Antennas · Volume 7
Multi-band & Specialty Dipoles
Off-center-fed (OCFD/Carolina Windom), fan dipole, trap dipole, doublet, G5RV, ZS6BKW, linked dipole, cage dipole — the multi-band wire-antenna problem and the dozen ways the ham community has solved it
Contents
1. About this volume
Volume 6 covered the half-wave dipole as a single-band reference antenna — the cleanest geometry, the cleanest math, the cleanest pattern. Real amateurs want to operate on more than one band. The textbook answer is “build one dipole per band and switch between them,” and that’s actually the right answer for a dedicated installation. But it’s also expensive, mechanically complex, and visually loud — three constraints most operators live under. The community’s response over 90 years has been a family of multi-band dipoles that compromise resonance, efficiency, or matching to cover more bands with a single wire.
This volume catalogs the six dominant solutions: the off-center-fed dipole (OCFD), the fan dipole, the trap dipole, the doublet, the G5RV / ZS6BKW family, the linked dipole, and the cage dipole (single-band but with the wider-bandwidth specialty case attached). Each gets a treatment of how it works, what it costs you in efficiency or convenience, where it wins, and where it falls down. The DIY build at §12 is a Buckmaster-style OCFD that covers 8 bands on a single feedline — the highest-utility-per-dollar multi-band wire antenna for a typical residential installation.
A note on scope: this volume is wire dipoles multi-banded by clever geometry. Multi-band vertical antennas (trap verticals, fan verticals) live in Vol 8 §7; end-fed half-waves with 49:1 matching transformers live in Vol 10; Yagi-Uda arrays multi-banded by trapping or LFA-style design live in Vol 11. The dividing line is structural: this volume covers center-fed (or near-center-fed) wire dipoles that span multiple bands.
2. The multi-band wire-antenna problem
A resonant half-wave dipole is single-band by construction (Vol 6 §5). Its feedpoint impedance is 73 Ω at its design frequency and rises to 300+ Ω at half that frequency (where the antenna looks like an inefficient end-fed quarter-wave) or 1500+ Ω at twice the design frequency (where it looks like a full-wave doublet at high impedance). Outside the resonant band, the SWR on 50 Ω coax climbs past 5:1 — beyond what a modern HF rig will tolerate without a tuner.
There are six structural approaches to giving a single wire multiple bands, each making a different trade:
| Approach | How it works | Trades away |
|---|---|---|
| Off-center feed (OCFD) | Shift the feedpoint off-center to find a position with similar impedance on harmonic resonances | Pattern symmetry, some power into BALUN heating |
| Parallel resonators (fan) | Several separate dipoles on a common feedpoint, each resonant on its own band | Mechanical complexity, element interaction |
| Series LC traps | Insert parallel-resonant LC tanks that act as open-circuits at the trap frequency | Per-trap power dissipation (0.5–1.5 dB per band), narrower per-band BW |
| High-impedance feed (doublet) | Feed a long resonator with low-loss balanced line into a tuner | Need balanced tuner, ladder line into the shack |
| Compromise length (G5RV / ZS6BKW) | Pick a specific length whose harmonic-resonance impedances happen to land within tuner reach | High SWR on some bands, tuner mandatory off-design |
| Switched links | Insert/remove jumpers to physically lengthen/shorten the antenna per band | Mechanical band-change; lower the antenna to switch |
A seventh approach — pure brute-force “wider bandwidth with fat elements or folding” — is also worth knowing, and the cage dipole in §9 covers it as a single-band specialty that buys 12–15% bandwidth on one band, useful for 80 m where the band itself is 500 kHz wide.
The decision tree:
What do you want?
│
┌──────────────┼──────────────┐
│ │ │
one-band a few bands many bands
wide BW (2-4 bands) (5+ bands)
│ │ │
cage dipole fan OCFD or
(§9) dipole doublet
(§4) (§3 or §6)
│
┌────────┴────────┐
│ │
"no tuner needed" "balanced ladder
Buckmaster line + tuner"
OCFD Doublet (§6)
(§3, §12)The four most common practical answers for a residential installation:
- Two or three bands, no tuner: fan dipole (§4) — clean per-band resonance, modest mechanical complexity.
- 80 / 40 / 20 / 15 m with one feedline: Buckmaster-style OCFD (§3, §12) — the canonical “modern multi-band wire.”
- All HF bands plus 6 m: doublet (§6) — needs ladder line and a balanced tuner, but highest efficiency.
- Field-portable, weight-conscious: linked dipole (§8) — superb per-band efficiency, slow band-change, lightest mechanical complexity.
3. Off-center-fed dipole (OCFD / Carolina Windom)
The OCFD is the dominant modern multi-band wire antenna because it does the most with the least: a single horizontal wire, one feedline, a 4:1 BALUN, eight bands of usable SWR. The trick is moving the feedpoint to a position where the harmonic resonance impedances happen to be similar.
3.1 How the off-center feed works
At a center feedpoint, a half-wave dipole presents 73 Ω, and its harmonics present 1500–4000 Ω (very high). At a 14% off-center feed (the standard Buckmaster position for an 8-band antenna), the same wire presents 200–300 Ω at its first resonance and 200–500 Ω at its second/fourth/eighth-harmonic resonances. The 200–300 Ω cluster maps to 50 Ω coax through a 4:1 BALUN (200/4 = 50 Ω), and the SWR stays usable across most bands.
short leg long leg
●═══════════●═══════════════════════════●
14% from end feedpoint at ~14% from one end
↑
FEEDPOINT (Z ≈ 200-300 Ω)
↑
4:1 current BALUN
↑
50 Ω coax to shackFor an 80 m design, the OCFD is approximately 134 ft (40.8 m) long, with the feedpoint 46 ft (14.0 m) from one end (the “short leg”) and 88 ft (26.8 m) from the other (the “long leg”). The 1.62:1 ratio between long and short legs is the key design constant — it positions the feedpoint at a current node on multiple harmonics simultaneously.
