Tools

Baluns (unbalanced ↔ balanced) at 1:1, 2:1, 3:1, 4:1, 6:1, 9:1, 49:1 and 64:1 and ununs (unbalanced ↔ unbalanced) at 1:1, 2:1, 3:1, 6:1, 9:1, 49:1 and 64:1, also with the Reisert crossover, on ferrite toroids: the turns, the impedance on each band and, for transformers, the core loss and temperature rise at your power level.

Below 500 Ω anywhere in the band, the choke barely stops common-mode current: add turns, use a bigger core or type 31 material, or stack two cores.

Between 500 and 1,000 Ω the choke helps, but below 1 kΩ some RF can still flow on the shield. If you can, stack another core.

The primary doesn't reach 4 times 50 Ω at the lowest frequency: the SWR will rise and the core will heat up. Add turns or use a bigger core.

These turns won't fit in a single layer inside the hole: choose a bigger core, thinner coax or wire, or fewer turns.

The flux density exceeds the recommended limit for ferrite at this frequency: the core will heat up in continuous-carrier modes. Stack another core, add primary turns or reduce power.

The core has to dissipate too much heat for its size: it can get hot enough to lose permeability (and crack). Stack another core or choose a larger one.

Iron powder has low permeability, so a choke would need a huge number of turns. For chokes, type 31 or 43 ferrite works much better.

A choke needs a high impedance that is mostly resistive: the solid curve is |Z| and the dashed one is the resistive part. Wind the coax in a single layer and keep the input and output ends apart (use a Reisert cross-over halfway if needed). Too many turns raise the winding capacitance and the choke falls off on the higher bands, which is why the automatic setting doesn't always use as many turns as will fit.

Sevick's rule: the primary reactance should be at least 4 times the impedance it sees (50 Ω) on the lowest band. On the higher bands, 49:1 and 64:1 ununs usually need a 100 to 150 pF capacitor (silver mica, 1 kV or more) across the primary to compensate for leakage inductance: adjust it while watching the SWR on 10 m. The flux limit is Amidon's guideline for ferrite (about 150/√f gauss); heating is based on the average power of the mode.

Model: each ferrite's complex permeability (µ' and µ'') comes from Fair-Rite's measured curves, the winding capacitance is fitted to VNA measurements (the Reisert crossover lowers it by 26%) and the temperature rise is worked out from the core's surface area. Ferrite tolerance is ±20%, so always measure the finished balun with an antenna analyzer (or a NanoVNA) before applying power.