Some Aspects of the Balun Problem (Adapted from QST, March 1983) in Antenna Impedance and SWR Measure- Ments Is Greatly Improved
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Chapter 21 Some Aspects of the Balun Problem (Adapted from QST, March 1983) in antenna impedance and SWR measure- ments is greatly improved. In addition, Sec 21.1 Introduction antenna-matching networks may be used hy all the mystery sur- with this choke balun, because no mis- rounding baluns? Here’s match limits are imposed by the balun. some straight talk to dis- Sec 21.2 Transformer pel the rumors! The balun W Accuracy — to use, or not to use — is one of today’s hottest topics in Amateur Radio. Because Using the General Radio 1606-A pre- certain aspects of the connection between cision impedance bridge and the Boonton a coaxial feed line and a balanced antenna 250-A RX meter, I have made measure- have been ignored, misunderstanding still ments of transformer-type baluns which exists concerning the function of baluns. prove that with a 50-ohm resistive load, Many commercial baluns embody some the transformers in typical 1:1 or 4:1 form of impedance transformer, promot- baluns do not yield a true 1:1 or 4:1 im- ing our tendency to misconstrue them as pedance transfer ratio between their in- little more than a matching device, while put and output terminals. This is because their primary function is to provide proper of losses, leakage reactance, and less than current paths between balanced and un- optimum coupling. My findings have been balanced configurations. substantiated by the work of the late John To help clarify the misunderstanding, Nagle, K4KJ (Ref 80). Furthermore, the this chapter explains some of the unde- impedance-transfer ratio of such baluns sirable effects that occur when a balun is degrades even further when used with an not used, and some that occur when antenna that is reactive from operation baluns employing coupling transformers away from its resonant frequency. This are used. In many cases, these effects degradation of impedance transfer asso- cause significant errors in measurements ciated with transformer-type baluns poses of antenna terminal impedance and SWR. no serious operational problems. However, This chapter also describes a simple and SWR curves plotted from measurements inexpensive method of loading the outside of an antenna using such a balun differ of a coaxial feed line with ferrite, which significantly from those using a choke effectively produces a well-balanced, wide- balun that has no impedance-transfer er- band choke balun. Because this configu- ror. Thus, when a precision bridge is used ration eliminates the coupling trans- to measure antenna impedance (R + jX), former (with inherent impedance-trans- the data will be erroneous with either a fer ratio errors), the accuracy obtainable transformer-type balun in the circuit, or Some Aspects of the Balun Problem 21-1 Fig 21-1—Illustration of the various current paths at a dipole feed point. with no balun at all. corresponding change in line attenuation. Sec 21.3 Should SWR Change Then why does the SWR sometimes change? If the SWR changes significantly with Line Length? with a change in line length, it means that We know that the feed-line input im- the load impedance terminating the line pedance changes with line length when is also changing. The load impedance can the load (antenna) is not matched to the change with line length? Yes. If a balun is line. Sometimes trimming the length of omitted when you feed a balanced an- the feed line helps to obtain a load im- tenna with coaxial cable, the load imped- pedance better suited to match the trans- ance will change when the line length is mitter. Theoretically the SWR should not changed, and of course the SWR will also change with line length — except for a change! To explain this commonly experi- barely perceptible change because of the enced phenomenon we must investigate 21-2 Chapter 21 how current flows in an antenna system. will be nearly equal. On the other hand, To understand the functions of a if the RF path to ground is a multiple of balun, it is essential to be familiar with λ/2, the impedance to ground will be fairly current paths at the feed point of the di- low, and current I3 may be substantial. pole. These paths are shown in Fig 21-1. This results in unequal currents in the Because of their symmetrical relationship, dipole arms and radiation from the feed the dipole arms couple energy of equal line. In many instances, this RF path to magnitude and opposite phase onto the ground includes the transceiver line cord feed line, thus canceling induced current and some house wiring, terminating at the flow