3.2 SWR profile across 8 bands
A well-built OCFD at the standard 14% feedpoint shows the following SWR pattern (with a 4:1 current BALUN, 50 Ω coax, h = 0.25λ on 80 m):
| Band | Resonance? | SWR at band center | Tuner needed? |
|---|---|---|---|
| 80 m | Yes — fundamental | 1.6–2.0:1 | No (1.5:1 in CW segment, 2.0:1 in phone) |
| 40 m | Yes — 2nd harmonic | 1.5–2.2:1 | No |
| 30 m | Compromised (off-harmonic) | 3–5:1 | Yes (tuner cleans it up easily) |
| 20 m | Yes — 4th harmonic | 1.4–2.0:1 | No |
| 17 m | Compromised | 2.5–3.5:1 | Maybe (depends on rig) |
| 15 m | Yes — 6th harmonic | 1.8–2.5:1 | No |
| 12 m | Yes (8th harmonic) | 2.2–3.0:1 | Borderline |
| 10 m | Yes (10th harmonic) | 1.8–2.5:1 | No |
| 6 m | Compromised | 3–5:1 | Yes |
The pattern: resonant or near-resonant on 80/40/20/15/12/10 m, compromised on 30/17/6 m. Calling it an “8-band antenna” is generous; “5 strong bands + 3 with-tuner bands” is more honest.
3.3 Carolina Windom variant — the choke is the antenna
The Carolina Windom (originally a Walter Maxwell W2DU design, marketed by Radio Works) adds a deliberate 22-foot length of vertical coax between the BALUN and a second common-mode choke, with the intent of radiating some power off the vertical coax segment. The thinking: vertical polarization on the descending coax gives a low-angle DX-favoring component, while the horizontal dipole element gives the main pattern. In practice this is a controversial design — the math says it works, but plenty of modeling-software runs show the vertical coax segment radiates only 1–2 dB worth of additional gain at low elevation angles, at the cost of common-mode noise pickup on receive and feedline pattern dependency.
The author’s view: a properly choked OCFD is simpler and equivalent in performance. The Carolina Windom is a useful historical design but offers limited practical advantage over a plain OCFD with two chokes (one at the feedpoint BALUN, one at the shack entry).
3.4 The BALUN — 4:1 current, sized for full power
The 4:1 BALUN at the OCFD feedpoint is the most-stressed component in the antenna system because it carries power on all 8 bands and has to maintain 4:1 impedance ratio across a 10:1 frequency span (3.5–30 MHz). The Mix-31 ferrite (2.4″ or 2.9″ toroid, bifilar-wound trifilar configuration) is the standard. Mix-43 is acceptable for 100 W operation but heats faster. Mix-52 (Fair-Rite’s newer HF/LF mix) is gaining popularity for its lower losses across the HF range.
| BALUN model | Topology | Power rating | Notes |
|---|---|---|---|
| Balun Designs 4115ocf | 4:1 current, Mix 31, 2.9″ toroid | 1.5 kW SSB | The canonical commercial choice; $110 |
| DX Engineering DXE-BAL050-H10-A4 | 4:1 current, Mix 31 | 1 kW SSB | $95 |
| Palomar Engineers PAR-EF-2K-MK2 | 4:1 current, dual toroid | 2 kW SSB | $130 |
| MFJ-916 | 4:1 current | 1.5 kW SSB | $55, lower build quality |
| DIY (FT240-31 × 2 trifilar) | 4:1 current, Mix 31 | 1.5 kW SSB | ~$45 parts; full instructions in Vol 16 §6 |
A second common-mode choke (string-of-beads or air-core CMC) at the shack entry, before the coax connects to the rig, is recommended even with a good BALUN at the feedpoint. The shack-end choke kills any common-mode current that snuck past the BALUN over the long feedline run.
4. Fan dipole — parallel resonators on one feedpoint
A fan dipole is the brute-force “two dipoles in parallel” solution: build separate half-wave dipoles for each band you want, solder them all to a common feedpoint, and arrange them so they fan out at slightly different angles. Each resonates on its own band; the inactive ones present high impedance and stay out of the way.
80m wire: ●═════════════════════════●═════════════════════════●
\
40m wire: ●══════════════━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━●
● ← common feedpoint
20m wire: ●━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━●
/
1:1 current BALUN
↑
50 Ω coax4.1 Pattern and impedance
On each band, the fan dipole behaves as if it were a single-band dipole — broadside figure-8 pattern, 73 Ω feedpoint (slightly modified by the inactive wires nearby), 5–8% bandwidth. The inactive wires present high impedance (1500+ Ω) on neighboring bands and act as “dead” structures with minor parasitic interaction.
The mechanical layout matters more than people realize. Wires spaced too close (<30 cm at the feedpoint, narrower than 30° fan angle) couple strongly and detune each other; wires spaced too far (>2 m at the ends, wider than 60° fan angle) reduce the available antenna real estate and look bad. The Goldilocks geometry: 30° fan angle between adjacent wires at the feedpoint, with end-spacers of light fiberglass rod every 1–2 m to keep the geometry stable in wind.
