The EH Antenna — Physics, Measurements, and Why It’s Inefficient
This is a technical deep dive. No roast, just Maxwell with a flashlight.
Claim vs. Reality
The claim: An EH antenna “synthesizes” in-phase E and H fields in a compact structure, so a ~1–2% λ device radiates like a full-size antenna, with little or no ground/counterpoise, and very high efficiency.
The reality: In time-varying fields, current over distance creates radiation. A physically small radiator has very low radiation resistance Rrad, large reactive energy storage, and requires heavy loading/matching. EH implementations behave like short, heavily loaded antennas with tuning capacitors and sometimes coils. A good 50 Ω match does not mean good efficiency.
What the EH Antenna Really Is (Equivalent Circuit)
- A very short conductor pair (or sleeve structure) → tiny Rrad (<< 1 Ω at HF for 1–2% λ height).
- Series/parallel capacitance to resonate out large reactance X.
- Sometimes a loading coil to achieve resonance.
- A matching network (often an L-match) to present ~50 Ω to the rig.
The box at the feedpoint is not a free-energy engine; it’s a matching/tuning network. It can hide the physics from your SWR meter, but not from field strength.
For an electrically short monopole of height h over a ground reference (h ≪ λ):
Rrad ≈ 160π² (h/λ)² Ω. As h/λ → 0.02, Rrad ~ 160π²·(0.02)² ≈ 0.63 Ω.
Any series loss (conductor, ESR of capacitors, coil loss, ground return) on the order of a few ohms will dominate, crushing efficiency.
Efficiency: Numbers That Don’t Care About Marketing
Consider an EH radiator ~2 m tall at 7 MHz (λ≈43 m → h/λ≈0.046): Rrad on the order of 1 Ω or lower. If the network and structure introduce a conservative 2–3 Ω of loss (capacitor ESR, coil copper/core loss, conductor resistance, ground/return loss):
η ≈ Rrad / (Rrad + Rloss) → 1 / (1+3) ≈ 25% (optimistic). Real-world builds often measure 5–15%, especially when the environment provides the “counterpoise” through lossy paths.
Compare to a λ/4 vertical on 7 MHz with a usable radial field: Rrad ≈ 36 Ω, modest loss a few ohms → η >90% is achievable.
Bandwidth and Q Tell the Tale
High efficiency antennas of the same size generally have broader fractional bandwidth than lossy resonators. EH setups often show very narrow bandwidth (e.g., a few kHz at HF), signaling high stored energy with significant resistive loss in the tuning network.
“E and H in Phase” — Why the Slogan Misleads
- In the radiating (far) field of any time-varying solution to Maxwell’s equations, E and H are inherently related and in phase for a plane wave. You don’t “dial” them independently at will in a tiny structure.
- Close to a tiny radiator, fields are predominantly reactive. Tuning can reshape currents but cannot create far-field radiation without adequate current over distance.
- The EH promise effectively rebrands small-antenna loading as “field synthesis.” The physics is unchanged.
Validation: Do-It-Yourself Experiments
- Field Strength A/B: Place the EH and a λ/4 vertical (with radials) equidistant from a measurement point. Normalize power. Expect the EH to be typically 10–20 dB down.
- Clamp-On Current: Measure RF current in the structure and along the feedline a few meters from the feedpoint. Significant shield current indicates the environment becomes the return path.
- Thermal Check: Run moderate power on a steady carrier. Heatsinking/ESR in the caps or matching components warms quickly → loss detected.
- Choke Test: Install a ≥5–10 kΩ 1:1 choke at the feedpoint. If tuning/”performance” collapses or changes drastically, feedline/common-mode was part of the radiator.
Why Some QSOs Still Happen
Any conductor with a match radiates something. With favorable propagation and enough power, contacts will occur. That’s not evidence of high efficiency—only that some fraction of your RF leaked into space.
Mitigations If You’re Stuck With One
- Add a purposeful return path (counterpoise on the target band, 0.05–0.1 λ).
- Use a serious common-mode choke at the feedpoint; sometimes add a second 0.05–0.15 λ down the line.
- Expect narrow bandwidth and re-tune frequently; keep component ESR low and coil Q high to minimize loss.
- Keep expectations realistic: EH is a compromise small antenna, not a substitute for full-size radiators.
Better Compact Alternatives
- Magnetic loop (small transmitting loop): Very compact with respectable efficiency when well-built (high-Q capacitor, low-loss conductor), albeit with narrow bandwidth.
- Loaded vertical with proper radials: Short but honest; performance scales with the quality/number of radials and loading strategy.
- EFHW/OCFD done right: When size allows, these deliver predictable patterns and efficiency—with proper choking and return paths.
Bottom Line
The EH antenna is not revolutionary physics—it is a small, loaded radiator with a matching network. A pretty SWR doesn’t equal efficiency. If you want repeatable performance, give RF a defined radiator and a defined return path; the far field will reward you.
Mini-FAQ
- Does an EH antenna really “synthesize” E and H? — No. E and H in radiating fields are coupled by Maxwell’s equations; tuning a tiny structure doesn’t bypass small-antenna limits.
- Why is the bandwidth so narrow? — High stored reactive energy and losses in the tuning network result in high Q and very small usable bandwidth.
- Can I improve it? — Provide a proper counterpoise, add robust common-mode chokes, minimize ESR/coil loss, and accept it’s still a compromise.
- What should I build instead in tight spaces? — A well-executed magnetic loop or a loaded vertical with real radials typically outperforms EH concepts.
Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes. Subscribe here.
Questions or experiences to share? Contact RF.Guru — we read everything.