Antenna Tuners for End-Fed and Longwire Antennas: Myths and Realities
Transmatches (often called antenna tuners) are frequently used to match end-fed or longwire antennas across multiple bands. While many boxes promise to “tune any wire,” there are physical and electrical realities you cannot ignore.
Myth: No Counterpoise Needed
In any RF system, current must return. In a center-fed dipole, the legs carry equal and opposite currents. With an end-fed/longwire, there’s one main conductor — the return must flow somewhere. If you don’t provide a counterpoise, the return will choose the coax shield, transmatch (tuner) case, mast, gutter, or shack wiring.
- Symptoms: common-mode current, RF-in-the-shack, hot mics, erratic SWR, RFI to nearby electronics.
- Risks: elevated chassis voltages, unpredictable patterns, and poor efficiency.
Rule: End-fed ≠ counterpoise-free. Provide a defined return path so your feedline doesn’t become the antenna.
Wire Lengths Are Not Arbitrary
Yes, a transmatch (tuner) can match wide impedances — but some lengths push it into extreme voltages/currents (|X| large, R very low or very high). Those lengths “match” on paper, but with heat and instability in practice.
| Band plan target | Avoid (typ.) | Often friendlier |
|---|---|---|
| 80–10 m coverage | Exact ½λ, 1λ, 2λ on any key band; very short <0.05λ | ~0.25–0.35λ or ~0.45–0.55λ w.r.t. lowest band of interest |
| 80/40 focus | ~20 m (near ½λ on 40 m at the tuner) | ~26–29 m longwire (well-known “plays nice” zone) |
| Multiband EF/longwire | Lengths that put voltage maxima directly at tuner terminals | Lengths that shift V-max away; keep R moderate and |X| manageable |
w.r.t. = “with respect to.” These are field-proven starting points, not absolutes. Local height and routing still matter.
SWR only indicates how well the transmatch makes the rig happy. It says nothing about radiation efficiency.
The real metric is ERP (Effective Radiated Power), the product of feedpoint power, efficiency, and antenna gain:
ERP = Pin × ηant × Gant
| Case | Feedpoint power | Efficiency | Gain (dBd) | ERP |
|---|---|---|---|---|
| Efficient dipole | 100 W | 90% | 0 dBd (1.0) | 90 W ERP |
| Vertical, poor radials | 100 W | 30% | +1.5 dBd (≈1.4) | ~42 W ERP |
| End-fed via long coax | 100 W | 50% (losses) | 0 dBd | 50 W ERP |
ERP is always w.r.t. a half-wave dipole. EIRP uses isotropic reference (add 2.15 dB).
Good Practices for End-Fed & Longwire Systems
- Provide a counterpoise: start with ~0.05–0.1 λ (lowest band). If space is tight, use multiple shorter wires. Place it away from the radiator to reduce coupling.
- Choke the feedline: put a high-CMR choke ~0.05 λ down the coax from the feedpoint (and another at shack entry if needed).
- Put the transmatch (tuner) at the feedpoint: matching here keeps coax near 50 Ω → minimal line loss. A shack-end tuner leaves high SWR on the line.
- Choose wire length wisely: avoid exact ½λ multiples and very short stubs; start around 26–29 m for broad HF coverage.
- Respect transformer limits: EF/longwire systems use 4:1, 9:1, 49:1 devices. These must handle voltage/heat; don’t assume “wideband” means efficient.
- Transformer (4:1, 9:1, 49:1): rough impedance translation at the feedpoint, must survive stress. Does not create a counterpoise.
- Transmatch (tuner): ensures rig sees ~50 Ω. Shack-end use still leaves high SWR on coax.
- Choke: stops return/common-mode currents turning your feedline into the counterpoise.
Marine Installations: Special Constraints
On boats, “ground” isn’t ground. The hull is isolated; saltwater is an excellent RF mirror only if you deliberately reference it. That means a defined counterpoise (copper foil, dynaplate, bonded rail with low-inductance straps), a transmatch at the base of the radiator, and aggressive choking of control/power lines. Poor marine installs invite CMC, autopilot glitches, and RFI to nav/AIS.
Reality Check: “Matched” ≠ “Efficient”
Transmatches make the PA happy. They don’t guarantee radiation efficiency. A “perfect 1:1” with the wrong wire length and no counterpoise often means heat in tuner and coax instead of ERP.
- 10 dB pad test: Insert attenuator. If spurs vanish, overload/CMC was the issue.
- Clamp meter sweep: Probe coax shield. High RF current = your feedline is the counterpoise. Fix with chokes/counterpoise.
- Feedpoint vs shack match: Compare. If reports improve with feedpoint transmatch, line loss was the culprit.
Summary
- End-feds/longwires need a counterpoise; otherwise the feedline and shack become the return path.
- Wire length matters. Avoid impedance extremes that stress the transmatch (tuner).
- Match at the feedpoint when possible; keep coax near 50 Ω.
- Choke early and often. Control return currents, then match.
A transmatch (tuner) is not a magic box. It’s one piece of the system. Define a return path, choose sane lengths, match in the right place, and choke the feedline. That’s how end-feds and longwires become effective antennas.
Mini-FAQ
- Do end-feds really need a counterpoise? — Yes. Without one, the feedline and shack become the return, causing CMC, RFI, and poor efficiency.
- Where should the transmatch (tuner) go? — At/near the feedpoint. Shack-end use leaves high SWR on coax → loss and CMC.
- What wire lengths should I avoid? — Exact ½λ multiples and very short <0.05 λ. Start with ~26–29 m for broad HF.
- Can a transformer fix everything? — No. It translates impedance but doesn’t supply a counterpoise or stop return currents.
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