EFHW and 49:1 transformer calculator

Where the mistake hides

The EFHW wire is a half wave, and that part is easy. The hard part is the transformer: if the primary has too little inductance for the lowest band, the SWR wanders and the core heats up. That is exactly what this calculator checks — how many turns you need on your particular toroid, rather than “seven to two and off we go”.

The transformer sits at the feedpoint — where the impedance of the wire is highest

Lowest operating frequency (MHz)
Transformation ratio	49:1 (2450 Ω) — the classic EFHW64:1 (3200 Ω)36:1 (1800 Ω)9:1 (450 Ω) — random wire
Core material	43526177
Toroid size	FT-23 — 5.8 × 3 × 1.5 mm, AL 158FT-37 — 9.5 × 4.7 × 3.2 mm, AL 350FT-50 — 12.7 × 7.1 × 4.8 mm, AL 440FT-50A — 12.7 × 7.9 × 6.4 mm, AL 480FT-50B — 12.7 × 7.9 × 12.7 mm, AL 965FT-63A — 16 × 9.6 × 6.4 mm, AL 520FT-82 — 21 × 13.2 × 6.4 mm, AL 470FT-82A — 21 × 13.2 × 11.9 mm, AL 885FT-87 — 22.1 × 13.7 × 6.4 mm, AL 485FT-87A — 22.1 × 13.7 × 12.7 mm, AL 970FT-100 — 25.4 × 15.5 × 6.4 mm, AL 500FT-100A — 25.4 × 15.5 × 8.1 mm, AL 645FT-100B — 25.4 × 15.5 × 12.7 mm, AL 1000FT-114 — 29 × 19 × 7.5 mm, AL 510FT-114A — 29 × 19 × 13.8 mm, AL 950FT-125 — 31.8 × 19 × 9.5 mm, AL 775FT-140 — 35.6 × 22.9 × 12.7 mm, AL 885FT-193A — 49.1 × 31.8 × 21.6 mm, AL 1075FT-240 — 61 × 35.6 × 12.7 mm, AL 1300
Stacked cores	OneTwoThree
Primary turns
Transmitter power (W)
Radiator
Wire length
Counterpoise length (0.05 λ)
Transformer
Impedance seen from the antenna
Turns ratio
Required primary inductance (X ≥ 4·Z)
Actual inductance
Minimum turns for this core
Voltage across the antenna winding

📐 The formulas

The wire:

L = k × 149.896 / f    [m],   k ≈ 0.95
Counterpoise ≈ 0.05 λ

The transformer:

n² = Z_antenna / 50
    49:1 → 2450 Ω      64:1 → 3200 Ω      9:1 → 450 Ω
turns = √(n²):  49:1 → 7:2,  64:1 → 8:2,  9:1 → 3:1

The inductance check — the whole reason this calculator exists:

X_L(primary) ≥ 4 × Z_antenna  at the lowest frequency
    →  L ≥ 4 × Z_antenna / (2π × f_min)
Actual: L = AL × N²          AL from the Amidon tables, nH/turn²

Voltage across the antenna winding:

U = √( P × Z_antenna )

Symbols:

• AL — the core’s inductance factor; stacked cores multiply AL by their number
• Z_antenna — the design impedance at the end of the wire

Where this calculator stops:

• 2450 Ω is a compromise: the real impedance at the end of a half-wave wire wanders between 2000 and 4000 Ω depending on height and counterpoise
• Core saturation at power is not checked — above 100 W use two stacked toroids
• Losses in the transformer itself are not accounted for: at 28 MHz they are noticeable and depend on the material
• Material 43 is good to 30 MHz; at kilowatt levels people more often use 52, or two cores

🎯 In practice

• A portable antenna for 40–10 m. One wire, one transformer, no tuner needed.
• An antenna for a summer place. Feeding from the end means the cable does not have to reach the middle of the wire.
• Checking a transformer you already have. Enter your core and your turns, and the calculator will tell you whether the inductance is enough.

Practical advice:

1. The counterpoise is not optional — 0.05 λ of wire, or at least a few metres. Without it the common-mode current comes into the shack along the braid.
2. A 100–150 pF capacitor across the primary flattens the SWR on 10 metres.
3. Wind the primary on a part of the toroid where it does not overlap the secondary; that keeps the stray capacitance down.
4. A choke on the cable is still worth having after the transformer: a transformer and a choke do different jobs.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator

UR3PKI. «EFHW Antenna Calculator and 49:1 Transformer Check». CyberDev.Space. https://cyberdev.space/en/radio/calculators/antennas/efhw/ (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.