4.2 Building a fan dipole — the per-band trim problem
The wires interact when they’re close together. Tune each wire individually in air before mounting the fan, and you’ll find that adding the second and third wires shifts each previously-tuned wire by 50–200 kHz — usually down in frequency (the parallel wires add capacitance, lowering the resonance). The build sequence:
- Cut all wires 1–2% long
- Mount all wires on the feedpoint and on the end-spacers, in their final geometry
- Sweep each band; trim the per-band wire to bring its resonance to target
- Re-sweep all bands after each trim (the trim of one band shifts the others slightly)
- Iterate until all bands are within ~30 kHz of target
Expect 3–5 iterations. The fan dipole is the most tuning-intensive build of the multi-band options, but the per-band performance once tuned is the closest to single-band-dipole-equivalent of any multi-band design.
4.3 Best fan-dipole configurations
| Configuration | Footprint | Pros | Cons |
|---|---|---|---|
| 80/40/20 m three-band | 40 m × 4 m fan | Most-used HF bands; modest size; cheap | Detuned-by-interaction during build |
| 40/20/15/10 m four-band | 20 m × 3 m fan | Smaller; CW + phone on the high-traffic HF bands | Doesn’t include 80 m |
| 80/40 m two-band | 40 m × 2 m fan | Simplest possible multi-band | Only two bands |
The 80/40/20 m three-band fan dipole is the most-recommended “first multi-band antenna” because it covers the three highest-traffic HF bands with one feedline and no traps. Power handling is limited only by the wire and BALUN (~1.5 kW with a Mix-31 1:1 BALUN), and the per-band efficiency is essentially indistinguishable from a single-band dipole.
5. Trap dipole — LC traps for band-by-band resonance
A trap dipole inserts a parallel LC tank (a “trap”) in each leg at a specific frequency. Above the trap’s resonant frequency, the trap acts as a near-open-circuit and electrically isolates the wire beyond it — the antenna behaves as if it ended at the trap. Below the trap frequency, the trap acts as a small series inductor, slightly lengthening the effective wire. With careful trap design, a single wire can have multiple resonances on a single feedline.
◄─── 40m resonance section ───►
●━━━━━━━━━━━━━━━━━━━(trap @ 7 MHz)━━━━━━━━━━━━━━━━━━━●
\
●━━━━━━━━━━━━━━━━━━━(trap @ 14 MHz)━━━━━━━━━━━━━━━━━━●● ← center feedpoint
/
◄──── 20m resonance section ────►5.1 Trap design and the loss penalty
A trap is a parallel LC tank with Q in the 50–150 range. At the trap’s resonant frequency, the tank presents very high impedance (10s of kΩ) and the wire beyond it is electrically decoupled. Off the trap frequency, the tank looks reactive — capacitive below, inductive above — and the magnitude of the reactance is what determines how much of the wire beyond the trap “counts” toward the lower-frequency resonance.
The fundamental loss: every trap dissipates power. A 100 W signal on a band where the trap is in-circuit (not at the trap’s own resonance) loses 0.5–1.5 dB per trap to dielectric heating in the capacitor and resistive heating in the inductor. For a 3-band trap dipole with traps for 20 m and 40 m, the 80 m operation passes through both traps and loses 1.5–3 dB — a quarter of your transmitted power, often more.
The trap also narrows per-band bandwidth. The capacitive loading on the wire below the trap shifts the per-band Q from ~50 (clean dipole) to ~75–120 (trap-loaded), with bandwidth dropping correspondingly to 1.5–2.5% per band.
5.2 The trap dipole’s niche
Trap dipoles dominated the 1970s–1990s consumer-amateur market and are still sold (MFJ, Hy-Power, Hustler) for their physical compactness: a single wire shorter than a full-size 80 m dipole can cover 80/40/20 m, fitting on a lot that couldn’t support a fan. The Hustler 4-BTV trap vertical uses the same principle and is covered in Vol 8 §7.
For a wire dipole installation, the modern recommendation is avoid traps if you have the space for a fan dipole or an OCFD. Trap dipoles only justify themselves when antenna real estate is the dominant constraint — and at that point, an end-fed half-wave (Vol 10) or a doublet (§6) is usually a better answer.
| Trap dipole | Bands | Length | Loss penalty | Typical SWR BW per band |
|---|---|---|---|---|
| Hy-Power 3B-1500 | 80/40/20 m | 32 m (105 ft) | 1–2 dB on 80 m | 50 kHz on 80 m |
| MFJ-1778 (G5RV-style, not trap) | 80–10 m | 31 m (102 ft) | — (no traps) | varies |
| MFJ-17758 | 80/40/20/15/10 m | 32 m (105 ft) | 1.5–3 dB on 80 m | 30 kHz on 80 m |
| Diamond W-8010 | 80/40/20/15/10 m | 32 m | 1.5–3 dB | 30–50 kHz |
| Cushcraft D-40 | 40 m only (not multi-band) | 20 m | — | 250 kHz |
6. Doublet — open-wire-fed, tuner-fed all-band
A doublet is conceptually the simplest multi-band wire antenna: a long wire (typically λ/2 at the lowest band, but the exact length doesn’t matter much) fed at the center with balanced ladder line into a balanced tuner. The tuner does the impedance matching on every band; the antenna itself is “just a wire.”
●═══════════════════════════●═══════════════════════════●
↑
BALANCED feedpoint
║
║ 450 Ω ladder line
║ (low loss, balanced)
║
║
┌─────────┐
│ Balanced │
│ Tuner │ ← matches to 50 Ω at the rig
└─────────┘
│
50 Ω to rig6.1 Why ladder line, not coax
A doublet works because the feedline loss is low even at high SWR. Coaxial cable’s loss is specified for 50 Ω match; at 10:1 SWR, the coax loss can triple (a 0.5 dB-per-100-ft coax becomes 1.5 dB-per-100-ft at 10:1 SWR). Open-wire ladder line at the same SWR loses essentially the same amount as at 1:1, because the loss mechanism is conductor resistance (very low for open wire) rather than dielectric dissipation (significant in coaxial PE/PVC dielectric).
