ShackWiki
Tools / Unun calculator

Unun calculator

Pick a transformer ratio, a ferrite core and your lowest band. You get the number of turns to wind, a check that the winding has enough inductance, and the SWR you can expect for a given antenna impedance.

Primary (50 Ω side)––
Secondary / total––
Primary reactance––
Ideal antenna impedance–What the transformer turns into 50 Ω
Typical power––

Primary turnsSecondary / total turnsPrimary inductanceReactance at lowest bandVerdict

What SWR will my antenna give?

Enter the impedance at the antenna end. An end-fed half-wave is usually 2,000 to 4,000 Ω. This assumes a purely resistive load and an ideal transformer.

Radio sees–Through the selected transformer
SWR––
Best ratio for this load––

How the calculator picks the turns

An unun is an autotransformer: one winding on a ferrite ring, tapped near the earthy end. The impedance ratio is the square of the turns ratio, so a 49:1 needs 7 times as many turns in total as in the primary, and a 64:1 needs 8 times as many.

Impedance ratio = (total turns ÷ primary turns)²
Primary inductance L = AL × N²
Primary reactance X = 2π × f × L

The primary has to look like a high impedance compared with the 50 Ω it's fed from, or it shunts power away and the SWR rises on the lowest band. A common rule is that its reactance should be at least four times 50 Ω, so 200 Ω, at the lowest frequency. Three times, 150 Ω, usually works in practice. That's why the classic 40m to 10m end-fed transformer has 2 primary turns on an FT-240-43, and an 80m version needs 3. AL values here are Amidon's figures in nH per turn², which have a tolerance of about ±25%.

49:1 or 64:1?

Both suit an end-fed half-wave. A 49:1 turns 2,450 Ω into 50 Ω and a 64:1 turns 3,200 Ω into 50 Ω. The real impedance at the end of a half-wave wire depends on its height, the wire, the counterpoise and the band, and is often between 2,000 and 4,000 Ω, so either can work well. Many builders find a 49:1 gives a better match on the higher bands, and a 64:1 on the lower ones. Use the SWR section above to compare them for your measured impedance.

The compensation capacitor

A 49:1 or 64:1 has leakage inductance that pushes the SWR up on 15m and 10m. A capacitor of about 100 pF across the primary (between the coax centre pin and ground) tunes it out. Use a ceramic capacitor rated at 1 kV or more, then adjust its value for the best SWR on 10m.

Cores and power

Mix 43 ferrite is the usual choice from 160m to 10m. The power figures shown are typical of what builders report for an end-fed transformer at a low SWR, not a guarantee. Ferrite transformers lose a few percent of the power as heat, and the core can crack if it gets too hot. FT8 and other data modes transmit continuously, so they heat the core far more than SSB at the same power. After a long transmission, check the core is no more than warm. If it is hot, use a bigger core or two stacked together. A 9:1 or 4:1 runs at lower impedance and handles more power on the same core.

Don't forget the counterpoise and choke

An end-fed antenna needs a return path. Often it's just the outside of the coax braid, which brings RF back into the shack. Fit a short counterpoise wire (about 0.05 λ on the lowest band) to the unun ground, and a common-mode choke on the coax a few metres from the unun. For lengths to use with each transformer, see the EFHW and random wire calculator.

Questions

How many turns for a 49:1 unun?

2 primary turns and 14 in total (12 more on top) on an FT-240-43 core for 40m to 10m. For 80m, use 3 primary and 21 in total, or two stacked cores with 2 and 14.

What's the difference between a balun and an unun?

A balun connects a balanced antenna, such as a dipole, to unbalanced coax. An unun connects an unbalanced antenna, such as an end-fed wire, to unbalanced coax. Both can also transform impedance.

Can I use a 9:1 unun with an end-fed half-wave?

Not well. A half-wave wire's end impedance is around 2,500 Ω, and a 9:1 turns that into about 280 Ω, an SWR of about 5.6. Use a 49:1 or 64:1.

Why does my unun get hot on FT8?

Data modes transmit at full power for the whole over, so the core heats up much more than on SSB. Use a bigger core, stack two cores, or reduce power.