on the outside of the feed line (Ref power-line ground! Thus, the amplitude 81). What is disturbing is the discovery of I3 varies with changes in feed-line that there are three paths for current flow length because of changes in the imped- in a coaxial feed line, instead of only two. ance between dipole arm 2 and ground. How can there be three current paths in Keep in mind that transmission-line only two conductors? At RF, skin effect iso- currents I1 and I2 cannot produce radia- lates the currents flowing on the inner and tion, because their fields are not only of outer surfaces of the coaxial shield. This equal magnitude and opposite phase, but effect, which does not occur significantly their fields are also confined within the at DC or low-frequency AC, prevents cur- shield of the coaxial cable. However, the rents that flow on the inner braid surface field developed by I3 does radiate, and from interacting with those on the outer thus, the outer surface of the coaxial braid surface, and vice versa. effectively becomes dipole arm 3, which As shown in Fig 21-1, while travel- is connected in parallel with dipole arm ing within the transmission line, current 2. I1 flows on the center conductor and I2 To clarify this equivalent connection flows only on the inner surface of the outer of radiators, I’ve simplified the circuit as shield. When antenna current is flowing shown in Fig 21-2. Since currents I1 and from left to right as shown, I1 flows out of I2 do not interact with any other currents, dipole arm 1, downward onto the center we may hypothetically place the RF gen- conductor, and returns to the generator. erator directly at the input terminals of Current I2, being of opposite phase and the antenna. Now that the coaxial cable direction, flows upward along the inside is no longer needed to transfer power from surface of the feed-line shield until it the generator to the antenna, the third reaches the junction of dipole arm 2. At conductor of the feed line (the outside sur- this junction, I2 divides into two separate face) can be replaced with a single wire paths to form I3 and I4. Current I3 flows connected between arm 2 and RF ground. back down the outside surface of the feed We have not changed the circuit electri- line, and I4, which equals I2 – I3, flows to cally because I3, which previously flowed the right onto dipole arm 2. The magni- on the outside of the coaxial cable, still tude of I3 depends on the impedance to flows to ground — but now on the single ground provided by the outside surface of wire shown. the coaxial shield. We know that, depending on height, If the effective path length to RF the impedance of a dipole at resonance is ground is an odd multiple of λ/4, the im- usually between 50 and 75 ohms, and is pedance to ground will be very high, mak- purely resistive. At frequencies above ing I3 negligible. In this case, I1 and I4 resonance, the resistance increases gradu- Some Aspects of the Balun Problem 21-3 Fig 21-2—Simplified electrical representation of Fig 21-1. ally, and series inductive reactance ap- components of the parallel combination of pears; below resonance, the resistance arms 2 and 3 are different than those of decreases and capacitive reactance ap- arm 1. Consequently, the dipole imped- pears. The impedance of each dipole arm ance is different than if arm 3 was not is half of the total dipole impedance. Since present. the far end of arm 3 is at RF ground, its Returning to Fig 21-1, we can now see impedance behavior follows that of a that, without a balun, changing the feed- short-circuited transmission line, with the line length also changes the antenna short appearing at ground. Thus, when length (arm 3), which in turn affects the the length of arm 3 is an odd multiple of impedance at the input end of the feed λ/4, its impedance is a parallel-resonant line. Therefore, the SWR measured at the maximum, a high resistance typically input of the transmission line does change from 2000 to 3000 ohms. This high resis- with line length when no balun is present tance in parallel with arm 2 has little ef- to eliminate I3. This phenomenon ex- fect on the total dipole impedance. How- plains a point that is often puzzling for ever, as the effective electrical length of the amateur who uses no balun, and who arm 3 departs from λ/4 (or odd multiples believes that he must trim his dipole each thereof), by changes in either its physical time the feed-line length is changed “to length or the generator frequency, the in- keep the SWR down.” put resistance of arm 3 decreases, and It is evident that, in coupling an un- reactance also appears in series with the balanced line to a balanced load (such as resistance.