Does feedline length matter?

Yes, feedline length can matter with multiband antennas, especially in terms of impedance transformation, common-mode currents, and overall system performance. Here’s a detailed explanation of why and how feedline length impacts multiband antennas:

1. Impedance Transformation

Standing Wave Effects:
  • If the antenna is not perfectly matched to the feedline (which is common in multiband antennas), standing waves are created on the feedline. These standing waves cause the impedance at the transmitter end of the feedline to vary depending on the feedline’s electrical length.
  • The impedance seen at the radio depends on how far along the feedline’s length the measurement is made, relative to the wavelength at the operating frequency.

Quarter-Wave Transformation:

  • Feedlines with lengths that are odd multiples of (quarter-wavelength) transform impedances:
    • Low impedance at the antenna becomes high at the transmitter, and vice versa.
    • This can complicate impedance matching for certain bands.

Mitigation:

  • Choose feedline lengths that avoid critical transformation points to prevent extreme impedance levels, ensuring they are not odd multiples of certain quarter wavelengths.

    To avoid an odd multiple, use the fundamental quarter-wavelength of the lowest frequency (longest wavelength) your multiband antenna operates on.

    Formula

    Quarter-wave length:

            (75 × Velocity Factor) / Frequency (MHz)

    Example for the 80m Band

    Using a velocity factor of 0.86:

            75 × 0.86 / 3.5 = 18.4 meters

    Safe Lengths

    Each even multiple of this length will avoid all odd multiples and is safe to use. For practical purposes, we round 18.4m to 18m. Safe lengths are approximately:

    • 18 meters
    • 36 meters
    • 72 meters
    • ...and so on (avoid 54, 90 etc ...)

    Special Note for End-Fed Antennas

    For end-fed antennas, measure the length starting from the point where the RF choke is installed, as this serves as the true feedpoint of your antenna. (if the cointerpoise is part of the coaxial feedline)

2. Common-Mode Currents

Imbalance Impact:

  • Multiband antennas (e.g., all type of end-feds, off-center-fed dipoles) often create imbalances that induce common-mode currents on the coaxial shield. The feedline length influences these currents.
  • A feedline length close to a resonant multiple (e.g., , ) can exacerbate common-mode current issues, leading to feedline radiation and noise pickup.

Choke Placement:

3. Multiband Considerations

Variable Impedance Across Bands:

  • Multiband antennas, such as end-fed half-wave (EFHW), end-fed off-center (EFOC), end-fed long-wire (EFLW),  off-center-fed dipoles (OCFD) and Multiband Verticals (Rybakoff) naturally have different feedpoint impedances on different bands. The feedline length can influence how these impedances are presented to the transmitter.

Feedline Loss:

  • High SWR on some bands (common with multiband antennas) increases feedline loss, especially for longer cables and at higher frequencies. Minimizing feedline length or using low-loss coaxial cable (e.g., EXTRAFLEX-BURY-7) can reduce these losses.

Optimal Length:

  • In some cases, adjusting the feedline length can “shift” high-SWR points to frequencies where losses are less impactful, improving overall efficiency.

    Feedline lengths that are not exact odd multiples of Lambda/4 will transform impedance differently. By changing the length, you can:
    • Shift the high-SWR points away from the primary operating frequencies.
    • Reduce the impact of mismatch on those frequencies.

4. Resonance and Interaction with Antenna

Ladder Line and Open Wire Feedlines: 

  • If a balanced feedline (e.g., ladder line) is used, its length often matters more, as it interacts with the antenna impedance and affects the tuner’s ability to match.

Coaxial Cable:

  • Coaxial feedline is less sensitive to length variations but still subject to impedance transformation effects, especially with multiband antennas.

5. Practical Tips for Managing Feedline Length

Avoid Critical Lengths:

  • Avoid feedline lengths close to odd multiples of Lambda/4 at the lowest operating frequency, as these can cause extreme impedance transformations.

Use a Tuner:

  • For multiband antennas, a tuner can compensate for impedance variations caused by feedline length.

Experiment for Best Results:

  • Adjust the feedline length in small increments to find a configuration that provides acceptable SWR across the desired bands.

Choke Placement:

Use Quality Coax:

  • For longer runs, use low-loss coax to minimize power losses, especially on higher bands.

6. When Feedline Length Matters Most

Feedline length is particularly important for:

  • End-fed antennas, where the feedline often acts as part of the counterpoise system.
  • Off-center-fed antennas, where feedline length impacts the feedpoint impedance transformation.
  • Multiband Verticals, like the Rybakoff  where feedline length impacts the feedpoint impedance transformation.
  • Non-resonant antennas, where standing wave effects on the feedline can introduce complications.

Conclusion

Feedline length can have a significant impact on multiband antenna performance. While it’s not always critical to have an exact length, avoiding problematic lengths and using tools like antenna tuners and common-mode chokes can help ensure efficient operation. For best results, experiment with feedline length while monitoring SWR and system performance. 

Contact us if you’d like help calculating optimal feedline lengths for your setup!