By Jerry Sodus, KM3K
In the spirit of the hobby, I pass along what I have learned and don’t profess to know all the answers.
1. The primary reason for this work is to explain how a particular
Transmission-Line-Transformer (TLT) known as a 1:1 Guanella-balun
Three other commonly used names to describe this balun are:
“current-balun”, “isolation-balun”, and “choke-balun”; I’d suggest
committing all four names to memory.
The 1:1 Guanella-balun is the basic building block for other types of types of Guanella-baluns or ununs (a Guanella-unun is just a Guanella-balun with the grounds in different places; a similar statement for Ruthroff-ununs cannot be made). Also, I’ve included ‘background’ info, which I thought may help in understanding.
2. About the word “balun”….It means “balanced-to-unbalanced”; Guanella (a Swiss engineer in 1944) needed to match a balanced (push-pull) amplifier to an unbalanced load; hence the name. However, these days we generally are going unbalanced-amplifier to balanced-antenna load. So, “unbal” would be a more appropriate term but we are stuck with balun.
3. It is most important, for the time being, to absolutely and completely put out of mind the concept of the conventional transformer and its flux-linkages.
4a. This section explains the very first 1:1 Guanella-balun. As part of his development work in 1944, Guanella coiled a transmission-line. The historical record, that I’ve accessed, is meager on the details of
the transmission-line that he used but it is reasonable to presume he
used coax (Heaviside had patented coax in 1880 & Bell Labs began
making coax in 1929). In coiling the line, Guanella intentionally formed a choke, whose
conductor was the exterior surface of the coax’s shield. Recall that
skin-effect is operative on a coax-shield at radio-frequencies, so the
transmission-line current is on the shield’s inside surface and the
harmful “common-mode currents” and so-called “antenna-current” are on
the shield’s outside surface. It is the choke that limits these harmful currents; these currents do cause flux in the core. As Guanella showed on his schematic, this choke is in parallel with the transmission-line.
This choke serves to isolate the input from the output and so suppresses any conventional transformer current and allows transmission-line current to flow. So that’s it; we have a 1:1 balun made with an air-core and, in that core, there are no flux-lines from the transmission-line currents.
4b. The seminal work by Joe Reisert W1JR in 1978 enhanced the 1:1 balun
idea further by winding that coil of coaxial transmission-line onto a
toroid-core. Here also, by virtue of the coax’s inherent shielding
feature, we certainly can see there are no flux-lines introduced into
the toroid-core when just balanced transmission-line currents are
flowing. W1JR also invented a unique way to wind the coil and here is a link to a site showing that winding-technique:
http://www.balundesigns.com/servlet/the-63/balun-designs-1-cln-1-Isolation/Detail. Notice the absence of a thick plastic jacket; easier to wind and can put on more turns.
4c. Without giving up much in performance, one can modify W1JR’s coax design to use a two-wire transmission-line where the two-wires are held together to maintain a desired characteristic impedance. The site called up by this link shows an example: http://www.balundesigns.com/servlet/the-98/isolation-choke-balun-common/Detail. A paragraph later in this work has some verbiage about the characteristics of two-wire lines.
4d. Another ingenious style “current-balun” is W2DU’s beaded-balun, where coax-cable is passed through ferrite-beads to form chokes; although simple in concept, proper core permeability choice is needed to prevent cores from breaking due to excess heating of the cores. (Editor note: this type also has the highest losses)
5. A different 1:1 balun is a “Ruthroff-balun”; named after its Bell-Labs inventor in 1959. Another name for the “Ruthroff-balun” is “voltage-balun”. As I see it, the voltage-balun generally is inferior to the Guanella-balun.
6a. We can make a 4:1 Guanella-balun by connecting two identical 1:1 Guanella baluns such that the inputs are in parallel and the outputs are series connected per either the ARRL’s 2007 Handbook or Antenna-book; a comment that neither book has any text on the importance of the transmission-line’s characteristic-impedance.
6b. Performance is enhanced at higher frequencies if the balun’s
transmission-line characteristic-impedance is made to an optimum level.
If matching a 50-ohm input to a 200-ohm load, let characteristic-impedance = 100-ohms.
If matching a 75-ohm input to a 300-ohm load, let characteristic-impedance = 150-ohms.
6c. The Guanella-balun can be reversed. So, if matching an unbalanced 50-ohm input to a balanced 12.5-ohm load for a Yagi beam, let the balun’s transmission-line have a characteristic-impedance of 25-ohms.
6d. Sevick’s book “Transmission-Line-Transformers” 2nd edition to 4th edition has multiple pages detailing how to make unique value characteristic-impedance coax (his Table 4.1 ranges from 12.5-ohm to 35-ohm) and two-wire lines (his Figure 5.2 ranges from ~32-ohm to ~175-ohm). Also provided are schematics/pictures/details for the surprisingly simple test-equipment he used in testing his cables and baluns.
6e. In instances where there may be a space or weight or cost problem with a two-core 4:1 Guanella-balun, the two-core 4:1 Guanella-balun can be wound with just one-core if the user is willing to trade frequency performance; the exact trade-offs depend on if the load is floating or grounded at its mid-point.
7a. Overall, the 1:1 balun is the most frequently used balun.
7b. Second place in usage is the previously mentioned 4:1 balun (for example, 200-ohm-to-50-ohm), which can be a Guanella or Ruthroff design; strong arguments can be made for each design approach.
