The Limitations of NEC
NEC (Numerical Electromagnetic Code) is a powerful tool for antenna modelling. Hams use it to simulate radiation patterns, input impedance, gain, and other characteristics of wire antennas. But it is crucial to understand what NEC is—and just as importantly, what it is not.
NEC is not a magic box that can simulate the entire messy reality of your installation. It is based on mathematical approximations (Method of Moments), idealisations, and simplifying assumptions. These work well in controlled scenarios, but quickly fall apart in real-world environments like boats, balconies, RVs, rooftops, or cluttered gardens.
NEC Is a Theoretical Tool
NEC solves Maxwell’s equations via the Method of Moments (MoM) for thin, perfectly conducting wires in free space or over a simple ground plane. That means:
- It assumes all wires are infinitesimally thin compared to wavelength
- It struggles with thick conductors or flat plates (e.g. hulls, balconies, railings)
- It cannot model volumetric surface currents or finite thickness
- It ignores dielectric properties of materials (wood, fiberglass, plastic, soil layers)
- It assumes perfect bonds and stable electrical connections
| NEC Handles Well | NEC Cannot Model |
|---|---|
| Thin wires in free space | Thick conductors, plates, hulls, curved rails |
| Ideal bonds and joints | Floating or lossy connections, corrosion |
| Flat ground plane or simple soil models | Layered ground, saltwater vs mud vs concrete |
| Far-field radiation patterns | Near-field inductive/capacitive coupling |
| Comparative lobes and nulls | Dielectric detuning from trees, plastic, fiberglass |
| Simple verticals, dipoles, loops | Complex clutter: boats, balconies, rooftops, attics |
Think of NEC as a clean lab tool. It sees the wire geometry in idealized surroundings, but it is blind to the messy, lossy, unpredictable real-world environment around your antenna.
What Happens on a Boat or Balcony
Many hams model their sailboat or balcony installs in 4NEC2, adding masts, rigging, railings, even water or rooftop edges. Then they trust the pattern as gospel. That’s wishful thinking.
On a fiberglass boat, many parts are “floating” electrically. The mast may not be bonded. Rigging may have intermittent connections. NEC cannot model lossy or uncertain bonds. Worse: it assumes fixed reference potentials and current paths. In reality, RF may flow unpredictably across hulls, railings, or coax shields.
Don’t Ignore the Non-Metal Stuff
Another misconception: “Only metal matters.” Wrong. NEC does not model detuning from nearby dielectrics like trees, fiberglass canopies, or even large plastic tanks. But in practice, they do matter. Anything comparable in size to a wavelength couples to the near field and shifts resonance, bandwidth, or patterns. NEC is blind to this.
NEC Is Far-Field Only
NEC gives you the far-field pattern after all near-field interactions. It cannot simulate the inductive or capacitive coupling between your antenna and nearby structures. Yet this is exactly what dominates in balconies, attics, rooftops, and boats: railing coupling, mast effects, detuned wires, ground loops. NEC simply doesn’t “see” those effects.
Adding floating structures into NEC sometimes makes the model look more “detailed” but actually less accurate. If NEC cannot model the coupling correctly, it may be better to leave those unknowns out and stick to a clean baseline.
| Parameter | NEC Prediction | Typical Real-World Deviation |
|---|---|---|
| Forward Gain | +6 dBi (ideal) | –2 to –3 dB lower due to losses |
| Front-to-Back | 25 dB | 10–15 dB shallower nulls |
| Elevation Angle | 20° | Shift ±10–15° from ground/obstacles |
| Resonant Frequency | 7.05 MHz | ±50–150 kHz detuning (supports, wires, clutter) |
| Bandwidth (2:1 SWR) | 300 kHz | 20–40% narrower |
These deviations are conservative, indicative values. Real-world installs often drift further depending on soil, clutter, and bonding. NEC is a guide, not gospel.
So Why Use NEC At All?
Because it is still very useful—if you use it right. NEC is excellent for understanding how a design should behave in clean conditions: estimating resonance, impedance, lobe shapes. But don’t expect it to predict your messy installation with precision.
What I Use Personally
I often use my own lightweight Python modelling code (NumPy + MoM) that assumes free space and ignores ground and clutter. Yet the patterns usually agree in shape with NEC. The absolute dB values differ, but the comparative lobes, nulls, and behavior trends are consistent.
The lesson: antenna modelling is about insight and trends, not chasing absolute decimals. Whether gain is 5.7 dBi or 6.2 dBi doesn’t change how you aim. What matters is understanding where lobes and nulls fall, so you can design and adjust intelligently.
Takeaways for Hams
- NEC assumes thin wires, perfect bonds, ideal ground
- It cannot model clutter, complex shapes, or floating conductors accurately
- It ignores dielectric detuning from non-metallic objects
- It only shows far-field, not near-field coupling
- Use NEC for comparisons and insight, not as final truth
- Expect at least ±2–3 dB deviation from reality in gain numbers
- When in doubt, measure → tweak → observe → iterate
When modelling antennas on boats, balconies, attics, or rooftops: NEC is an approximation, a starting point—not a guarantee. Floating conductors and complex clutter are better left out than “guessed in.” The real work is in measuring and adjusting on site.
Mini-FAQ
- Can NEC model my balcony antenna exactly? — No. It cannot account for unknown bonds, clutter, and dielectric loading.
- Is NEC useless? — Not at all. It’s very useful for baseline design and comparative modeling.
- How accurate are NEC gain numbers? — Usually off by at least 2–3 dB from reality in messy environments.
- What should I trust more? — NEC for shapes and trends; real measurements for truth.
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