Open vs Closed Antennas — Resonance vs Traveling Wave
Updated August 20, 2025 — Technical article for advanced amateurs
Open vs Closed Antennas — Resonance, Loops, and Traveling Waves
Antennas can be grouped by how they handle energy at their boundaries. Some rely on reflections and standing waves, others circulate energy in a closed resonant path, and a third class absorbs energy in a termination to support a traveling wave. For the technically curious ham, these distinctions explain why some antennas excel at efficiency while others trade efficiency for bandwidth and pattern stability.
1. Open Resonant Antennas (Reflective)
These are “open-ended” structures where the RF current is reflected at the element ends, creating a standing wave. Energy oscillates back and forth, establishing sinusoidal current maxima and minima along the wire. Their efficiency and gain come from constructive interference of these standing-wave currents.
- Examples: Half-wave dipole, inverted V, 1/4λ vertical, Yagi-Uda
- Physics: Reflections at the ends enforce boundary conditions (current ≈ 0 at open ends, voltage maximum)
- Traits: High efficiency, narrowband, resonant impedance dips
2. Closed Resonant Antennas (Loops)
Unlike open-ended dipoles, loops form a continuous closed path for current. A full-wave loop supports a circulating standing wave around its perimeter. Energy is not absorbed in a load — it is entirely re-radiated. The result is a resonant, efficient antenna with different polarization and pattern behavior than linear dipoles.
- Examples: Delta loop, quad loop, full-wave horizontal loop
- Physics: Boundary condition is continuity of current around the loop; fields are contained and radiate in predictable lobes
- Traits: High efficiency, often lower noise pickup, broader pattern control (e.g., vertical vs horizontal polarization)
Note: While loops are “closed,” they are not traveling-wave terminated designs — they are resonant systems, typically as efficient as or more efficient than a dipole of similar size.
3. Closed Terminated Antennas (True Traveling Wave)
In this family, the antenna wire is terminated with a resistor or load. Energy launched at the feedpoint propagates one way, is not reflected, and is instead absorbed at the far end. This prevents standing waves and supports a unidirectional traveling wave along the wire. The price is reduced efficiency since some RF is lost in the termination, but the benefits are exceptional bandwidth and stable radiation patterns.
- Examples: Beverage, terminated folded dipole (T2FD), rhombic with load
- Physics: Traveling wave absorbed in termination; little to no back-reflection
- Traits: Wideband impedance, stable patterns across frequency, lower radiation efficiency
Comparing the Three Families
| Aspect | Open Resonant | Closed Resonant (Loops) | Closed Terminated (Traveling Wave) |
|---|---|---|---|
| Energy behavior | Reflected, standing waves | Circulating standing wave in closed path | One-way traveling wave absorbed in load |
| Bandwidth | Narrow, high Q | Moderate, often wider than dipole | Very broad |
| Efficiency | High near resonance | High, comparable or superior to dipoles | Lower (termination loss) |
| Pattern stability | Varies with frequency | Stable with smoother lobes | Very stable across wide ranges |
| Best use case | Monoband TX/DX | Efficient multi-band TX/RX with low noise | Wideband RX, military/commercial monitoring |
Why the Distinction Matters
For transmit efficiency and gain, open resonant and loop designs dominate. For wideband receiving and predictable patterns, terminated traveling-wave designs shine. Recognizing which category an antenna belongs to explains its behavior and sets realistic expectations: a Beverage is unbeatable for low-band reception, but won’t match the efficiency of a delta loop when transmitting.
Mini-FAQ
- Are loops traveling-wave antennas? — No. Loops are closed resonant structures with circulating standing waves, not terminated traveling-wave antennas.
- Why do terminated antennas sound quieter? — Their non-resonant nature reduces local noise pickup and provides smoother impedance to the receiver.
- Do loops always outperform dipoles? — Not always, but they often offer lower noise and can deliver gain advantages when configured properly.
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