A 30 m run of 450 Ω ladder line has total round-trip loss of about 0.15 dB at any SWR up to 20:1. The same 30 m of LMR-400 coax at 10:1 SWR has 1.5 dB of round-trip loss. The tuner at the shack end takes whatever impedance the antenna+feedline presents (often a few hundred ohms reactive) and transforms it to 50 Ω for the rig — and because the feedline didn’t lose much, the rig delivers nearly all its power to the antenna regardless of band.
6.2 Doublet lengths and per-band behavior
The classic doublet lengths are based on convenient multiples:
| Doublet length | Lowest band | Bands covered | Notes |
|---|---|---|---|
| 31 m (102 ft) | 80 m (just barely) | 80–10 m | Classic “all-band” doublet |
| 40 m (135 ft) | 80 m comfortably | 80–10 m | More efficient on 80 m |
| 20 m (66 ft) | 40 m | 40–10 m | ”Half-size” doublet for smaller lots |
| 13 m (44 ft) | 20 m | 20–10 m | QRP/portable doublet |
A 31 m doublet works on all amateur bands from 80 m through 10 m, with the impedance the tuner sees varying from very-low (~15 Ω on 80 m where the antenna is electrically short) to high (~3000 Ω on 20 m where the antenna is full-wave). A good balanced tuner (Palstar AT2K, MFJ-976, LDG RT-100 with balanced output) handles this range; an unbalanced tuner with a 4:1 BALUN at the output is the common workaround but introduces 0.5–1 dB of BALUN loss.
6.3 Doublet pros and cons
Pros:
- Genuinely all-band: 80 m to 6 m with one antenna, one feedline.
- Highest efficiency of any multi-band wire antenna (no traps, no compromise BALUN, no off-resonance heat dissipation in lossy elements).
- Cheap to build: wire + ladder line + balanced tuner you already own.
- The tuner does the matching work on the bench, where you can tweak it for each band — fast band-change once tuner settings are known.
Cons:
- Requires balanced ladder line, which can’t be coiled or buried like coax. Routing into the shack requires a feed-through panel and short coax pigtail (with attendant common-mode chokes).
- Balanced tuner ($300–600 used market for a quality one — LDG RT-100, Palstar AT2K, Johnson Matchbox) — not cheap if you don’t already own one.
- Ladder line picks up RFI more than coax, and any imbalance in the ladder line’s spacing or routing throws off the balance.
- 31 m of horizontal wire is visible and noisy in the wind.
The doublet is the purist’s multi-band antenna. If you have a balanced tuner, a clean ladder line run into the shack, and the real estate for a 31 m wire, it outperforms an OCFD on efficiency at the cost of mechanical fuss. For most installations, the OCFD’s “plug coax into the rig and go” convenience wins over the doublet’s marginal efficiency advantage.
7. G5RV and ZS6BKW — the 102-foot solutions
The G5RV is the most-famous and most-misunderstood multi-band wire antenna. Designed by Louis Varney (G5RV) in 1946 and published in 1958, the G5RV is a 102-foot horizontal wire fed at the center by 31 feet of 450 Ω ladder line which then transitions to coax. Varney designed it as a 20 m antenna — the geometry gives 3 half-waves on 20 m (collinear, with ~3 dB gain over a single dipole on 20 m), and the ladder-line section acts as an impedance transformer. The multi-band coverage is a fortunate accident of geometry, not Varney’s original design intent.
7.1 Why the G5RV works on multiple bands
The 102-foot horizontal wire is approximately:
- 0.62λ on 80 m (electrically short, very high SWR at the feedpoint)
- 1.24λ on 40 m (1.5 wavelengths, moderate impedance)
- 2.5λ on 20 m (the original design — collinear 3 half-waves)
- 3.7λ on 15 m
- 5.0λ on 10 m
The 31 feet of 450 Ω ladder line is a quarter-wavelength on 20 m, half-wavelength on 10 m, and various fractional wavelengths on the other bands. At each band, the combination of antenna impedance and feedline transformation lands in a roughly tuner-friendly range — but never at 1:1 SWR on coax.
The ladder-to-coax transition is where the G5RV’s behavior gets controversial. Without a BALUN, the unbalanced coax is loading the balanced ladder line and introducing common-mode currents on the coax shield. With a 4:1 BALUN, the impedance step helps on some bands but compromises others. With a 1:1 current BALUN at the ladder-to-coax junction, common-mode is controlled but the impedance match worsens. The “right” answer depends on which band you operate most.
7.2 G5RV SWR profile
A typical G5RV (102 ft + 31 ft ladder + coax + 1:1 BALUN) shows the following SWR at the coax end:
| Band | SWR | Tuner needed |
|---|---|---|
| 80 m | 5–10:1 | Yes |
| 40 m | 3–5:1 | Yes |
| 30 m | 6–10:1 | Yes |
| 20 m | 1.5–2.5:1 | No (the design band) |
| 17 m | 4–6:1 | Yes |
| 15 m | 2–3:1 | Maybe |
| 12 m | 4–6:1 | Yes |
| 10 m | 2.5–4:1 | Maybe |
Honest assessment: the G5RV is a 20 m antenna that happens to be usable on other bands with a tuner. Calling it “all-band” is marketing. The original G5RV at full-size is still a fine 20 m antenna; the “G5RV Jr.” (half-size, 51 ft + 15 ft ladder) covers 40–10 m with the same design philosophy.
