The Illusion of Resonance: When Coax Becomes the Antenna
In the world of compact antennas, some designs achieve resonance in a surprisingly small footprint. A well-known example is the Isotron-type LC resonant antenna, which uses a coil and capacitive plates to form a tuned circuit. These antennas, and similar “resonant point” designs, can present a low SWR on an analyzer despite having no substantial counterpoise or radial system. But in many cases, their apparent resonance hides an unintended participant: the coaxial feedline.
Electrically short and lacking a dedicated return path, these antennas often recruit the coax shield as part of the radiating structure. The resonance you measure isn’t just from the compact radiator — it’s from the combined system of radiator plus several meters of feedline.
RX vs TX: What Actually Happens
- On receive (RX): the imbalance between the radiator and the missing counterpoise increases common-mode pickup on the coax braid. The braid acts like an unintended sensor, pulling in local noise and nearby fields.
- On transmit (TX): the same imbalance forces return current to flow on the outside of the braid. That return current makes the feedline part of the radiator, altering impedance and pattern. Insert a proper choke at the feedpoint and the antenna may seem to “stop working” — because the coax can no longer serve as the missing return path.
Why This Matters
- Narrow bandwidth — Often <50 kHz usable; small frequency or environmental changes detune the system.
- Radiation pattern instability — The feedline’s routing (bends, coils, drape) changes the pattern and match.
- Noise on RX — Elevated common-mode pickup on the braid pulls more shack/house noise into the receiver.
- Efficiency trade-offs — A lot of power is stored reactively or lost in the matching network rather than radiated.
Matched vs. Efficient: Key Differences
| Characteristic | Well-Matched but Inefficient Antenna | Efficient, Properly Designed Antenna |
|---|---|---|
| SWR Reading | Often 1:1 at design frequency | Near 1:1 or within tuner range |
| Bandwidth | Extremely narrow (<50 kHz typical) | Wider usable range, more stable tuning |
| Dependence on Feedline | High — braid carries return current and radiates | Low — feedline carries current but does not radiate significantly |
| Radiation Efficiency | Low — much energy stored or lost as heat | High — majority of applied power radiated |
| Pattern Stability | Unstable — changes with feedline routing and surroundings | Stable and predictable |
| Suitability for QRO | Poor — high losses, heat buildup in matching | Good — can be designed for high duty-cycle/power |
The Takeaway
Isotron-type and similar LC-resonant compact antennas can be useful for QRP, indoor, or ultra-limited spaces — but understand their limits. A slightly larger radiator with a real counterpoise and a proper feedpoint choke will usually deliver better efficiency, pattern stability, and lower RX noise. Resonance alone is not proof of efficient radiation.
If an antenna’s performance depends on the coaxial cable carrying the return current and radiating, it isn’t a standalone radiator. Resonance ≠ efficiency.
Mini-FAQ
- My compact antenna shows 1:1 SWR. Is it efficient? — Not necessarily. SWR is match, not efficiency.
- Why did a feedpoint choke “kill” the antenna? — It blocked the braid from acting as the return path, revealing the underlying imbalance.
- How can I improve it? — Add a proper counterpoise/radials, use a current balun/line isolator at the feedpoint, and minimize feedline radiation.
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