OK1M 7el 20-15-10m

Performance (MMANA-GAL / OK1M-PyminiNEC3)

Computed at 12 m height over real ground (ε = 13, σ = 5 mS/m), horizontal polarization, model v0.4.

Band	Elements	Frequency	Gain	F/B	SWR	Elevation	SWR < 1.5	SWR < 2
20 m	2	14.175 MHz	11.4 dBi	11.7 dB	1.03	23.7°	whole band	whole band
15 m	2	21.225 MHz	11.1 dBi	10.6 dB	1.30	16.2°	171 kHz	344 kHz
10 m	3	28.350 MHz	12.3 dBi	16.1 dB	1.17	12.2°	371 kHz	640 kHz

Band	Range	SWR	Gain	F/B
20 m	14.000–14.350	1.03–1.34	11.2–11.7 dBi	9.8–13.3 dB
15 m	21.000–21.450	1.27–2.61	10.7–11.6 dBi	8.7–10.8 dB
10 m	28.000–28.700	1.06–2.15	12.3–12.4 dBi	15.7–16.6 dB

Model v0.4 = v0.3 with the REF 10m length retuned, 5 100 → 5 198 mm. In v0.2/v0.3 the 10 m reflector came out shorter than the 10 m driven element (5 100 vs 5 166 mm) and sat on the edge of an F/B cliff — at 5 040 mm the F/B at 28.000 MHz falls to 8.9 dB. v0.4 raises the F/B at the bottom of the band from 13.4 to 15.7 dB for 0.1 dB of gain and leaves about 100 mm of margin to the cliff; SWR is practically unchanged and 20 m and 15 m do not react to this element at all.

v0.3 introduced lighter 15 m elements (Ø20 centre + Ø16 tips instead of Ø25/20/16) with the REF 15m and DE 15m lengths and positions retuned. The thinner 15 m tubes cost a little bandwidth: SWR < 2 over 21.078–21.422 MHz, 2.6 / 2.2 at the band edges.

Computed with OK1M-PyminiNEC3 (MININEC 3 core). v0.4 was re-computed in MMANA-GAL pro: over 14 points across the three bands the two agree to R ±0.1 Ω, jX ±0.2 Ω, Ga ±0.04 dB, F/B ±0.14 dB, with the same segmentation (333 pulses at 28.5 MHz).

Built: Pantelleria, CQ WW DX SSB 2026

The first antenna built to this design was assembled on site on Pantelleria (JM56) in October 2026 and worked the contest as IH9/OK1M. All seven elements went together from the cut list below without changes to the model dimensions.

Assembly

Element plates, tube clamps and stainless hardware laid out on a table — Element plates marked by band, the Ø16–Ø30 tube clamps and the A2 hardware.

Mast-boom plate and an element plate bolted to the square boom — Mast-boom plate and an element plate on the 35×35×2 mm boom — see Plates.

The mast-boom plate from above with the U-bolt saddles fitted — The same plate with the mast U-bolt saddles fitted.

The boom with element plates resting on trestles, sea in the background — The one-piece 4 360 mm boom with the element plates in place.

Close-up of the split 20 m dipole with the current balun and the 10 m driven element beside it — The feed point: the split 20 m dipole with the 1:1 current balun, and the 10 m driven element 155 mm behind it — the open-sleeve coupling that feeds 15 m and 10 m from the same coax.

The complete seven element antenna assembled on trestles — All seven elements on the boom, ready to go up.

On the mast

The antenna on a guyed mast above the plateau, sea on the horizon — Up on the guyed mast, clear horizon to the sea.

The antenna from below against a deep blue sky, all seven elements visible — From below — the seven elements on the 4.36 m boom.

The antenna and the guy ropes against a sky with cirrus clouds — The 20 m pair sets the span — 11 m from tip to tip.

The stone house with the operating position under the pergola and the mast behind it — The station — operating position under the pergola, the mast behind the house.

Elements

Position is measured from REF 20 m (MMANA X from DE 20 m in brackets) and from the rear end of the boom. Lengths are the total tip-to-tip element length (model v0.4 rounded to mm).