7.3 ZS6BKW — the optimized alternative
Brian Austin (ZS6BKW) published an optimized variant in 1980: a 95-foot wire fed by 39 feet of 400 Ω feedline. The geometry shifts the harmonic impedances so that 5 of the 6 most-used bands (40/20/17/12/10) present <2.5:1 SWR without a tuner. The remaining bands (80, 30, 15) still need a tuner, but the no-tuner band count doubles compared to the G5RV.
The ZS6BKW’s design philosophy: instead of accepting the G5RV’s “good on 20 m, mediocre elsewhere” tradeoff, deliberately engineer the geometry for multiple acceptable bands. This works, but the antenna becomes slightly more sensitive to height-above-ground and end-effect than the G5RV.
A modern installation choice between G5RV and ZS6BKW: if you have a tuner and want a known-good design with 70 years of community knowledge, build a G5RV. If you want to operate barefoot (no tuner) on as many bands as possible and have the patience to tune the antenna carefully to its design dimensions, build a ZS6BKW.
8. Linked dipole — manual band-swap by inserting links
A linked dipole is conceptually the simplest multi-band design: build a dipole long enough for the lowest band, with small electrical “links” (jumpers, alligator clips, or proper detachable pins) at each band’s resonant point. To operate on a higher band, physically lower the antenna, open the links beyond that band’s resonant length, and re-hoist.
80m resonance (links closed):
●═════════●═════●═════●═════════●═════════●═════●═════●═════════●
↑ ↑ ↑ ↑ ↑
link link feedpoint link link
open open open open
for 40m for 20m for 20m for 40m
On 40m (links @ 40m position OPEN, others CLOSED):
●═════════●═════●═════●═════════●═════════●═════●═════●═════════●
↑
feedpoint, antenna now 40m half-wave8.1 Why the linked dipole has a niche
The trade is straightforward: per-band efficiency is full half-wave (no traps, no off-resonance current distribution, no BALUN compromise) at the cost of slow band-change (climb to the antenna, or lower it, to change links). For most fixed installations the band-change time is unacceptable. For two specific use cases, the linked dipole is the right answer:
- SOTA (Summits On The Air) and POTA (Parks On The Air) activations — operators set up at a single fixed location, work one or two bands, then move on. A linked dipole at full per-band efficiency is the SOTA-community-favorite portable HF antenna.
- Field Day-style multi-station operation — when each band has its own station and the antennas are deployed for the duration of an event, the linked dipole’s per-band efficiency dominates over multi-band compromise antennas.
8.2 The SOTABEAMS Band Hopper reference design
SOTABEAMS (UK) sells a 40/30/20 m linked dipole kit (the Band Hopper) that has become the de-facto SOTA standard. The geometry:
- 40 m wire length: 20 m of #20 stranded wire (lightweight for portability)
- 30 m links: insertable jumpers at 14.0 m from the feedpoint
- 20 m links: insertable jumpers at 10.0 m from the feedpoint
- Feedpoint: small fiberglass center insulator with 1:1 current BALUN (Mix 43 toroid, lightweight)
- End insulators: lightweight plastic
- Weight: ~250 g total
- Cost: ~$90
The linked dipole design has been extended in the SOTA community to include 80 m (with another set of links), and to use a single mast at the apex for inverted-V deployment. The Sotabeams kit is the canonical reference; many operators build their own with lighter wire (#22 or #24 magnet wire for ultra-low-weight backpack carry).
8.3 DIY linked dipole
Build from scratch: cut wire long enough for the lowest band, splice in screw-terminal blocks or banana-plug jumpers at each band’s resonant length (cut the resonance point with the antenna in air, then trim 0.5–1% short to account for end-effect at the higher band’s resonance). The screw-terminal-block approach is field-serviceable; banana-plug jumpers are faster to switch but less reliable in wet conditions.
A typical SOTA-style 40/30/20 m linked dipole BOM:
| Part | Specification | Mid-2026 price |
|---|---|---|
| #20 stranded copper wire, 20 m | DX Engineering DXE-ANTW-20 (~$0.45/ft) | $30 |
| 2× ceramic end insulators | Glen Martin EI-1 | $10 |
| 4× link jumpers | Pomona test-clip with 5 mm banana plug | $20 |
| Center insulator (lightweight, fiberglass) | SOTABEAMS / DIY 3D-printed | $15 |
| 1:1 current BALUN (Mix 43, lightweight) | DIY FT114-43 with 8 turns #20 enamel | $15 |
| Halyard rope (Dacron 3 mm, 30 m) | Synthetic Textiles 1/8″ | $15 |
| Coax pigtail (RG-316, 1 m, with BNC) | $20 | |
| Total | ~$125 |
Lighter than a full-size 80 m dipole; per-band performance equal to a single-band dipole; the band-change penalty is “lower the dipole, swap a link, re-hoist” — about 5 minutes including re-trimming for changing wind.
9. Cage dipole — wide-bandwidth single-band
The cage dipole is the only single-band entry in this volume, but it lives here because it’s the multi-band-adjacent answer to the question “I want to cover all of 80 m with one antenna and a tuner-free SWR.” Instead of multi-banding by traps or feed shifts, the cage dipole widens the bandwidth of a single-band dipole enough that the entire band lives inside the 2:1 SWR window.
9.1 The geometry: parallel wires forming a “cage”
A cage dipole is a half-wave dipole built from 4–6 parallel wires arranged in a cylindrical “cage” pattern, shorted at the ends and tied together at the center feedpoint. The parallel wires electrically form a much fatter conductor — equivalent diameter is roughly the cage’s circumscribed cylinder diameter (10–30 cm typical, depending on wire spacing).
End ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● End
╱ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ╲
╱ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ╲
╱ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ╲
●─━━━━━●━━━●━━━●━━━●━━━●━━━●━━━●━━━●━━━●━━━●━━━●━━━●━━━━●━●
feed spreaders every ~1 m hold the wires at constant spacing
pointThe bandwidth-widening mechanism: the dipole’s Q is proportional to its length-to-diameter ratio. A #14 wire dipole has Q ≈ 50 (bandwidth ~5%). A cage dipole with effective diameter 20 cm has Q ≈ 15 (bandwidth ~15%). The radiation pattern is essentially unchanged.
9.2 When the cage dipole wins
The cage dipole is overkill for any HF band narrower than its native bandwidth — a #14 dipole easily covers all 350 kHz of 40 m at 2:1, so the cage approach adds nothing. The cage dipole earns its keep on:
- 80 m — a full-size 80 m dipole’s 130 kHz at 2:1 misses 75% of the 500 kHz band. A cage dipole’s 600 kHz at 2:1 covers the entire band including the 75 m phone segment.
- 160 m — same problem as 80 m, only more acute. A cage dipole on 160 m can cover both CW and phone segments.
- 6 m wide-coverage — for the rare operator who works the full 50–54 MHz range with one antenna.
- VHF/UHF reception — wideband receivers (scanner, SDR) benefit from a cage dipole’s flatness across a wide frequency range.
9.3 The mechanical reality
A cage dipole is not a simple build. The wire-spacer geometry must be maintained over the antenna’s full length, and the spacers are wind-loaded. A typical 80 m cage dipole has 30+ fiberglass spreaders (PVC works for low power; fiberglass is the proper material), each ~30 cm long, distributed every 2 m along the antenna. The build is fussy and the wind load is significant — count on 2–3× the wind load of a plain wire dipole.
Despite the complexity, the cage dipole is the cleanest “wide-bandwidth single-band” solution. The doublet’s all-band approach is more flexible, but the cage dipole’s no-tuner-needed-anywhere-in-band convenience is unmatched for the operator who lives on a single band.
10. Best-case and worst-case use across the variants
Summary matrix — pick the row that matches your constraints:
| Variant | Best when | Worst when | Per-band gain | Mech complexity |
|---|---|---|---|---|
| OCFD | Multi-band convenience, single feedline, no tuner | Pattern symmetry critical | -0.5 to 0 dBd | Low |
| Fan dipole | 2–3 bands, clean per-band performance | Mechanical fuss is unacceptable | 0 dBd | Medium |
| Trap dipole | Real estate limits demand a shorter antenna | Per-band efficiency matters | -1.5 to -0.5 dBd | Medium |
| Doublet | All bands, highest efficiency, tuner already owned | Ladder line into shack is impossible | 0 dBd (across bands) | Medium |
| G5RV | 20 m primary band, occasional other bands | All-band performance expected | -1 to +1 dBd (varies) | Low |
| ZS6BKW | 5 no-tuner bands wanted, willing to trim carefully | Carelessness will detune the design | -0.5 to +0.5 dBd | Low |
| Linked dipole | Field portable, slow band-change OK | Fixed installation, fast band-change needed | 0 dBd | Low |
| Cage dipole | Single band, full-band SWR coverage needed | Multi-band coverage wanted | 0 dBd | High |
The two strongest defaults: OCFD for fixed installations with a single feedline run, doublet for fixed installations with a balanced tuner and ladder-line tolerance. For portable, linked dipole. For 80 m no-tuner full-band coverage, cage dipole.
11. Power handling
Multi-band dipoles all share the same three power-handling limits as single-band dipoles (Vol 6 §9): wire diameter, end-insulator dielectric strength, and BALUN saturation. The added concern: per-band losses in trap-loaded and BALUN-shifted designs heat the BALUN harder than a single-band dipole would.
| Antenna | Limit | Typical max continuous |
|---|---|---|
| OCFD with Mix-31 4:1 BALUN | BALUN saturation | 1.5 kW SSB / 1 kW CW |
| OCFD with Mix-52 4:1 BALUN | BALUN saturation | 2 kW SSB / 1.5 kW CW |
| Fan dipole with Mix-31 1:1 BALUN | BALUN saturation | 1.5 kW SSB / 1 kW CW |
| Trap dipole (3-trap) | Trap dissipation | 800 W on lowest band (heaviest trap load) |
| Trap dipole (5-trap) | Trap dissipation | 500 W on lowest band |
| Doublet (ladder line + balanced tuner) | Tuner & ladder spacing | Tuner-limited (often 1 kW for amateur tuners) |
| G5RV | BALUN + ladder line | 1 kW PEP |
| Linked dipole (ultralight #20 wire) | Wire heating | 200–400 W |
| Cage dipole (8× #14 in cage) | BALUN | 2 kW SSB easily |
The amplifier-running 1.5 kW SSB amateur should preferentially build an OCFD with a 2 kW-rated BALUN (Palomar PAR-EF-2K-MK2 or Balun Designs 4115ocf-2K) or a doublet with a high-power balanced tuner (Palstar AT5K). Trap dipoles in particular don’t scale well to amplifier power — the traps become the limit, and a saturated trap fails by burning open mid-transmission (audible “ping” followed by SWR going to infinity on the trap’s design band).
12. DIY build — a Buckmaster-style OCFD for 80/40/20/17/15/12/10/6 m
This is the canonical modern multi-band wire antenna build. About 3 hours of work plus tuning time. Total parts cost ~$200 USD at mid-2026 prices.
