Why you need a choke
Coax carries RF on the inside of the braid and on the centre conductor, with equal and opposite currents. When the antenna isn't perfectly balanced, or it's fed at one end, some current flows on the outside of the braid as well. This is common-mode current. It turns your feedline into part of the antenna, which distorts the pattern, raises the noise you hear, and brings RF back into the shack, where it can make microphones buzz and computers misbehave.
A common-mode choke puts a high impedance in the way of that outside current while leaving the signal inside the coax untouched. Wound on a ferrite toroid, the same thing is often called a 1:1 current balun.
Choosing a ferrite mix
| Mix | Best range | Use it for |
|---|---|---|
| 31 | About 1 to 30 MHz | The best all-rounder for HF, and the first choice for 160m to 40m. |
| 43 | About 5 to 30 MHz, useful to 50 MHz | Good on 40m to 10m. Less effective than 31 on the low bands. |
| 52 | Upper HF and low VHF | The upper HF bands and 6m. |
| 61 | About 30 to 300 MHz | VHF. Often a few clip-on cores on the coax are enough. |
Ferrite chokes work mainly by being lossy. At HF, mix 31 and 43 cores turn the unwanted current into a little heat instead of reflecting it. That's what makes them work well across several bands. If the choke impedance is too low, the core has to absorb a lot of power and can overheat, so aim high: at least 1,000 Ω, and 5,000 Ω or more for end-fed antennas and high power.
How the calculator picks a core
The core has to be big enough for your coax to pass through the number of turns you need in one layer, and big enough to stay cool at your power. FT240 cores (61 mm outside, 35.6 mm hole) suit RG-213 and anything over 100 W. FT140 cores (35.6 mm outside, 22.9 mm hole) are fine for thinner coax at lower power. The turns figure is a starting point based on widely used designs. The number that fits is worked out from the size of the hole and the thickness of your coax.
The only way to know what a choke really does is to measure it. Connect a NanoVNA across the outside of the coax, from the braid at one end to the braid at the other, and read the impedance on each band. Add or remove a turn and measure again. Crossing the coax over halfway, so the two halves of the winding are on opposite sides of the core, lowers stray capacitance and improves the higher bands.
Coiled coax chokes
A coil of coax with no ferrite is an inductor. Its reactance rises with frequency until the capacitance between turns makes it self-resonate. Around that point it chokes very well, and above it, it stops working and can even make things worse. So a coax coil is a good single-band choke, and a poor one across many bands. The inductance uses Wheeler's formula for a single-layer coil. Treat reactance above 500 Ω as useful, but remember that it says nothing about where your coil self-resonates.
Questions
How many turns of RG-58 on an FT240-31?
For 80m to 10m, 10 to 12 turns is a common starting point, crossing over halfway. For 160m, use 14 or more turns, or stack two cores. Always check with an analyser.
Mix 31 or 43 for a choke balun?
Mix 31 for anything that includes 80m or 160m, and as a general HF all-rounder. Mix 43 works well from 40m to 10m. If in doubt, choose 31.
Do I need a choke on an end-fed half-wave?
Yes. An EFHW uses the outside of the coax as part of its counterpoise, so a choke a short distance down the coax, often about 0.05 wavelength on the lowest band, stops the rest of the feedline radiating. See the EFHW calculator for the distance.
Is a coiled coax ugly balun any good?
On one band, it can work well. Across several bands it's unreliable, because its self-resonance makes it effective over only a narrow range. A ferrite choke is better for multi-band antennas.