Feedlines, coaxial and balanced feedlines (ladderlines)
Feedlines: Coaxial vs Balanced (Ladderline)
Antenna feedlines are the RF “transmission highway” between station and antenna. Choosing the right medium — coaxial cable or balanced feedline (ladderline/open wire) — directly affects loss, matching, and noise immunity. Below we compare structures, losses, matching behavior, and when to use each.
1) Types of Feedlines
Coaxial cable (unbalanced)
- Structure: Center conductor, dielectric, shield (braid/foil), outer jacket.
- Impedances: 50 Ω (HF/VHF gear), 75 Ω (TV/CATV, some RX).
- Pros: Easy to route, weather-jacketed, inherently shielded from E-field pickup.
- Cons: Loss rises with frequency and especially with SWR; braid can carry common-mode if not choked.
Balanced feedline (ladderline/open-wire)
- Structure: Two parallel conductors with spacers (typ. 300–600 Ω).
- Pros: Extremely low loss, even with high SWR; handles wide impedance swings — ideal for multiband doublets.
- Cons: Needs clear routing (away from metal), weather care at transitions, and a balanced or isolated match at the shack.
2) Impedance & Matching Essentials
For maximum power transfer the system wants matched impedances (Tx ≈ line ≈ antenna). In practice, multiband antennas aren’t matched on every band — your line must tolerate mismatch without burning watts.
- Coax: Low loss when SWR ≈ 1:1; with SWR and long runs, attenuation and heating increase quickly.
- Ladderline: Loss remains very low even at high SWR; tuner in the shack (or at the transition) handles the match.
Quarter-wave stubs & half-wave repeats (narrowband tools)
- ¼λ matching stub: Transforms impedances at a single frequency. Narrowband; velocity factor and frequency shifts detune it.
- ½λ line: Repeats load impedance at the input. Also narrowband; practical mostly for fixed-frequency systems.
3) Coax vs Ladderline — Practical Comparison
| Feature | Coaxial | Balanced (ladderline) |
|---|---|---|
| Ease of install | Very easy; route anywhere | Needs spacers/stand-offs; mindful routing |
| Loss at low SWR | Low–moderate, depends on type/run | Very low |
| Loss at high SWR | Rises quickly on long runs | Remains low |
| Noise immunity | Good if common-mode choked | Excellent when balanced & isolated |
| Weathering | Jacketed; robust | Excellent if kept clear; care at transitions |
| Tuner need | None if antenna matched | Typically required for multiband |
| Best use | Resonant antennas; short/medium runs | Multiband doublets; long runs; QRO |
4) Balanced Feedline: Benefits & Caveats
- Ultra-low loss with SWR: Perfect for multiband doublets where impedance swings wildly band-to-band.
- Wide Z handling: 100–>2 kΩ loads can be transported with minimal loss to the tuner.
- Balanced currents: Proper symmetry minimizes feedline radiation and RFI.
Don’t “shield” ladderline. Adding a conductive tube/casing creates imbalance, capacitive coupling, and loss — defeating the advantage.
- Routing & installation: Keep ≥5–10 cm from metal, gutters, siding, and tower legs; cross at right angles if needed.
- Weather: Use quality spacers; drain holes in transition boxes; keep transition points sealed and strain-relieved.
- Tuner interface: Prefer a balanced tuner or an asymmetrical tuner + high-power 1:1 current balun/isolator at the tuner output.
5) Choosing the Right Feedline
Use coax when…
- Running resonant antennas with low SWR on each band.
- Install needs simple routing, tight spaces, or portable ops.
- Feedline lengths are short so loss remains negligible.
Use ladderline when…
- You want multiband capability with one antenna (doublet) and long runs to the shack.
- You operate QRO and want minimal heating and loss at mismatch.
6) Tips to Optimize Any Feedline
- Length choices: Avoid exact multiples of ½λ (electrical) when mismatch exists — it can move a nasty impedance right to the tuner. Likewise, some ¼λ lengths can help or hurt depending on the system; model or test.
- Chokes & baluns: On coax, always add a common-mode choke at the antenna and at the shack entry. For ladderline transitions, use a robust 1:1 current balun at the tuner panel.
- Protect junctions: Seal outdoor transition boxes and connectors (cold-shrink or proper tape stack) and add a drip loop.
Conclusion
Coax offers convenience and good performance with resonant antennas and short runs. Balanced feedlines offer ultra-low loss and mismatch tolerance, making them the best choice for long-run, multiband systems. With correct routing, isolation, and tuner interfacing, either option can deliver a quiet, efficient station.
Mini-FAQ
- Can I run ladderline into the shack directly? — Yes, if you have a balanced tuner and maintain clearance from metal; otherwise use a 1:1 current balun at the entry panel.
- Why is my coax hot with RF? — You need a common-mode choke at the antenna and often at the shack entry to stop outer-shield currents.
- Does SWR always mean loss? — On coax, high SWR increases loss; on ladderline, loss remains low even with SWR — the tuner handles the match.
- Should I shield ladderline? — No. Shielding unbalances and increases loss; keep it clear of metal and use standoffs.
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