12.1 The geometry
A standard 8-band OCFD has total length 134 feet (40.84 m), with the feedpoint at 14% of total length from one end:
- Long leg (from feedpoint to far end): 86% × 134 = 115.2 ft (35.13 m)
- Short leg (from feedpoint to near end): 14% × 134 = 18.8 ft (5.73 m)
Slightly different proportions exist in the literature (12% vs 14% vs 17%), each optimized for a different band coverage. The 14% configuration is the Buckmaster standard for an 8-band antenna and gives the broadest SWR coverage. The 17% configuration (“Carolina Windom” variant) puts the feedpoint slightly closer to the end and slightly improves 80 m at the cost of 6 m.
12.2 Bill of materials
| Part | Specification | Source | Mid-2026 price |
|---|---|---|---|
| Antenna wire | #14 AWG copper, 7-strand insulated, ~135 ft (41 m) | DX Engineering DXE-ANTW-14B ($0.92/ft) or Wireman 534 hard-drawn bare ($0.55/ft) | $40–55 |
| End insulators (2×) | Ceramic egg, 30 kV rated | DX Engineering DXE-ISD-25 ($4.50 each) | $9 |
| Center insulator + BALUN enclosure | Combined unit | Balun Designs CI-4115 (center insulator + 4:1 1.5 kW BALUN combo, $145) | $145 |
| Alternatively (DIY BALUN + plain center insulator): | |||
| - Plain center insulator | Plastic body with strain-relief eyes | DX Engineering DXE-COA-1 | $25 |
| - 4:1 current BALUN, Mix 31 | DIY FT240-31 × 2 trifilar configuration | Toroids + #14 enamel wire | $45 |
| - Weatherproof BALUN enclosure | Hammond 1554 series polycarbonate | $20 | $90 (DIY route) |
| Halyard rope, Dacron 1/8″, 30 m total | UV-stable, low-stretch | DX Engineering DXE-DACRON | $25 |
| Coax pigtail | RG-8X, 2 m, with field PL-259 | Times Microwave | $20 |
| Coax-Seal + 3M 33+ | Weatherproofing | $12 | |
| Total (commercial BALUN combo) | ~$251 | ||
| Total (DIY BALUN) | ~$200 |
The commercial Balun Designs combo is the conservative choice — pre-built, weatherproof, full power-rated. The DIY route saves $50 at the cost of a winding session and a more careful weatherproofing job.
12.3 Step-by-step construction
Cut the wires. Cut a short leg of 19.0 ft (5.79 m) and a long leg of 116.5 ft (35.51 m). Both are about 0.5% long for trim headroom. Cut long, trim later.
Terminate the ends. Each wire end loops through the ceramic insulator’s hole, doubles back 15 cm, and is wrapped 6–8 turns around the standing wire then soldered. Same procedure as a single-band dipole (Vol 6 §10.2).
Terminate the inboard ends. Each leg’s inboard end terminates at the BALUN’s antenna terminals. For the Balun Designs CI-4115 combo, the antenna terminals are bolts; bond each leg’s wire with a soldered ring lug.
Install the BALUN. Mount the BALUN at the center insulator location. For a commercial CI-4115 the center insulator and BALUN are one unit; for the DIY approach, mount the BALUN box adjacent to the center insulator with a short jumper between the antenna terminals and the BALUN’s antenna inputs. The BALUN’s coax output goes to the feedline via a PL-259 or N connector.
Add halyards. Each ceramic end insulator gets a Dacron halyard, sized for the antenna’s deployment geometry. Plan for the long-leg end to be ~38 m from the support point and the short-leg end to be ~8 m from a closer support.
Hoist and measure. Deploy at 12–15 m (40–50 ft) for fixed installation. Sweep 3.0–30.0 MHz with the NanoVNA at the feedline end. Confirm the SWR profile matches §3.2’s table — resonances near 3.65 / 7.15 / 14.2 / 21.2 / 28.5 MHz should all show SWR < 2.5:1.
Trim. OCFD trimming is uneven: most of the trim happens to the long leg (95% of the wire’s electrical length is in the long leg, so most adjustments go there). If the 80 m resonance is at 3.95 MHz (too high), the antenna needs lengthening — add 60 cm to the long leg. If it’s at 3.45 MHz (too low), trim 60 cm from the long leg. The short leg requires only fine adjustment after the long leg is in place.
After 80 m is tuned, check the other bands. The 14% geometry is self-consistent: 80 m being correct means the other harmonic-resonant bands (40, 20, 15, 10) will also fall close to their design frequencies. If 80 m is right but 20 m is off by >100 kHz, the geometry is off (the 14%/86% ratio isn’t right) — re-measure leg lengths.
Lock and weatherproof. Same procedure as the single-band build (Vol 6 §10.2). Particular attention to the BALUN-to-coax junction; the OCFD’s heavy off-band loading on the BALUN means any moisture intrusion at the connector causes accelerated corrosion.
12.4 Tuning verification
A successful OCFD build has the following NanoVNA signature:
- 80 m: SWR minimum at 3.50–3.85 MHz (depending on the operator’s preferred CW vs phone bias), with the 2:1-SWR window covering ~150 kHz around the minimum.
- 40 m: SWR minimum at 7.05–7.20 MHz, 2:1 window ~200 kHz.
- 20 m: SWR minimum at 14.05–14.30 MHz, 2:1 window ~300 kHz.
- 15 m: SWR minimum at 21.00–21.30 MHz, 2:1 window ~300 kHz.
- 12 m: SWR minimum at ~24.9 MHz, 2:1 window ~200 kHz.
- 10 m: SWR minimum at ~28.4 MHz, 2:1 window ~400 kHz.
- 30 m, 17 m, 6 m: SWR > 2:1 across the band, but typically < 4:1 — tuner-friendly.
