Why We Use Simplified Antenna Models—and When We Use NEC
Why We Use Simplified Antenna Models—and When We Use NEC
A model earns its place by answering a defined question. Sometimes the clean equation is the right instrument; sometimes the currents must be solved; the installed antenna still has to face measurement.
RF.Guru often uses compact analytical models for radiation plots. That is a deliberate engineering choice, not a rejection of NEC. A transparent model can expose the mechanism immediately; NEC becomes valuable when the geometry, coupling and current distribution are themselves part of the question.
The mistake is to rank models by complexity. A model with more segments is not automatically more truthful. A short equation is not automatically crude. Both can be excellent, and both can be confidently wrong, when their assumptions do not match the claim.
Start with the Question, Not the Software
“What does this antenna do?” is too broad for one plot. Pattern shape, feedpoint impedance, accepted power, efficiency, realized gain, common-mode current, null depth and installed receive SNR are different measurands. Each needs different inputs and a different evidence chain.
| Question | Useful first model | What still needs proof |
|---|---|---|
| Why does changing height move elevation lobes? | Element pattern plus a declared ground-reflection or image model | Real-earth properties, terrain, installation geometry and measured field response |
| How do spacing and phase steer an ideal array? | Array factor with declared element currents | Mutual coupling, feed-network errors, element patterns and calibration |
| What impedance appears at the feedpoint? | A current-solving electromagnetic model with a declared source plane | Real conductor, loss, connection and installation tolerances |
| Will the coax exterior carry current? | A model that explicitly includes the exterior and its return path | Current-probe or equivalent measurements on the installed system |
| Which antenna gives better receive SNR here? | A system model that includes pattern, external noise and receiver limits | Controlled A/B/A measurements at the same time, bandwidth and reference plane |
What a Simplified Model Can Show Cleanly
A closed-form element pattern, image-theory height factor or prescribed-current array sum is powerful because its assumptions are visible. Change height, wavelength, phase or spacing and the cause of the pattern change remains in view. This is ideal for teaching a mechanism, checking signs and conventions, and running sensitivity sweeps.
That clarity is conditional. A prescribed-current calculation answers: if these currents flow at these positions with these phases, what field follows? It does not establish that a real feed network creates those currents. An ideal image plane shows direct-and-reflected interference; it does not recreate a particular garden, roof or radial field.
What NEC Adds
NEC is a family of electromagnetic codes for wire and conducting-surface structures. NEC-2 solves integral equations numerically for induced currents and can return currents, charge, fields, coupling and other quantities for the geometry and environment represented in the input. Later NEC versions extend or improve parts of that modelling framework.
That current-solving step matters when the currents cannot be prescribed credibly in advance. Bends, close conductors, asymmetric feeds, loading, parasitic elements and mutual coupling can all change amplitude and phase along the structure. Those are good reasons to move beyond an ideal element pattern or array factor.
NEC does not remove the modeller from the result. Wire radius, segmentation, junctions, source placement, loads, ground choice and the geometry that was omitted remain engineering decisions. LLNL's NEC-5 validation material separates numerical modelling errors from physical modelling errors caused by simplifying the real antenna and its environment. A converged answer can therefore be numerically stable while still describing the wrong installation.
The Conductors You Omit Do Not Disappear Outdoors
An ideal feedpoint is useful when the question is about the intended radiator. It is incomplete when the coax exterior, mast, radial system, control wiring, station bonds or nearby metal can carry meaningful RF current. If that path is absent from the model, its pattern and impedance contribution is absent from the answer.
This is especially important at HF, where the physical installation is often electrically large and asymmetrical. The appropriate response is not to distrust every model. It is to state which current paths are included, add the important ones when the conclusion depends on them, and measure exterior current or pattern behaviour when the installed boundary is uncertain.
Ground and Height Must Be Declared Together
Height-dependent elevation lobes do not arise merely because an antenna has a coordinate called “height.” They arise from the antenna's relationship with a reflecting or lossy environment. Translate an isolated antenna through free space and its intrinsic far-field pattern does not change.
A perfect conducting plane is an excellent way to expose the direct-and-image interference mechanism. Real earth adds conductivity, permittivity, polarization and angle dependence; terrain and nearby structures add more geometry. A clean height plot can explain the trend, but a site-specific takeoff-angle or gain claim needs a site model or a suitable measurement.
Convergence Tests the Calculation, Not the Story
Any numerical model should be refined until the quantities used for the decision stop changing beyond a chosen tolerance. That may mean increasing wire segmentation, refining integration points, changing an angular grid or tightening a solver setting.
Convergence answers one question: has this numerical approximation stabilized for this model? It does not prove that the assumed current distribution, ground, loss, feed network or omitted environment represents the physical antenna.
How RF.Guru Builds an Evidence Chain
- Name the measurand. Decide whether the claim concerns pattern shape, impedance, current, gain, efficiency or receive SNR.
- Choose the smallest adequate model. Keep the mechanism and every important assumption visible.
- Test sensitivity. Vary frequency, geometry, phase, amplitude, ground and other uncertain inputs far enough to find conclusions that can reverse.
- Check numerical convergence. Refine the calculation and watch the quantities used in the decision.
- Add electromagnetic detail where it changes the currents. Use NEC or another suitable solver for coupling, impedance and geometry-dependent current distribution.
- Include the return path. Model or measure feedline exterior, mast, radials and other conductors when they can join the antenna.
- Measure at the right reference plane. Pattern, accepted power, current, gain and receiver SNR do not share one universal test fixture.
- State uncertainty and scope. Explain what the result establishes and what it does not.
IEEE 149 treats antenna-pattern measurement as a disciplined test-facility problem, while NIST guidance emphasizes uncertainty sources and their combination. That is the final guardrail: a simulation should lead to a measurable proposition, not replace one.
The Point Is Understanding, Not Software Loyalty
For an educational plot, the analytical model may be the strongest answer because the cause and effect are visible. For a tightly coupled or geometrically complicated antenna, a current-solving tool may be essential. For an installed performance claim, neither is the finish line.
Analytical modelling shows the mechanism. NEC tests a declared electromagnetic implementation. Measurement tests the hardware and site. Used together, they form an evidence chain. Used as competing badges of authority, they mostly produce prettier arguments.
Primary and authoritative references
- Burke and Poggio — Numerical Electromagnetics Code, Method of Moments: theory, code and user's guide
- Lawrence Livermore National Laboratory — NEC-5 Validation Manual
- Lawrence Livermore National Laboratory — NEC v5.0 capabilities and update notes
- IEEE 149-2021 — Recommended Practice for Antenna Measurements
- NIST — Estimating Uncertainties in Antenna Measurements
Mini-FAQ
- Does RF.Guru avoid NEC? No. We use analytical models for transparent mechanism studies and current-solving tools when geometry, coupling or impedance requires them.
- Can a simplified model be accurate? Yes, for a clearly bounded question whose important physics and assumptions are included. It should not be stretched into an installation claim it cannot support.
- Does NEC automatically include common-mode current? No. The relevant exterior conductor and return path must be represented for their current and radiation to appear in the solution.
- Can a normalized pattern compare antenna efficiency? No. Normalizing each pattern to its own maximum removes the absolute amplitude needed for an efficiency or gain comparison.
- Does convergence prove that a model is physically correct? No. It shows that the numerical result has stabilized for the stated model; physical assumptions still need validation.
- What proves an installed antenna result? A controlled measurement of the relevant quantity, with a declared reference plane, conditions and uncertainty.