Three ledgers of efficiency: modeling the POTA PERformer
Greg Mihran KJ6ER’s PERformer is one of the most popular published portable antennas: an elevated quarter-wave vertical for 40M–6M — a 17’ telescoping stainless whip with the feedpoint 52” up on a tripod or spike, and two elevated tuned radials sloping down to stakes. The free plans include 4NEC2 model results and a headline claim: over 90% efficient, versus “only 37%” for a typical ground-mounted quarter-wave with four ground-coupled radials.
verticals.pota_performer models it faithfully —
the plans’ 15M reference geometry, per-band variants straight from the
whip/radial tables, the 90° directional and 180° omni radial spans, and
VA3KOT’s single-radial simplification. This page is about what happens
when you check the numbers: the claims duplicate, and reading the
efficiency number in full teaches the most useful lesson in portable
antenna modeling.
The claims duplicate
Section titled “The claims duplicate”Four models from three modelers — spanning two genuinely independent solver cores, the NEC-2 family and momwire — on the 15M configuration, two radials, average ground:
| Model | Peak gain | Takeoff | F/B (90° span) | El. beamwidth |
|---|---|---|---|---|
| KJ6ER, 4NEC2 (published) | +0.31 dBi | 24° | 3.37 dB | 46° |
| VA3KOT, EZNEC (independent) | +1.19 dBi | 25° | 3.34 dB | 47° |
| antennaknobs, momwire | +1.06 dBi | 24° | 2.91 dB | 44° |
| antennaknobs, PyNEC | +1.02 dBi | 23° | ~3 dB | 44° |
The beam shape is unanimous: takeoff 23–25°, a mild ~3 dB front-to-back toward the radial span, mid-40s elevation beamwidth. The directional-vs-omni delta duplicates too (KJ6ER +0.98 dB, we measure +0.76 dB), and so do VA3KOT’s single-radial trends (more F/B, wider elevation, narrower azimuth). Absolute peak gain spreads about 0.9 dB across engines, with KJ6ER’s own figure the lowest of the four — ordinary cross-model variation in ground constants and whip idealization.
The SWR story holds up as well: at the plans’ exact 15M lengths the directional feedpoint solves to 45 + 1j Ω — SWR 1.10, matching the “better than 1.1:1” field measurements (and the omni span solves lower, just as the plans say elevating radials should).
The efficiency claim: true, in its ledger
Section titled “The efficiency claim: true, in its ledger”“Over 90% efficient” is a structural efficiency: input power that isn’t burned in conductors and components. We confirm it. Solving the same geometry with lossless wire and taking the ratio: the stainless whip plus radials cost only ~2–3%, structural efficiency ≈ 98%. KJ6ER’s 90.8% is a few points lower — plausibly the real whip’s thin stainless tip sections, which our uniform mid-taper radius flatters (his −0.12 dB choke is itemized separately in his table, not inside the 90.8%). And the physics behind his 90%-vs-37% comparison is real: elevated tuned radials remove the ground-coupled loss resistance that sits in series with a ground-mounted vertical’s feed and eats over half its power in-circuit.
But there is a second ledger, and it’s already visible in the table
above. Gain and efficiency are the same measurement. Gain is power
density per input watt; average linear gain over the sphere is exactly
Pradiated/Pinput (radiated_fraction in the
library computes it from any pattern). Equivalently: this beam shape has
a directivity of ~6.5 dBi — that’s what the peak would read if every
input watt were radiated. The peaks actually read about +1 dBi:
| Model | Peak gain | Implied radiated fraction |
|---|---|---|
| KJ6ER, 4NEC2 | +0.31 dBi | ~24% |
| VA3KOT, EZNEC | +1.19 dBi | ~30% |
| momwire (integrated directly) | +1.06 dBi | 29% |
| PyNEC (integrated directly) | +1.02 dBi | 34% |
So for 100 W in: ~2 W warms the stainless, ~68 W warms the ground within a few wavelengths, ~30 W leaves as sky wave. The missing 4.5–6 dB between “+5.5 dBi if lossless” and “+1 dBi as modeled” is the Fresnel-zone ground absorption, and it appears in nobody’s “efficiency” figure — including the 90.8% — because structural efficiency by definition stops at the antenna’s terminals-and-metal. KJ6ER’s own published +0.31 dBi peak is a ~25% radiated fraction, stated in dB.