Element	Band	Pos. from REF 20 m	From DE 20 m	From boom rear end	Length	Half	Tubes
REF 20m	20 m	0	−3 000	30	10 974	5 487	30 / 25 / 20 / 16
REF 15m	15 m	409	−2 591	439	7 130	3 565	20 / 16
REF 10m	10 m	1 452	−1 548	1 482	5 198	2 599	16
DE 15m	15 m	2 734	−266	2 764	6 846	3 423	20 / 16
DE 20m (feed)	20 m	3 000	0	3 030	10 294	5 147	30 / 25 / 20 / 16
DE 10m	10 m	3 155	+155	3 185	5 166	2 583	16
D1 10m	10 m	4 300	+1 300	4 330	4 788	2 394	16

Element design — Al tubes

Telescopic tubes, overlap 150 mm in every joint. Visible lengths measured from the boom axis:

Band	Center	2nd tube	3rd tube	Tip
20 m	30/2 — 2 × 1 200 (one piece 2 400)	25/2 — 1 200 (cut 1 350)	20/1.5 — 1 200 (cut 1 350)	16/1 — half − 3 600 (+150)
15 m	20/1.5 — 2 × 1 500 (one piece 3 000)	16/1 — half − 1 500 (+150)
10 m	16/1 — one tube, full length

DE 20 m is split in the center (2 × 30/2 1 200 mm) and mounted insulated on the dipole plate — this is the only feed point. DE 15 m and DE 10 m are continuous elements (open sleeve) coupled from DE 20 m.

Joints: slot the end of the outer tube and secure with a stainless hose clamp. Tip lengths are cut as listed; final trimming is done during SWR tuning.

Boom

One piece 35×35×2 mm, 4 360 mm — no joints, 30 mm overhang past the outer elements (cut from a 6 m bar).
Boom mass 3.1 kg — lighter than a stepped 30/35/30 boom (3.5 kg incl. the 325 mm joint overlaps), with the same max. static bending stress (approx. 38 MPa at the mast).
Mast plate at the computed center of gravity 1 900 mm from the rear end (1 130 mm behind DE 20 m) — check by balancing the finished antenna before final tightening.

Boom drilling drawing - hole positions measured from the rear end of the boom — Boom drilling — all hole positions measured from the rear end of the boom. Element plates are fixed by two M5 through bolts 40 mm apart (±20 mm from the element axis), the mast plate by four horizontal M6 bolts.

Construction drawing — elements

Element layout and half-element build details for 20, 15 and 10 m — A) element layout, B) 20 m half element, C) 15 m half element, D) 10 m elements, E) cut list.

Cut list

Element tubes

Tube	Qty	Length	Used in
30/2	1	2 400	REF 20m
30/2	2	1 200	DE 20m
25/2	4	1 350	REF 20m, DE 20m
20/1.5	2	3 000	REF 15m, DE 15m
20/1.5	4	1 350	REF 20m, DE 20m
16/1	1	5 166	DE 10m
16/1	1	5 198	REF 10m
16/1	1	4 788	D1 10m
16/1	2	2 215	REF 15m
16/1	2	2 073	DE 15m
16/1	2	2 037	REF 20m
16/1	2	1 697	DE 20m

Total tube length

30/2	4.80 m
25/2	5.40 m
20/1.5	11.40 m
16/1	31.20 m

Boom

35×35×2

1 × 4 360 (one piece)

Plates

Element plate 200×60×5	6
Dipole plate 300×60×5	1
Mast-boom plate 180×100×4	1
Clamp RAP430X (Ø30, A4)	6
Clamp RAP220X (Ø20, A2)	4
Clamp RAP216X (Ø16, A2)	6

Plates

Laser-cut aluminium plates (same as the 9el), DXF for cutting and PDF drawings in Downloads.

Element plate 200×60×5 mm — all elements except DE 20 m; slotted holes for U-bolts of Ø16–30 mm tubes.
Dipole plate 300×60×5 mm — DE 20 m, two insulated halves.
Mast-boom plate 180×100×4 mm.

Insulators — tube clamps

Element tubes are held by polypropylene tube clamps DIN 3015-1, light series A (e.g. Parker RAP…X), which also insulate the elements from the plates. Two clamps per element plate (160 mm apart), four on the dipole plate. The bolt distances match the slotted holes of the element plate.

Tube	Clamp	Size	b × c × width	Bolt distance	Qty	Elements
Ø30	RAP430X	A4	59 × 42 × 30	40	6	REF 20m (2), DE 20m (4)
Ø20	RAP220X	A2	42 × 33 × 30	26	4	REF 15m, DE 15m
Ø16	RAP216X	A2	42 × 33 × 30	26	6	REF 10m, DE 10m, D1 10m

Bill of materials

Complete parts list for one antenna (model v0.4). Mass approx. 15.9 kg without coax and balun. Element tube cut lengths are in the cut list.

