Controlled Feeder Radiation: The Multi-Band HF Antenna with NO Traps (ZS6BKW style) – Has anyone tried this?
That is a fascinating topic, especially for those looking to avoid the losses and narrow bandwidth associated with traditional traps. Controlled feeder radiation usually points toward a G5RV or a ZS6BKW style antenna. Instead of using traps to “cut” the antenna to length electrically, these designs use a specific length of balanced feeder (ladder line) as a matching transformer. This allows the antenna to be resonant or easily matched on multiple bands while allowing the feedline itself to contribute—in a controlled manner—to the radiation pattern. I am using a homebrew ladder line with my 40m loop for multiple bands. The ZS6BKW style seems to be quite interesting, looking forward to your feedback before trying it out.
The Theory: Impedance Transformation
The “magic” happens in the matching section. On some bands, the antenna is a half-wave dipole; on others, it’s multiple half-waves. This creates wildly different impedances at the feedpoint.
By using a specific length of 450Ω or 300Ω ladder line, the high or low impedance at the antenna is transformed to something close to 50Ω at the end of the matching section.
- Non-Resonant Radiator: The wire elements are typically not a resonant length for any single band.
- Standing Waves: We want standing waves on the ladder line because it is extremely low-loss compared to coax.
- The 1:1 Current Balun: Vital at the transition from ladder line to coax to prevent RF from traveling down the outside of your coax and into the shack.
The Build: The ZS6BKW (Optimized G5RV)
The ZS6BKW is widely considered the superior “trap-free” multi-bander because it provides a near-perfect 50Ω match on 40m, 20m, 17m, 12m, and 10m without a tuner.
Dimensions
| Part | Length (Side 1) | Length (Side 2) |
|---|---|---|
| Top Radiator | 13.75 meters | 13.75 meters |
| Matching Section (450 Ω) | 12.2 meters |
Construction Steps
- The Center Insulator: Use a robust T-piece. Connect the two halves to the two conductors of the ladder line.
- The Ladder Line: Let this hang vertically. If it must turn, keep it at a 90-degree angle from the radiator and away from metal objects (gutters, towers).
- The Transition: At the bottom of the 12.2m ladder line, install a 1:1 Current Balun.
- Coax Run: From the balun, you can run high-quality 50Ω coax (like RG-213 or LMR-400) to your rig.
Why “Controlled” Radiation?
In a standard dipole, we want the feeder to be “cold” (no RF). In these designs, the ladder line acts as a linear transformer. Because the system is balanced, the radiation from the two closely spaced wires of the ladder line cancels out mostly, but not entirely. This subtle radiation can sometimes “fill in” the nulls in the antenna’s radiation pattern, making it a more effective all-rounder for DX and local contacts.
Tuning and Troubleshooting
- Height Matters: This antenna performs best when the center is at least 10–12 meters high.
- SWR Check: If your SWR is high on 20m, the ladder line might be reacting to nearby metal. Use plastic spacers to keep it away from the mast.
- The “Short” 80m: A ZS6BKW is too short for 80m resonance, but with a high-quality internal or external tuner, you can often force it to tune, though efficiency will drop.