Most 1:1 current chokes and commercial 4:1 baluns are not wound transformers at all. They are lengths of transmission line on a core, and the energy travels along the line rather than crossing a magnetic field — so the turns barely matter and the line impedance is everything. This gives you the line to wind with, how many turns, and a drawing you can wire from.
Guanella and Ruthroff — wired terminal by terminal.
Most 1:1 current chokes and most commercial 4:1 baluns are not wound transformers. They are lengths of transmission line on a core, and the energy travels along the line rather than crossing a magnetic field. That changes what matters: the turns barely describe the thing, and the characteristic impedance of the line is everything.
If you are winding a tapped autotransformer instead — a 49:1 for an end-fed, or an odd ratio like 300 ohms — that is the Balun & Unun tool. This one is for the transmission-line family.
n equal lines, paralleled at the low-impedance end and in series at the high-impedance end. Each line is choked on its own core.
The cores are not coupling anything. Their only job is to stop current flowing on the outside of the line, which is exactly what makes it a current balun — it forces equal and opposite currents in the two conductors. Share one core between lines and they stop being independent, and you lose that.
4:1 on one core, from a single bifilar winding. Wire 1 runs A to C, wire 2 runs G to D, and D is joined back to A. The line carries its differential voltage end to end, so C sits one line-voltage above D — which is already one above ground — and the output taken from C to G is twice the input. Twice the voltage is four times the impedance.
That is also why it looks like a tapped autotransformer with one turn in and two out, and is not one.
You pay for the single core twice:
Build a Ruthroff when size and simplicity matter more than the top band and feedline behavior. Build a Guanella otherwise.
This is the step people skip and the one that decides the result.
Put an ohmmeter across a piece of 50 Ω coax and you read a dead short or an open, depending which end you touch. Characteristic impedance is the ratio of voltage to current for a wave traveling along the cable, and it is set purely by geometry — how thick the two conductors are, how far apart they sit, and what insulation is between them. Six inches and six hundred feet of the same cable are both 50 Ω. There is no component in there to measure at DC.
Each line in the balun has to be √(Zin × Zout). For a 4:1 from 50 Ω that is √(50 × 200) = 100 Ω.
Coax is manufactured in 50 Ω and 75 Ω and essentially nothing else. So apart from the 1:1 — where the line is 50 Ω and you simply wind the feedline itself through the ring — you make the line yourself:
Tighter twist or thicker wire lowers the impedance; wider spacing or thinner wire raises it.
Wind about a meter of your intended line. Short the far end and measure impedance at the near end (Zsc). Open the far end and measure again (Zoc). Then:
Z₀ = √(Zsc × Zoc)
Reads 130 Ω and you wanted 100? Twist tighter and measure again. This is worth the ten minutes: wind a Guanella with whatever bifilar wire is to hand and its impedance will be whatever that wire happens to be — usually near 100 Ω, which is right for a 4:1 from 50 Ω and wrong for everything else. That is why so many home-made 9:1 ununs measure well on 80m, where the line is electrically short enough not to matter, and badly at the top of HF where it is not.