7c. The reader should be aware there is a myriad of other impedance-ratio possibilities made up from combinations of current-balun and/or voltage-baluns; one use is to match vertical-antennas.
8a. In a TLT, energy is transferred from input to output by means of a transmission-line
Although hams are very adept in using transmission-lines like coaxial-cable or two-wire ladder-line to move the transmitter’s output to an antenna, in a balun it is a difficult to think of the conductors as transmission-lines. One tends to think they are just two wires wound around a core.
8b. Concerning a transmission-line, the US-Army manual TM11-675 (Aug
1951) has that “…the greater portion (about 90%) of the transmitted
energy is in the electromagnetic waves that the line conductors guide
through the space between them. In general, less than 10% of the
transmitted energy is actually in the conductors of which a
well-designed line is comprised.”
(Comment by KM3K: I have not seen these statements anywhere else.)
8c. In a transmission-line, there are two conductors (spaced as in two-wire-line or as in coax). There is an electric-field between the two wires. Each wire has its own magnetic-field. These fields are related in a way that only the Divine Creator fully
understands but Heaviside, drawing on Maxwell’s work, was able to
express mathematically and we are not going there at all. For two-wire-line, the energy is confined to the immediate region around those wires.
For coax, the energy is confined between the center-wire and the shield (assuming a good shield). The electromagnetic signals we will be using will be Transverse-ElectroMagnetic (TEM) waves; that is, the standard HF and VHF ham-band stuff.
9. The TLT can be made in several different ways: a) coiling loops of coax (which means we have an air-core),
b) winding either coax or two-wire on a toroid-core or ferrite-rod. To quote Dr. Sevick W2FMI (SK) about the TLT, “….it is both a choke (a lumped element) and a transmission-line (a distributed element).” The toroid-core has enhanced performance over the other methods, although the other methods have appealing attributes like simplicity of winding and cost.
10. In his seminal ARRL 1985 article, Roy Lewallen W7EL introduced the terms of “current-balun” and “voltage-balun”. Here is a link; it is a good read…. http://www.eznec.com/Amateur/Articles/Baluns.pdf Both baluns work by the transmission-line mode. The current-balun (Guanella) puts equal current into/out-of its output terminals; highly desired feature. The voltage-balun (Ruthroff) puts equal magnitude voltages at its output terminals.
11. It is important to comment on the impressive frequency range of a well-designed current-balun. Earlier in this work, I provided a link to a toroid wound with coax that is a commercially produced 1:1 current-balun at 50-ohms, wound on a toroid-core with coax, has a passband from 1 to 54 MHz with very, very low insertion-loss and very high choking-impedance.( http://www.balundesigns.com/servlet/the-63/balun-designs-1-cln-1-Isolation/Detail)
12. Moving energy in a balun from input to output by the
transmission-line mode on a toroid-core means that theoretically there
is no flux in the toroid-core. Now, in actuality, because we cannot make a perfect transmission-line,
we can expect there will be some accidental flux but nowhere near the
magnitude from a conventional transformer.
It is easy to understand why there is no flux for well made coaxial-cable. But it may not be so clear for two-wire transmission-line.
So here is why there is no flux in the toroid-core for a theoretical two-wire transmission-line: a) the current in each wire is equal but going in opposite directions; should be 180 deg out-of-phase. b) this means the magnetic-fields caused by those currents are also opposite and cancel each other out everywhere except in the very immediate region between the two wires. c) because the magnetic-fields largely cancel everywhere, there is no net flux in the core, which in turn means no concerns about core saturation and all the problems that can bring.
13a. If the balun-designer is not careful, it is possible for the balun to conduct conventional transformer currents rather than the desired transmission-line currents. Winding the transmission-line on the toroid-core makes a choke; hence the name “choke-balun”. The choke has a reactance and it serves to isolate the input from the output and works to prevent conventional transformer currents; we want that reactance to be at least 5 times (some authors say 10x) the load resistance. Although more turns makes more reactance, it adds parasitic capacitance between the windings and that harms the high frequency responses.
13b. In a voltage-balun, in spite of good design efforts, invariably at the lower frequencies there will be conventional transformer currents and attendant flux-linkages in the core; this causes the low frequency range to suffer performance (typically higher insertion-loss).
14. If the 1:1 current-balun is used in a 50-ohm system, then the transmission-line’s characteristic-impedance used in the balun is 50-ohm. One factor in the high-frequency limit of the balun is determined by how close to 50-ohm is that transmission-line’s characteristic-impedance; parasitic generally are absorbed but can be another factor.
15. Key uses for the 1:1 current-balun: a) is to marginalize the “inverted-L current” in the transmission-line feeding a dipole-antenna. This will prevent a radiating Feedline and prevent distorting the antenna’s radiation pattern.
There will be some flux in the core but it’ll be minor due to the small current causing it. This problem is covered at length in the 2007 ARRL Antenna-Book. b) drive balanced antennas (e.g. dipole or Yagi) with equal currents.
16. A quick comment about notation “1:4” and “4:1”; which is right?. The reader may see impedance-ratios written by an author as 1:4 or as 4:1. From my reading, I noted the only consistency is inconsistency in notation (even with the same author). So my own rule is to read the text and glean from it what is the author’s meaning.
I hope this was of some interest and help to my fellow hams.
73 Jerry Sodus KM3K; FN10je; member of the South-Mountain-Radio-Amateurs Club