200 Ω. Fig 3—A schematic of Ruthroff’s 4:1 voltage balun. 30 Sep/Oct 2005 within about 10% of each other. through ferrite beads as suggested by tain balance and can operate effec- Imbalance does not impose any con- Walt Maxwell8 for the choke, Rauch’s tively and efficiently at much higher straint on the maximum impedance measurements (see Note 4) show that impedance levels than could a current a voltage balun can drive. This is de- beaded coax chokes have a large re- balun. In turn, the choke presents a termined by efficiency considerations sistive component to their common- balanced load impedance to the volt- and is usually greater than the mag- mode impedance. Equation 13 shows age balun preceding it, despite any nitude of the winding inductance, so that this will increase the choke’s imbalances in the antenna system, the voltage balun can drive balanced power dissipation and reduce the ef- allowing the hybrid balun to drive loads at least five times larger than ficiency of the balun. The voltage unbalanced loads more effectively can a current balun with the same balun found on many ATUs can be than could the voltage balun alone. winding impedance. converted to a hybrid balun by sim- Let VC be the common-mode volt- Unfortunately, supposedly ply adding a common-mode choke age across the choke windings as balanced antennas may present un- after the balun, without need for shown in Fig 5. Since VC also appears balanced loads due to the presence modification to the ATU or balun. across the winding impedance ZW in of nearby conductors or asymmetry Although the common-mode choke Lewallen’s model, and the current in the antenna installation imposed appears identical to the 1:1 current flowing through the winding imped- by site constraints. Kevin Schmidt5 balun, the term “common-mode choke” ance is the common-mode current describes one such case: is preferred, as it is driven by the bal- I1 – I2, “One of my antennas is a ‘dipole’ anced voltage at the output of the volt- VC = (I1 – I2) ZW (Eq 5) about 60 feet on a leg running around age balun, instead of having one side the outside of my 1-story house. It is fed grounded. This means the voltage The voltages at the input side of the Ω by about 30 feet of 300 TV twinlead. across the windings is no longer the choke are VS and –VS, so The legs of the ‘dipole’ are not straight, full source voltage V , but only a por- S I = (V – V ) / Z (Eq 6) and it isn’t symmetric about the feed 1 S C 1 tion of VS that depends on the degree point, since the shack is at a corner of of imbalance of the load. Consequently, and the house.” the common-mode choke does not suf- I = (V + V ) / Z (Eq 7) He measured the differential and fer from the limitation of the current 2 S C 2 common-mode impedances of the an- balun—that |Z2| must be less than Substituting Equations 6 and 7 into tenna at 3.52 MHz using professional one tenth of |ZW| in order to main- Equation 5 and solving for VC, instruments and obtained the results shown in Fig 4.6 If this antenna were driven by a voltage balun, IMB would be 0.22. This shows that the inherent Z = 191 + 358j balance of amateur antennas cannot 1 Z 2 = -33 + 176j be assumed, so antenna tuner baluns must be able to drive unbalanced loads effectively. Fig 4—The The Hybrid Balun Z = 887 + 622j impedances of 3 Schmidt’s dipole. The preceding sections have shown that the current balun is limited by its inability to maintain current balance when driving all but low-impedance loads, while the voltage balun qx0509-roos04 cannot maintain current balance in unbalanced loads.

Load more