Simulating a 5-Element 2m Yagi with NEC2

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 what you expected, and then rebuilding the whole thing — that's expensive and annoying. For weak-signal work at 144 MHz, whether you're doing EME (Earth-Moon-Earth moonbounce) or tropospheric scatter, a 1 dB error in gain isn't some academic rounding issue. When you're dealing with EME path loss around 252 dB, every single dB matters. You feel it in the noise floor.

NEC2 (Numerical Electromagnetics Code) has been the reference wire-antenna simulator for about 40 years now. It solves the Method of Moments (MoM) integral equation for current distribution on wire structures, spitting out far-field patterns, gain, front-to-back ratio, and feedpoint impedance in seconds. The Antenna Sim tool puts NEC2 right in your browser — no Linux install, no compiling ancient Fortran, none of that.

The Design: 5-Element Yagi at 145 MHz

Why 5 elements instead of 3? A 3-element Yagi on 2 metres typically delivers around 7.5–8 dBd gain with a front-to-back ratio of maybe 20–22 dB. That's fine for local SSB work, but it's not enough for EME, where you need every dB you can squeeze from a single boom. Front-to-back ratio matters too, because ground noise from the back lobe directly raises your system noise temperature — and that kills your ability to hear weak signals.

A well-optimised 5-element design hits approximately 10 dBd gain with F/B of 26–28 dB. That's a meaningful 2+ dB improvement over the 3-element version, which is equivalent to more than doubling your transmit power on receive. For a single-Yagi station, that difference is huge.

Simulation Inputs

Here's what we're feeding into NEC2 for the initial model:

The element lengths follow the typical Yagi pattern: reflector is longest, driven element is slightly shorter, and the directors progressively decrease in length as you move forward. The 12 mm diameter is a common aluminium tube size — easy to source, stiff enough for 2.3 metres of boom, and thick enough that you're not worried about wind loading.

For the real-ground simulation, we add ground parameters that model typical soil:

These ground parameters represent average soil — not the worst case, not the best case. If you're on sandy soil near the coast, your conductivity might be lower. If you're on clay, it might be higher. But 0.005 S/m is a reasonable middle ground for most locations.

Free-Space Results

Running the antenna in free space first gives us a clean baseline without any ground effects mucking up the numbers. NEC2 returns:

That feedpoint impedance of 47 + j3 Ω is essentially perfect for a direct 50 Ω coax feed — no matching network needed, no balun losses to worry about. The folded dipole naturally transforms the low radiation resistance of a parasitic-loaded driven element up to the coax impedance range. This is one of the main reasons we use folded dipoles on Yagis instead of simple dipoles.

The free-space gain follows 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 $G \approx 10 \log_{10}(8.56) \approx 9.3\,\text{dBd}$. That's a rough ballpark estimate. The NEC2 result of 10.1 dBd reflects the more precise optimisation of element spacing and lengths — the formula doesn't account for the fine-tuning you can do with director spacing.

The beamwidth numbers tell you how forgiving the antenna is for pointing accuracy. A 38° E-plane beamwidth means you've got about ±19° of slop before you're down 3 dB. For EME work where you're tracking the Moon, that's tight but manageable with a decent rotator. Most operators end up peaking the signal manually anyway.

Real-Ground vs. Free-Space: The Surprise

Now here's where it gets interesting. Switch the simulation to real ground with σ = 0.005 S/m, εr = 13, and the antenna at 6 m height (about 2.9λ), and the picture changes dramatically:

The ground reflection adds approximately 3 dB of gain at low elevation angles — exactly what troposcatter and EME paths need. The Moon is typically between 5–30° elevation when it's accessible from mid-latitudes, so that 12° peak gain angle is right in the sweet spot. This ground gain is essentially free; you get it just from siting the antenna at the right height above ground. It's the same effect that gives you multiple lobes in a vertical plane when you model any horizontal antenna over real ground.

The reduced F/B in the real-ground case happens because back-lobe ground reflections partially fill in the null. You lose about 7 dB of front-to-back compared to free space, but 19.8 dB is still more than acceptable for most applications. The feedpoint impedance shifts slightly — you pick up a few ohms of reactance — but you're still under 1.15:1 VSWR, which is negligible.

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 fundamentally changes your link margin calculations. Most people forget to account for this when they're planning an EME station and then wonder why their calculations don't match reality. Use the RF Link Budget calculator with EIRP based on the real-ground peak gain to compute the full EME path budget — otherwise you're leaving performance on the table.

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

Running the 3-element version in the same NEC2 setup (1.0 m boom, same 12 mm element diameter) gives us a direct comparison:

The 5-element wins by 2.5 dB of actual path gain and 6 dB of front-to-back ratio. For a single-Yagi station attempting EME, the 5-element is the minimum sensible choice. Most serious EME operators stack four or more of them to get another 6 dB from the array gain, but even a single 5-element Yagi will let you hear your own echoes off the Moon with a decent preamp and low-noise coax.

That extra 1.3 metres of boom length is a small price to pay for 2.5 dB. Mechanically, both antennas have similar wind loading — the boom weight goes up, but the element count only increases by two. If you can mount a 3-element, you can mount a 5-element.

Practical Build Notes the Simulation Surfaces

Element-to-boom insulation matters. NEC2 models elements as continuous wires floating in space. If you mount aluminium elements directly on a conductive aluminium boom, you short the element midpoint to the boom and completely detune the array. You'll lose gain, the feedpoint impedance will shift, and the pattern will distort. Either insulate each element from the boom with plastic blocks or use non-conductive fibreglass tube for the boom — the simulation assumes the latter. Most builders use fibreglass because it's simpler and more weatherproof than a bunch of insulating hardware. Driven element clearance. The folded dipole needs about 15 mm of clearance around the feed gap. If you crowd it with metal mounting hardware or let the coax shield touch the element too close to the feedpoint, you'll shift the impedance. The NEC2 model uses thin-wire approximation, and 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 simulation tool will warn you if you violate this, but it's worth checking manually. Weatherproofing the feedpoint. The simulation gives you 47 Ω at the feed under ideal conditions. In practice, even 5–10 mm of moisture ingress at the feedpoint can add 2–5 Ω of resistive loss. That's invisible in the simulation but very visible in F/B degradation over a winter. Water in the coax connector or around the folded dipole feedpoint will kill your performance. Seal it properly with self-amalgamating tape and heat shrink, or use a proper weatherproof enclosure. This is one of those things most people skip and then regret later when they're trying to figure out why the antenna doesn't perform as well as it did in September. Boom sag and element droop. A 2.3 metre boom will sag under its own weight and the weight of the elements, especially if you're using aluminium. The simulation assumes perfectly straight elements in perfect alignment. In reality, a few millimetres of boom sag or element droop won't kill your performance, but 10–20 mm will start to shift the pattern and reduce gain. Use a stiff enough boom or add a truss to keep everything straight.

Simulate first, cut second. The Antenna Sim tool gives you the full NEC2 result — gain, pattern, impedance, elevation plot — in under a minute. That's a lot cheaper than a miscut boom or a set of elements that don't resonate where you thought they would. You can tweak element lengths, adjust spacing, and see the effects immediately. Once you've got a design that hits your gain and impedance targets in simulation, then you cut metal.

Simulate your Yagi with NEC2