Tuning Advice for an End-Fed Off-Center (EFOC) Antenna

An End-Fed Off-Center (EFOC) antenna is a versatile solution for multiband HF operations. Its design incorporates a feedpoint that is off-center from the antenna’s total length, paired with a 4:1 transformer to match impedance across multiple bands. While EFOC antennas are relatively forgiving, proper tuning is essential to maximize performance across all intended bands.

Understanding the EFOC Antenna Design

1. Feedpoint Position:

  • The feedpoint is located at approximately 30–40% of the total length from one end. This position ensures the antenna is resonant on harmonics of the fundamental frequency.
  • Unlike EFHW antennas, EFOC designs have less extreme impedance swings, making them easier to tune on multiple bands.

2. Transformer:

  • A 4:1 UNUN is commonly used to step down the high impedance at the feedpoint to a manageable level (typically 200–300 ohms to 50 ohms).

3. Length of the Radiator:

The antenna’s length is critical for resonance on desired bands:
  • EFOC8: ±8 meters, optimal for 20m–10m.
  • EFOC17: ±17 meters, optimal for 40m–10m.
  • EFOC29: ±29 meters, optimal for 80m–10m.

Steps to Tune Your EFOC Antenna

1. Initial Setup

  • Choose a clear location: Avoid nearby structures, metal objects, or trees that could detune the antenna.
  • Mounting height: Aim for 6–10 meters above ground for optimal performance on most bands. Lower heights may favor NVIS (Near-Vertical Incidence Skywave) operation on lower bands (e.g., 80m and 40m).

2. Measure SWR Across All Bands

  • Use an antenna analyzer or SWR meter to check the antenna’s standing wave ratio (SWR) across the desired bands.
  • Record the resonant frequency and SWR for each band.
  • As this is a near-resonant type of antenne ... do not focus on the dips, focus on the overall SWR near resonance on the desired bands !

3. Adjusting the Antenna Length

If the antenna is too long:
  • The resonant frequencies will appear below the desired bands. Shorten the wire in small increments (5–10 cm) and re-measure.
  • Alternatively, fold back the excess wire at the end of the antenna instead of cutting it. Secure the folded section with cable ties, tape, or other non-conductive materials. Read more: Folding Back vs. Cutting Wire Antennas: Essential Tips for Optimal Performance
  • Again  this is a near-resonant type of antenne ... do not focus to much on the dips, focus on the overall SWR near resonance on the desired bands !

If the antenna is too short:

  • The resonant frequencies will appear above the desired bands. Add wire (if feasible)

4. The Common-Mode Choke:

Place a common-mode choke (e.g., ferrite beads or a coaxial choke) on the feedline near the balun to minimize RF currents on the coax shield.
 Proper placement of the choke depends on the antenna type:
  • • For EFOC8: Place the choke approximately 2.2 meters from the feedpoint.
  • • For EFOC17: Place the choke approximately 3.7 meters from the feedpoint.
  • • For EFOC29: Place the choke approximately 12.2 meters from the feedpoint.
These distances help reduce common-mode currents by targeting current nodes along the feedline and ensuring efficient RF suppression.

5. The Common-Mode Choke Placement and Impedance

Closer to the Antenna:
    • Placing the choke closer to the feedpoint (antenna) typically raises the feedline impedance seen at the feedpoint, as it limits the length of the coax contributing to the antenna system.
    • This helps prevent the coax from acting as part of the radiating system but could increase the mismatch in some cases.
Farther from the Antenna:
    • Moving the choke farther away lowers the impedance seen at the feedpoint.
    • This happens because the longer portion of the coax between the antenna and choke contributes to the system, effectively behaving like a counterpoise.
    • This can improve the match, particularly for non- and near- resonant antennas, where a lower impedance feedpoint may be desired.

6. Feedline Length:

  • Avoid multiples of half-wavelengths at the operating frequencies to prevent feedline resonance.
  • This ensures the feedline does not interact with the antenna’s impedance, which could lead to RF hot spots or unwanted radiating feedlines.

7. Tuning Specific Bands

Low Bands (80m, 40m, 30m): (EFOC29/17)
  • These bands require the entire antenna length to radiate effectively.
  • Ensure sufficient height above ground to reduce ground losses, especially for 80m.
Mid Bands (20m, 17m, 15m): (EFOC8/17)
  • These are the most efficient bands for EFOC antennas due to their harmonic alignment with the radiator length.
  • If the SWR is slightly high, fine-tune the length or adjust the feedline choke.
High Bands (12m, 10m): (ALL EFOC)
  • These bands may show multiple lobes in the radiation pattern due to the antenna being several wavelengths long.
  • Performance is typically acceptable without adjustments, but SWR may rise slightly due to minor impedance mismatches.

8. Environmental Adjustments

  • Seasonal changes (humidity, vegetation growth) can slightly affect tuning, so periodic rechecks are recommended.
  • Proximity to Ground: Lower heights improve NVIS for lower bands but may increase ground losses for DX on higher bands.

9. Using an Antenna Tuner

  • While an antenna tuner is not strictly required, it can help fine-tune mismatches and improve performance across multiple bands.
  • Use a tuner capable of handling the expected power level and impedance range.

10. Common Issues and Solutions 

Ensure sufficient clearance between metallic objects (such as poles or masts) and your wire antenna. The end of the wire and its broad side should be at least 0.1λ away, with a recommended distance of 0.25λ (a quarter wavelength). For the 80m band, this translates to a clearance of 8–20 meters.

 

Issue Cause Solution
Low efficiency on 80-40M bands Insufficient height above ground Raise the antenna or improve the ground plane
Poor SWR performance on 10-15M bands Excessive length causes detuning Consider shortening the wire and use a tuner for the 80M band

 

Tuning an EFOC antenna involves balancing the radiator length, feedpoint matching, and environmental factors to achieve resonance and low SWR across multiple bands. With careful setup and periodic adjustments, an EFOC antenna can provide excellent performance for both DX and regional communications.