How to design an EFHW for 80m to 6m

| May 25, 2026 | Amateur Radio (Ham Radio) | No Comments

Designing an End-Fed Half-Wave (EFHW) antenna for 80m through 6m is an excellent homebrew project. Because a half-wavelength wire on 80m is roughly a full wavelength on 40m, one and a half wavelengths on 30m, and so on, it naturally resonates on the harmonic bands. However, the impedance at the end of a half-wave wire is extremely high—typically between 2000 and 3000Ω. To bring this down to the 50Ω expected by your transceiver, you will need a broadband matching transformer. Getting this setup to perform well all the way up to 6m requires careful attention to the transformer’s stray capacitance.

Here is the step-by-step design process.

1. The 49:1 Impedance Transformer (Unun)

The heart of the EFHW is the transformer. A 49:1 ratio transforms a 2450Ω impedance down to 50Ω.

If you are pushing this with a 100W, a single FT240-43 ferrite toroid is the standard choice. It handles 100W SSB/CW comfortably without saturating.

  • Winding Ratio: You need a 1:7 turns ratio. The impedance transformation is the square of the turns ratio: 72 = 49.
  • Turns: Use 2 primary turns and 14 secondary turns. You can use 1.2mm (18 AWG) enameled copper wire.
  • The Crossover: Wind 7 turns, cross the wire over the toroid to the opposite side, and wind the remaining 7 turns. This reduces stray capacitance, which is critical for making the antenna usable on 10m and 6m.

2. The High-Voltage Compensation Capacitor

To flatten the SWR curve on the higher frequency bands (15m, 10m, and especially 6m), you must add a capacitor across the primary winding (in parallel with the coaxial connector).

  • Value: Typically 100 pF to 150 pF.
  • Type: It must be a high-voltage ceramic or silver mica capacitor, rated for at least 3 kV. Even at 100W, the RF voltages at the feedpoint are high enough to fry standard capacitors.

3. The Radiating Wire

The fundamental length of a half-wave wire is calculated using the standard formula, where f is your target frequency in MHz:

L (meters) = 142.5/f

L (feet) = 468/f

For the lower end of the 80m band, the theoretical length is roughly 40.1 meters (131.8 feet).

Cut the wire about 1 meter longer than calculated. You will fold the end back on itself to tune the antenna for the lowest SWR on 80m. Because the higher bands are harmonics, tuning the base frequency generally brings the rest of the bands into alignment.

4. Counterpoise and Common Mode Choke

Even though it is “end-fed,” the antenna still requires a return path for the RF currents.

  • Counterpoise: The shield of your coaxial cable naturally acts as a counterpoise.
  • Choking: To prevent common-mode RF current from traveling all the way back into your shack and causing interference, install a 1:1 common mode current choke on the coax.
  • Placement: Place the choke approximately 0.05 wavelengths (roughly 4 meters) away from the transformer. This allows that short section of coax shield to act as the counterpoise while blocking RF from coming further down the line.

5. Managing the 80m/40m Harmonic Shift

You may find that tuning the wire perfectly for 80m makes the 40m, 20m, and 10m bands resonate slightly too high in frequency. This is due to the “end effect” of the wire.

To fix this, you can insert a small compensation coil (about 110 μH) roughly 2 to 3 meters from the transformer end of the wire. This coil electrically lengthens the wire for 80m but acts as an RF choke for the higher frequencies, allowing you to tune 80m independently of the harmonic bands.

Here is a straightforward, practical guide to building a highly effective 1:1 common-mode choke capable of handling 100W across the 80m to 6m bands.

1. The Core Material

For a broadband HF choke covering 80m to 6m, a single FT240-43 (2.4-inch diameter, Mix 43 ferrite) is an excellent choice. It provides a good balance of high choking impedance across those specific bands without saturating or overheating under 100W of power.

2. The Coaxial Cable

You will wind the actual coaxial cable around the toroid.

  • RG-58: The standard, affordable choice. It handles the power easily, though it can be somewhat stiff to wind tightly around the core.
  • RG-316 or RG-400 (Recommended): These are Teflon-dielectric coax cables. They are much thinner, highly flexible, handle heat exceptionally well, and allow for a very neat, tight wind on the toroid.

3. Winding the Choke (The W1JR Method)

To maintain good choking performance on the higher bands (10m and 6m), you must minimize stray capacitance between the input and output ends of the coax. The “W1JR crossover” method is the standard way to achieve this.

  • Turns: You need 12 to 14 turns in total.
  • First Half: Pass the coax through the core and wind 6 or 7 turns, keeping them snug and adjacent to one another.
  • The Crossover: Take the cable directly across the open diameter of the toroid to the opposite side.
  • Second Half: Wind the remaining 6 or 7 turns, ensuring you continue winding in the same direction through the core.
  • Result: Your input and output coax connectors will end up on opposite sides of the toroid, keeping the “hot” and “cold” ends physically separated.

4. Housing and Placement

  • Enclosure: Mount the finished toroid in a weatherproof PVC or ABS junction box. Solder the ends of the coax to SO-239 chassis connectors on either end of the box.
  • Location: For an EFHW, do not place the choke directly at the 49:1 transformer. The system needs a short length of the coax shield to act as a counterpoise to complete the circuit. Place the choke about 3 to 4 meters down the coax line from the transformer. Everything past the choke (heading toward your radio) will be shielded from common-mode current.