Yes, feedline length can significantly impact multiband antenna performance, particularly in terms of impedance transformation, common-mode currents, and overall system efficiency. Here’s a detailed explanation of why feedline length matters and how it affects your setup:

1. Impedance Transformation

a. Standing Wave Effects

In multiband antennas, the antenna is rarely perfectly matched to the feedline, resulting in standing waves on the feedline. These standing waves cause the impedance at the transmitter end to vary depending on the feedline’s electrical length relative to the wavelength of the operating frequency.

• The impedance presented to the transmitter changes based on where along the feedline the measurement is made, relative to the signal’s wavelength.

• This variation becomes more pronounced in multiband antennas operating across a wide range of frequencies.

b. Quarter-Wave Transformation

Feedlines with lengths that are odd multiples of a quarter-wavelength (λ/4) act as impedance transformers. For instance:

• A low impedance at the antenna feedpoint is transformed into a high impedance at the transmitter, and vice versa.

• This transformation can complicate impedance matching, leading to higher SWR on certain bands.

c. Mitigation Strategies

To avoid problematic impedance transformations:

1. Avoid Odd Multiples of λ/4: Choose feedline lengths that are not odd multiples of a quarter-wavelength at the lowest operating frequency.

2. Calculate Quarter-Wave Lengths: Use the formula below to calculate critical lengths:

Example for the 80m Band (3.5 MHz):

Using a velocity factor of 0.86 for the coax:

75 × 0.86 / 3.5 = 18.4 meters

For practical purposes, round to 18 meters.

3. Safe Feedline Lengths: Avoid critical odd multiples of λ/4 by using even multiples of the quarter-wavelength. For the 80m band:

• Safe lengths: 18m, 36m, 72m, etc.

• Avoid problematic lengths: 54m, 90m, etc.

d. Special Note for End-Fed Antennas

For end-fed antennas, measure the feedline length starting at the point where the RF choke is installed, as this is the effective feedpoint of the antenna. If the coaxial shield acts as a counterpoise, the length becomes critical for mitigating common-mode currents.

2. Common-Mode Currents

a. Imbalance and Its Impact

Multiband antennas—such as end-fed half-wave (EFHW), end-fed off-center (EFOC), off-center-fed dipoles (OCFD), and long-wire antennas—often create imbalances at the feedpoint. This imbalance induces common-mode currents on the coaxial shield.

• Feedline lengths near resonant multiples (e.g., λ, λ/2) can exacerbate common-mode current issues, causing:

• Unwanted feedline radiation.

• Increased noise pickup, degrading reception performance.

b. Choke Placement

A common-mode choke can suppress these currents and improve system performance. Place the choke:

• Near the antenna feedpoint to address the source of the imbalance.

• Midway along longer feedlines to suppress residual currents.

(For more details, read about common-mode chokes.)

3. Multiband Considerations

a. Variable Impedance Across Bands

Multiband antennas naturally exhibit different feedpoint impedances on each band. The feedline length determines how these impedances are transformed and presented to the transmitter.

b. Feedline Loss

High SWR, common in multiband antennas, increases feedline loss, particularly at higher frequencies or with longer feedlines. To mitigate losses:

• Use low-loss coaxial cable, such as EXTRAFLEX-BURY-7.

• Minimize feedline length where possible.

c. Optimizing Feedline Length

Adjusting the feedline length can:

• Shift high-SWR points to frequencies where losses are less critical.

• Reduce the impact of impedance mismatches on key operating bands.

4. Resonance and Interaction with the Antenna

a. Ladder Line and Open-Wire Feedlines

Balanced feedlines, such as ladder line, interact directly with the antenna impedance. Their length is critical as it affects how the tuner matches the system across multiple bands.

b. Coaxial Cable

Coaxial feedlines are less sensitive to length variations compared to ladder lines. However, impedance transformation effects still occur and should be considered, especially in multiband setups.

5. Practical Tips for Managing Feedline Length

• Avoid Critical Lengths: Avoid feedline lengths that are odd multiples of λ/4 at the lowest operating frequency.

• Use a Tuner: A tuner compensates for impedance variations caused by feedline length, ensuring a better match.

• Experiment with Lengths: Small adjustments to feedline length can optimize SWR and overall performance.

• Add Chokes: Use common-mode chokes to suppress unwanted currents at strategic points along the feedline.

• Use High-Quality Coax: For longer feedline runs, opt for low-loss coaxial cables to reduce power losses, especially on higher bands.

6. When Feedline Length Matters Most

Feedline length is particularly important for:

• End-Fed Antennas: The feedline often acts as part of the counterpoise system.

• Off-Center-Fed Antennas: Feedline length significantly impacts impedance transformation.

• Multiband Verticals (e.g., Rybakoff Antennas): Feedline length influences the feedpoint impedance.

• Non-Resonant Antennas: Standing wave effects on the feedline can complicate the tuning process.

Conclusion

Feedline length plays a critical role in the performance of multiband antennas. While exact lengths are not always necessary, avoiding problematic lengths and using tools like tuners and common-mode chokes can ensure efficient operation. For best results, experiment with feedline lengths and monitor SWR and system performance.

If you need help calculating the optimal feedline length for your setup, contact us! We’re happy to assist.

This enhanced version clarifies key points, emphasizes practical steps, and ensures consistency throughout. Let me know if you’d like further adjustments!