Simulating a 5-Element 2m Yagi with NEC2 Before Cutting Any Aluminium

Why Simulate Before You Cut?

Cutting aluminium tubing for a Yagi is cheap. Cutting it wrong, discovering the gain is 1.5 dB short of the textbook claim, and rebuilding is not. More importantly, for weak-signal work at 144 MHz — EME (Earth-Moon-Earth moonbounce) or tropo scatter — a 1 dB error in gain is not a rounding issue. At EME path loss of roughly 252 dB, every single dB is meaningful.

NEC2 (Numerical Electromagnetics Code) has been the reference wire-antenna simulator for 40 years. It solves the Method of Moments (MoM) integral equation for current distribution on wire structures, giving you far-field patterns, gain, front-to-back ratio, and feedpoint impedance in seconds. The Antenna Sim tool puts NEC2 in your browser — no Linux install required.

The Design: 5-Element Yagi at 145 MHz

Why 5 elements? A 3-element Yagi on 2m delivers around 7.5–8 dBd gain with a front-to-back ratio of 20–22 dB. That is adequate for local SSB but not for EME, where you want every dB you can get from a single boom, and F/B matters because ground noise from the back lobe directly raises your system noise temperature.

A well-optimised 5-element design hits approximately 10 dBd gain with F/B of 26–28 dB, representing a meaningful 2+ dB improvement over the 3-el — equivalent to more than doubling your transmit power on receive.

Simulation Inputs

For the real-ground run:

Free-Space Results

With the antenna in free space, NEC2 returns:

Feedpoint impedance of 47 + j3 Ω is essentially ideal for a direct 50 Ω coax feed — no matching network needed. The folded dipole naturally transforms the low radiation resistance of a parasitic-loaded driven element up to the coax impedance range.

The free-space gain obeys the approximate formula for Yagi gain as a function of boom length:

With $L_\text{boom} = 2.3\,\text{m}$ and $\lambda = 2.07\,\text{m}$ at 145 MHz, this gives $G \approx 10 \log_{10}(8.56) \approx 9.3\,\text{dBd}$ — a rough estimate; the NEC2 result of 10.1 dBd reflects the more precise optimisation of element spacing and lengths.

Real-Ground vs. Free-Space: The Surprise

Switch the simulation to Real ground (σ = 0.005, εr = 13) with the antenna at 6 m height (2.9λ) and the picture changes:

The ground reflection adds approximately 3 dB of gain at low elevation angles — exactly what tropo-scatter and EME paths need (Moon elevation is typically 5–30° when accessible from mid-latitudes). This ground gain is free; you get it just from siting the antenna at the right height. The reduced F/B in the real-ground case is because back-lobe ground reflections partially fill in the null — still more than acceptable.

For EME operators, this means the effective system gain is 13.4 dBd at 12° elevation, not the free-space 10.1 dBd. That 3.3 dB difference changes the link margin calculus significantly. Use the RF Link Budget calculator with EIRP based on the real-ground peak gain to compute the full EME path budget.

Comparing 3-El vs. 5-El at This Height

Running the 3-element version in the same NEC2 setup (1.0 m boom, same element diameter) gives:

The 5-element wins by 2.5 dB of actual path gain and 6 dB of F/B. For a single-Yagi station attempting EME, the 5-el is the minimum sensible choice; most serious EME operators stack four or more of them.

Practical Build Notes the Simulation Surfaces

Element-to-boom insulation matters. NEC2 models elements as continuous wires. If you mount aluminium elements directly on a conductive aluminium boom, you short the element midpoint to the boom and detune the array. Either insulate each element from the boom or use non-conductive fibreglass tube — the simulation assumes the latter. Driven element clearance. The folded dipole needs about 15 mm clearance around the feed gap. The NEC2 model uses thin-wire approximation; real-world element diameter effects are handled by the segment-diameter ratio. Keep the segment length-to-diameter ratio above 4:1 in your model (the tool warns you if you violate this). Weatherproofing the feedpoint. The simulation gives you 47 Ω at the feed. In practice, 5–10 mm of moisture ingress at the feedpoint can add 2–5 Ω of resistive loss — invisible in simulation but very visible in F/B degradation over a winter. Seal it properly.

Simulate first, cut second. The Antenna Sim tool gives you the full NEC2 result — gain, pattern, impedance, elevation plot — in under a minute. That is a lot cheaper than a miscut boom.

Simulate your Yagi with NEC2