Non-Resonant Traps (NRTs) and Their Simulation in NEC Tools

Non-Resonant Traps (NRTs) are a powerful yet often misunderstood technique for building efficient multiband wire antennas. Unlike classic resonant traps, which rely on parallel LC circuits tuned to a specific frequency, NRTs are designed as broadband impedance discontinuities—typically inductors—that create current breaks or redirections across a broad frequency range. Their simplicity, power handling, and predictability make them highly attractive for robust antenna designs. However, simulating them properly is non-trivial.

What Makes an NRT Different?

NRTs do not behave as tuned circuits. Instead, they are narrow inductive sections—usually air-core or ferrite-core coils—in series with the antenna conductor. Below a certain frequency (often set by the electrical length of the trap wire past the NRT), they act as an ordinary wire. Above that frequency, the inductor creates an increasing impedance, forcing the RF current to drop significantly or stop entirely. The trap's location then becomes the effective endpoint of the radiator.

This behavior is continuous and frequency-dependent. The impedance isn't infinite above the "trap frequency," but it becomes large enough that current is minimized, which allows the wire beyond the trap to become electrically invisible.

Why MMANA Can’t Model NRTs Directly

MMANA-GAL is based on a simplified NEC2 core and does not support defining lumped-element inductors in series with wires. It only supports loadings at wire segments (e.g., lumped loads or short transmission lines), which is not equivalent to how a true NRT behaves.

Moreover, MMANA does not allow inserting frequency-dependent RLC components. Since NRTs are inherently broadband and depend on impedance variation with frequency, this limits MMANA’s usefulness for NRT modeling.

Simulating NRTs in NEC2/NEC4

To realistically simulate an NRT in a more capable NEC environment (like 4NEC2, EZNEC Pro/4, or nec2c), you need to:

  1. Insert a series inductor at the point where the NRT is located, using a frequency-dependent impedance model.
  2. Use the LD card with type 5 (complex RLC loads) to define frequency-dependent loading if the NEC variant allows it.
  3. Alternatively, run frequency sweeps with different fixed inductor values approximating the behavior at specific bands.
  4. Use NEC cards to place a wire at the NRT location with a tiny radius and add a loading coil, either real or idealized.

In nec2c or 4NEC2, you can define an RLC load with:

LD 5 tag segment 0 0 R L C

Where:

  • R is the series resistance (usually small, <0.5 Ohms)
  • L is the inductance in µH (e.g., 10 µH)
  • C is the capacitance (0 for a pure inductor)

Do You Need NEC to Design Antennas?

In several of our previous articles, we’ve clearly stated that you don’t need a NEC engine to design great antennas. Practical antenna building often starts with real-world constraints, heuristics, proven design ratios, and iterative measurements—not simulation.

But it’s not just about field testing — it’s also about understanding the logic and math behind simple resonant structures:

  • A quarter-wave radiator is λ/4 long and relies on a ground system (or counterpoise) to complete the image.
  • A half-wave dipole works because it presents high voltage and low current at the ends, and a current maximum in the center.
  • A 5/8 wave vertical gives low-angle radiation due to its phase relationship with ground reflection.

These antenna types are based on clear, repeatable math. You can calculate their lengths, impedance behaviors, and current distribution without needing to "draw wires" in a simulator. For example:

  • Quarter-wave length (in meters): 71 / f (MHz)
  • Half-wave length (in meters): 143 / f (MHz)
  • 5/8 wave: 178 / f (MHz)

Simple antennas follow simple physics. Good design comes from understanding principles, not necessarily from building a perfect NEC model. If you understand current distribution, matching, and the influence of environment — you’re already well ahead of most simulations.

That said, NEC is still a useful tool for understanding pattern shape or identifying major flaws in a concept—especially when you understand its limitations. But it’s not a silver bullet.

Practical Use

Most NRTs used in end-fed or trap dipoles have values in the range of 5–15 µH. You can test several options in your NEC model and observe how the current distribution changes across frequencies. Look for the band where the current sharply drops past the NRT location — this tells you where the trap becomes effectively "opaque" to RF.

Conclusion

While MMANA is a fantastic tool for many antenna simulations, it lacks the ability to handle broadband traps like NRTs. For accurate modeling of such systems, turn to full-featured NEC engines and define inductive elements with frequency-dependent behavior. Once you understand the current distribution across the antenna at various bands, you’ll see the true power of NRTs in creating multiband antennas with minimal compromise.

Tip: You can also measure the real-world impedance of your NRT using a VNA and then use that complex impedance (Z = R + jX) as a substitute in NEC simulations for even better accuracy.

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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.