Why You Might Not Need NEC: The Surprising Accuracy of Simple Models for Active RX Arrays

If you're designing phased receive arrays with active probes, you might assume that NEC (Numerical Electromagnetics Code) simulations are mandatory for accuracy. But what if a much simpler model gives you nearly identical results?

In fact, if you're working with active E-field probes (like our EchoTracer or VerticalVortex), you can often get highly reliable pattern predictions using simple free-space models based purely on geometry and phasing. This is not just a convenient shortcut — it's grounded in electromagnetic fundamentals.

1) Electrically Small = Predictable and Linear
Active E-probes typically measure less than λ/12 in physical length. This places them firmly in the electrically small regime, where they behave as linear voltage sensors. Their interaction with the incident field is essentially point-like and governed by first-order terms in Maxwell's equations. No resonance -> no reactive near-field effects.

2) No Standing Waves or Energy Reflection
Unlike passive antennas, these active elements do not rely on resonance for efficiency. Since they are terminated with a high input impedance buffer amplifier, they absorb the local E-field rather than reflecting it. This eliminates standing wave formation and secondary radiation, making the probe's presence nearly invisible to neighboring elements.

3) Negligible Mutual Coupling
Because the elements are non-resonant and non-radiating, mutual impedance between elements is minimal. This sharply contrasts with passive arrays, where mutual coupling can drastically alter input impedance and radiation patterns. In active probe arrays, the geometry and delay lines (or hybrid phasing) dictate the result, not antenna loading.

4) Pattern Determined by Phase Geometry
In receive-only mode, what matters is how the received voltage from each element adds up after phase shifting. For E-probe arrays, the far-field pattern is defined almost entirely by array geometry and phasing, not by the individual probe's characteristics. This means a simple vector-sum model in free space can predict beam directions, nulls, front-to-back ratio, and even RDF with high fidelity.

5) Validated by NEC: Deviations Are Marginal
We've compared our idealized Python model with full NEC2/NEC4 simulations using real ground, proper probe models, and accurate interconnections. The results consistently show that:

  • Beam directions and null positions differ by less than 5 degrees
  • Front-to-back and RDF vary by < 2 dB
  • Elevation lobe shape remains nearly identical for arrays <5 m above ground

These differences are smaller than the real-world variability due to terrain, obstructions, or even coax routing.

6) Massive Gains in Design Speed
With a geometry-phase model, you can sweep across frequency ranges, spacing values, and azimuth orientations in seconds. Overlays, RDF plots, null angle optimizations, and elevation cross-sections become scriptable and interactive — allowing rapid prototyping that would take hours or days in NEC.

When Should You Still Use NEC?

  • To model interactions with the ground (e.g., low-angle DX work)
  • When your elements are resonant (e.g., short dipoles, full-size loops)
  • When deploying near large conductive objects or lossy soil
  • For verifying gain normalization or absolute E-field levels

Bottom Line

If you're building RX-only phased arrays with active, non-resonant, electrically small elements, a simple geometry-based model is not just good enough — it's practically optimal for design and optimization. NEC remains a valuable validation tool, but your time is better spent understanding phase geometry and array topology. The physics says so. The measurements confirm it.

 

Interested in more technical content like this? Subscribe to our notification list — we only send updates when new articles or blogs are published: https://listmonk.rf.guru/subscription/form

Questions or experiences to share? Feel free to contact RF.Guru or join our feedback group!

Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.