Feedlines, coaxial and balanced feedlines (ladderlines)

Antenna feedlines play a critical role in transmitting RF energy from your transmitter to the antenna and vice versa. Choosing the right feedline type—coaxial cable or balanced feedline (such as ladderline)—is essential for achieving efficient power transfer, minimizing losses, and handling impedance mismatches effectively.

1. Types of Feedlines

Coaxial Cable

Structure: Coaxial cable consists of a center conductor, an insulating dielectric, a braided shield, and an outer protective jacket.
Characteristics:
    • Unbalanced: The shield provides a reference ground.
    • Common Impedances: 50 ohms (e.g., RG-58, RG-213) or 75 ohms (e.g., RG-6 for TV applications).
    • Ease of Use: Simple to route and install; does not require spacers like ladderline.

Losses: Higher losses compared to balanced lines, especially at high frequencies or long runs.

Balanced Feedline (Ladderline)

Structure: Two parallel conductors separated by spacers, typically with an impedance of 300–600 ohms.

Characteristics:

    • Balanced: Equal and opposite currents flow through each conductor, reducing radiation from the feedline.
    • Low Loss: Extremely low loss, even at high frequencies or in the presence of high SWR.
    • Flexibility: Requires a tuner for impedance matching but is forgiving with mismatched loads.

2. Impedance and Matching

The feedline’s impedance should ideally match the transmitter’s output impedance (typically 50 ohms) and the antenna’s feedpoint impedance. Mismatched impedances result in reflected power, causing higher SWR and efficiency losses.

1/4-Wave Matching Stub
  • Purpose: Matches two impedances using a feedline that is one-quarter wavelength long at the operating frequency.
  • How It Works:
  • The stub acts as a transformer, converting the load impedance to match the feedline.
  • Works best for narrowband applications, as it’s frequency-sensitive.
  • Catch: Changes in frequency or feedline velocity factor affect matching accuracy. 
1/2-Wave Feedline
  • Purpose: Repeats the load impedance at the transmitter end of the feedline.
  • How It Works:
  • A feedline cut to exactly half a wavelength electrically will present the same impedance at both ends.
  • Useful for bridging mismatched impedances, but precise cutting is required.
  • Catch: Changes in frequency and feedline characteristics can cause misbehavior.

3. Benefits of Each Feedline Type

 

Feature Coaxial Cable Balanced Feedline (Ladderline)
Ease of Installation Simple to install, route, and terminate. Requires spacers and careful routing.
Losses Higher, especially at high SWR or frequency. Very low, even with high SWR.
Weather Resistance Generally good, with a protective jacket. Requires care to prevent moisture intrusion.
Flexibility with SWR Requires low SWR for efficiency. Performs well even with high SWR.
Tuning Requirements Minimal if impedance is matched. Requires a tuner for multiband operation. *
Common Uses HF/VHF/UHF; portable and mobile setups. HF long runs, multiband operation.

 *There are multiband antennas designed to utilize ladderline for enhanced efficiency while eliminating the need for a tuner (we plan to launch such antennas in 2025).

4. Benefits of Balanced Feedlines

1. Low Loss:
  • Ladderline has significantly lower losses compared to coaxial cable, especially at high frequencies or with high SWR. This makes it ideal for multiband antennas with mismatched impedances.
2. Wide Impedance Range:
  • Balanced feedlines can handle a wide range of antenna impedances without excessive losses, making them versatile for experimental or multiband antennas.
3. RF Radiation Suppression:
  • Properly balanced currents minimize RF radiation from the feedline, reducing interference and improving efficiency.

 5. Challenges of Balanced Feedlines

1. Routing and Installation:
  • Ladderline requires careful routing to avoid nearby conductive surfaces, which can cause detuning and losses.
2. Weather Vulnerability:
  • Exposed spacers can collect water or snow, affecting performance.
3. Tuning Dependency:
  • Most balanced feedline systems require an antenna tuner to handle mismatches effectively.

 6. Choosing the Right Feedline

 When to Use Coaxial Cable:
  • Ease of Use: Coax is ideal for situations where simplicity and ease of installation are priorities, such as portable operations or installations in confined spaces.
  • Short Runs: For short distances, the higher losses of coax are negligible.
When to Use Ladderline:
  • Low-Loss Applications: Ladderline is the best choice for long runs or high-power setups.
  • Multiband Antennas: For antennas with significant impedance swings, such as off-center-fed dipoles, ladder line offers unmatched efficiency.

 7. Tips for Optimizing Feedline Performance

1. Length Considerations:
  • Avoid cutting the feedline to a length that is a multiple of half-wavelengths to prevent resonance issues.
2. Chokes and Baluns:
  • Use a current balun to transition from balanced feedline to coaxial cable when needed.
3. Protect Connections:
  • Use weatherproofing materials to seal connectors and junctions from the elements. 

Conclusion

Understanding the differences between coaxial and balanced feedlines is essential for designing an efficient antenna system. While coaxial cable offers simplicity and convenience, ladderline provides unmatched performance in terms of loss and impedance handling. Choosing the right feedline depends on your operating needs, installation environment, and performance goals. With careful planning and installation, either option can serve as a reliable conduit for your RF energy.