When designing an efficient multi-band antenna system, many radio amateurs consider the choice between 600-ohm open-wire and 450-ohm ladder line. Additionally, the choice of tuner—whether a balanced Z-match tuner or an asymmetrical tuner with a balun—can greatly impact overall efficiency. This article explores why a 600-ohm doublet paired with a Z-match tuner outperforms a 450-ohm system with an asymmetric tuner, particularly in terms of loss reduction and power transfer.

1. Lower Losses in the Transmission Line

Higher Impedance = Lower Current = Reduced I²R Losses

Power loss in a transmission line occurs due to conductor resistance and dielectric losses. Since power (P) is related to current (I) by P= I²R, a higher-impedance line naturally reduces current, minimizing resistive losses.

A 600-ohm feeder:

• Carries less current for the same power level compared to a 450-ohm feeder.
• Has lower I²R losses, making it more efficient over long runs.

Dielectric Loss Considerations

• 600-ohm open-wire typically has a wider conductor spacing and is mostly surrounded by air, which is nearly lossless.
• 450-ohm ladder line, often made with polyethylene or PVC spacers, suffers from slightly higher dielectric losses, especially when wet or dirty.

The combination of lower resistivity and dielectric losses makes the 600-ohm feeder the superior choice in terms of efficiency.

2. Tuner Efficiency: Z-Match vs. Asymmetrical Tuner

Z-Match Tuners Provide a Direct, Low-Loss Match

• A Z-match tuner is a balanced design, eliminating the need for a balun.
• It directly couples to a balanced antenna system, avoiding additional insertion loss.

Asymmetric Tuners Introduce Additional Losses

• Many asymmetrical tuners require a 4:1 balun to interface with balanced feedlines.
• These baluns introduce core losses, which increase as SWR rises.
• High-current and high-voltage conditions can lead to core heating and saturation, further reducing efficiency.

Since Z-match tuners avoid these losses entirely, they provide a much more efficient tuning solution for balanced doublets.

3. Lower Mismatch Losses

A 600Ω Line Matches High-Z Antenna Conditions Better

• A doublet antenna, particularly when used on harmonics, often presents a high impedance (2000–5000Ω).
• A 600Ω feeder provides a better impedance transformation compared to a 450Ω feeder, reducing mismatch losses and improving overall power transfer.

With a 450Ω line, the mismatch at higher frequencies can result in higher transmission line loss, reducing efficiency. A 600Ω line handles these conditions more effectively.

4. Higher SWR Handling Capability

Open-Wire Feeders are Designed for High SWR Operation

• Both 450Ω and 600Ω open-wire lines are capable of handling high SWR.
• However, 450Ω ladder line experiences slightly more loss when SWR is high due to its construction (closer conductor spacing and additional dielectric).
• A 600Ω open-wire line maintains efficiency better, especially on high-impedance bands.

Conclusion: The 600Ω + Z-Match Advantage

• 600Ω line reduces resistive and dielectric losses
• Z-match tuners are more efficient than asymmetric tuners with baluns
• Better impedance matching improves power transfer
• Handles high SWR with minimal efficiency loss

For radio amateurs seeking maximum efficiency, a 600-ohm open-wire fed doublet with a Z-match tuner provides superior performance over a 450-ohm system with an asymmetric tuner. Whether you’re operating QRP or high power, this setup ensures that more of your RF energy reaches the antenna rather than being lost in transmission and tuning components.

Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.