Order — Al stock bars 6 m (EN AW-6060 / 6063 T6)

Profile	Bars	Cutting plan (kerf ~3 mm)	Offcut
Ø30×2	1	2 400 + 1 200 + 1 200	~1 190
Ø25×2	1	4 × 1 350	~590
Ø20×1.5	2	each bar 3 000 + 2 × 1 350	2 × ~290
Ø16×1	6	5 198 / 5 166 / 4 788 / 2 × (2 215 + 2 073 + 1 697) / 2 037 + 2 037	802, 834, 1 212, ~1 920
□35×35×2	1	4 360 (boom, one piece)	~1 640

Plates, insulators, hardware

Part	Qty	Used for
Element plate 200×60×5 (Al, laser cut)	6	all elements except DE 20m
Dipole plate 300×60×5	1	DE 20m
Mast-boom plate 180×100×4	1	mast
Clamp RAP430X (Ø30, A4)	6	REF 20m (2), DE 20m (4)
Clamp RAP220X (Ø20, A2)	4	REF 15m, DE 15m — check type in the catalogue
Clamp RAP216X (Ø16, A2)	6	REF 10m, DE 10m, D1 10m
Bolt M5×55 A2	12	Ø30 clamps
Bolt M5×45 A2	20	Ø20 and Ø16 clamps
Bolt M5×50 A2	14	plates to the boom (vertical)
Bolt M6×50 A2	4	mast plate to the boom (horizontal)
Bolt M5×40 A2 + ring cable lug M5	2	feed terminals DE 20m
Self-locking nut M5 / M6	48 / 4
Washer M5 / M6	80 / 8
U-bolt M8 with nuts and washers	2	mast clamp — choose for the mast diameter
Hose clamp W4 9 mm, 25–40 / 20–32 / 16–27	4 / 4 / 8	joints 30/25, 25/20, 20/16
End cap Ø16 / Ø30 / □35	14 / 2 / 2	element tips, DE 20m inner ends, boom ends
Contact grease for Al joints	1	all telescopic joints
1:1 current balun 50 Ω	1	DE 20m feed point
Coax 50 Ω, UV-resistant cable ties	—	as needed

SWR and field diagrams

Model v0.4, 12 m above real ground, computed with OK1M-PyminiNEC3 (MMANA-GAL pro conventions).

The patterns are a replica of the MMANA-GAL pro “Plots → Far fields” window (radial scale r/R = 10^dB/40, rings −3 / −10 / −20 / −30 / −40 dB) and show the total field; the Ga and F/B figures are for horizontal polarization.

20 m

20 m SWR, impedance, gain and F/B — SWR, impedance, gain and F/B 14.000–14.350 MHz

20 m radiation pattern — Pattern 14.175 MHz

15 m

15 m SWR, impedance, gain and F/B — SWR, impedance, gain and F/B 21.000–21.450 MHz

15 m radiation pattern — Pattern 21.225 MHz

10 m

10 m SWR, impedance, gain and F/B — SWR, impedance, gain and F/B 28.000–29.700 MHz (whole 10 m band; SWR < 2 over 28.047–28.687 MHz)

10 m radiation pattern — Pattern 28.350 MHz

Downloads

OK1M-7el-20-15-10m_v0.4.maa — MMANA-GAL model v0.4, REF 10m 5 198 mm, verified in MMANA-GAL pro (this page)
OK1M-7el-20-15-10m_v0.3.maa — MMANA-GAL model v0.3, REF 10m 5 100 mm
OK1M-7el-20-15-10m_v0.2.maa — MMANA-GAL model v0.2, 15 m elements 25/20/16 mm, verified in MMANA-GAL pro
Bill of materials — complete parts list, also BOM.md in the repository
boom.png, boom_drilling.png, build.png — construction drawings
element.dxf, element.pdf — element plate 200×60×5
dipole.dxf, dipole.pdf — dipole plate 300×60×5
mast-boom-plate.dxf, mast-boom-plate.pdf — mast-boom plate 180×100×4
drawings.py — Python script generating the construction drawings
Git repository — source of all files