The tax everyone pays
Section titled “The tax everyone pays”Ground absorption at these heights is nearly identical for every portable vertical. For calibration, the same integration on a 7 m-high inverted vee gives 72% radiated on 20m, 72% on 15m, 69% on 10m — the classic reason horizontal wire beats a vertical over real ground when you have the supports, and the vertical’s counter-argument is the low takeoff angle (23° here vs the vee’s 51° on 20m), not efficiency. You cannot pack better dirt. That’s precisely why the convention in antenna write-ups is to quote structural efficiency: it’s the part the builder controls, and on that score the PERformer is genuinely excellent — the elevated-radial design earns its numbers, and the relative claims (beats ground-mounted, 3 dB of steerable F/B from a 90° radial span, resonant with no tuner) all survive independent modeling.
The lesson is about reading claims, not doubting this antenna: when a spec sheet says “90% efficient” and the same page’s pattern plot peaks near 0 dBi, both numbers are correct — they are different ledgers. The gain plot is the one the ionosphere sees.
The rest of the trio: Challenger and Dominator
Section titled “The rest of the trio: Challenger and Dominator”KJ6ER publishes two more antennas in the same family, and both are now
in the tree: verticals.challenger (off-center-fed
halfwave vertical: 25’ whip is ~77% of the halfwave, a short ~10% λ
counterpoise completes it through a 4:1 unun) and
verticals.dominator (a true vertical EFHW: the
whip is the whole halfwave, fed through a 49:1 with a long ~33% λ
counterpoise). Same duplication exercise, same result — the claims hold:
| Claim (4NEC2, 15M) | Published | antennaknobs |
|---|---|---|
| Challenger peak / takeoff / el BW | −0.32 dBi / 20° / 33° | +0.14 dBi / 21° / 34° |
| Dominator peak / takeoff / el BW | +0.60 dBi / 18° / 27° | +0.34 dBi / 17° / 26° |
| Takeoff ordering, trio | 18° < 21° < 24° | 17° < 21° < 23° |
The transformers are where these two get interesting, because KJ6ER
measures their insertion losses and itemizes them honestly — and our
Transformer branch puts the same numbers in the power budget: the
stock 49:1 burns −0.96 dB (~20% of input power) — the same black
box interrogated in the end-fed question — the
Challenger’s 4:1 only −0.34 dB, with the “plus” upgrades at −0.40/−0.24 dB
(plus variants; the magnetizing branch is calibrated to the measured
loss at 15M, not derived from core datasheets). His own comparison
table shows the interesting tension — the Dominator has the trio’s
highest “structural efficiency” (99.5% — it’s just one aluminum tube)
and the trio’s lossiest component — and to his credit both rows sit
side by side on his page 14. Our power budget simply adds them, plus
the dirt’s share, into one delivered-watts number.
And the third ledger says: Challenger radiates 25%, Dominator 25% (transformer included), PERformer 29%. Nobody escapes the dirt. What actually separates these three antennas — exactly as KJ6ER’s “primary reach” table frames it — is the takeoff angle, and that claim reproduces perfectly.
Try it
Section titled “Try it”Open verticals.pota_performer in the
simulator. Flip radial_span_deg between 90 and 180 and watch the
front-to-back appear and vanish; drop n_radials to 1 for VA3KOT’s
fast-deploy config; walk the band variants (band20 … band6) and
watch the droop angle steepen as the radials shorten against the fixed
stake height — exactly the table in the plans.
The third ledger is on screen while you do it: with a finite ground selected, the solve readout’s radiated (incl. ground) row (and the radiated % readout on the pattern views) fills in whenever the knobs settle — ~29% for this antenna, the number this page is about. See the workbench reference for how it’s computed.
Sources: KJ6ER’s PERformer plans (rev 2025-02); VA3KOT, “Testing and modifying the POTA PERformer antenna” (2025-05).