If the 80 m and 40 m minima are correct (both within ~50 kHz of design) but the 20 m and higher bands are off, the BALUN itself is suspect — it’s not maintaining 4:1 ratio across the HF range, and a different ferrite mix or more turns is needed. Mix-31 with the standard trifilar configuration works for almost everyone; if it doesn’t work for you, consider Mix-52 (newer, better HF ratio stability).
13. Commercial buys
Sorted by price tier (USD, mid-2026). Vendor coverage is North-America-centric; European equivalents noted where relevant.
| Tier | Model | Bands | Price | Notes |
|---|---|---|---|---|
| Budget | MFJ-1778 (G5RV-style) | 80–10 m (tuner-dependent) | $90 | Real G5RV at 102 ft; works but the BALUN is modest. |
| Budget | Carolina Windom (Radio Works) | 80–10 m | $120 | The original Carolina Windom; vertical-coax-radiates design. |
| Budget | MFJ-1740 multi-band trap dipole | 80/40/20/15/10 m | $130 | Five-band trap dipole; OK build quality, lossy traps. |
| Budget | SOTABEAMS Band Hopper 4 | 40/30/20/17 m linked | $130 | EU-popular linked-dipole kit for SOTA/POTA. |
| Mid | Buckmaster 7300-OCF | 80/40/20/17/15/12/10/6 m | $230 | The reference OCFD. Buckmaster designed and tested the 14% geometry; built to amateur-radio quality standards. |
| Mid | DX Engineering Resonator OCF | 80/40/20/15/10 m | $280 | DX Engineering’s house design; well-built, slightly different geometry from Buckmaster. |
| Mid | Balun Designs OCFD 8-band kit | 80–10 m | $320 | Includes the 1.5 kW Mix-31 BALUN; user-assembled. |
| Mid | Alpha Antenna ProMaster | 80/40/20/15/10 m | $300 | Multi-band antenna with included matching network. |
| Mid | NCG Ferrite OCFD | 80–10 m | $290 | Multiple ferrite mix options; user-selectable for environment. |
| Premium | Carolina Windom 80SSD | 80–10 m | $450 | Premium Carolina Windom build — heavy-duty hardware, high-quality BALUN. |
| Premium | Inrad 7-band kit | 80/40/20/15/12/10/6 m | $500 | Higher-power BALUN (2 kW), better hardware. |
| Premium | Force 12 monoband dipole pair (fan-mounted) | per-band pair | $400–700 | Two single-band Force 12 dipoles mounted on common feedpoint = fan dipole equivalent at premium build. |
| Premium | InnovAntennas wire OCFD | 80/40/20/15/10/6 m | $400 | UK manufacturer; clean build, well-documented. |
What to avoid:
- Cheap “8-band miracle antennas” without a published BALUN saturation rating — these are usually 5-band antennas with marketing claims for 8.
- “Compact 80m+ antennas” shorter than 60 ft — physics says these are bottom-loaded antennas with significant efficiency loss; they work but they’re not what’s advertised.
- “Multi-band trap dipoles” with more than 4 traps — every trap adds 0.5–1.5 dB of loss; a 5-trap antenna is losing 3–7 dB across the bands. Use an OCFD or doublet instead.
- “All-band antennas with no tuner needed” — physics says this can’t be true for HF over a 10:1 frequency range. The honest “no-tuner antennas” cover 3 strong bands and 3 marginal ones.
EU/UK alternatives: SOTABEAMS Band Hopper series (linked dipoles), Wimo G5RV, Diamond G5RV-OCF, Cobwebb cubical-quad-style multi-band loop antennas (different topology from this volume but achieve similar multi-band coverage).
14. Companion gear
Multi-band wire antennas need slightly more support than single-band dipoles:
- At least one mast or central support capable of holding the BALUN/feedpoint at the designed height — 12–15 m typical. A free-standing mast (Spiderbeam 12 m fiberglass, Jackite 31′) works; a tree branch with a properly halyarded support point works equally well.
- Two end supports for the OCFD’s asymmetric geometry — the short leg’s end support is closer to the center, the long leg’s end support is farther. For Yagi-style multi-band antennas with horizontal layout, three supports (one per end and one center) is the cleanest geometry.
- Robust halyards — Dacron 1/8″ minimum; UV-stable polyester is acceptable; never nylon. For OCFD installations on residential lots, plan for ~80 m of total halyard length.
- A balanced tuner if going the doublet route — Palstar AT2K, MFJ-976, LDG RT-100 with balanced output, or a vintage Johnson Matchbox. Budget $300–600 for a quality used unit.
- A second common-mode choke at the shack entry — even with a 4:1 BALUN at the OCFD feedpoint, a string-of-beads (50× Mix 43) choke at the shack-entry feedthrough kills residual common-mode current and isolates the shack from feedline radiation pickup.
- Lightning protection — same polyphaser arrestor and single-point ground topology as in Vol 6 §12 — applies equally to multi-band antennas. The OCFD’s extra band coverage doesn’t change the lightning-protection requirements.
15. Common gotchas and myths
- “OCFD doesn’t need a tuner” — usually true on 7 of 8 bands; on the 8th (typically 30 m or 17 m) the SWR is 3:1+ and a tuner is needed. “No tuner needed” is marketing; “no tuner needed on most bands” is honest.
- “G5RV is multi-band” — true with a tuner. Without a tuner, the G5RV is a 20 m antenna with bonus bands at high SWR. The “all-band G5RV” claim assumes the operator has a tuner; it doesn’t mean the antenna works barefoot on every band.
- “Traps don’t lose power” — traps absolutely lose power: 0.5–1.5 dB per trap per band of off-resonance operation. A 3-trap antenna on 80 m loses 3–4 dB worth of transmitter power to trap heating. The losses are real, measurable, and reproducible.
- “The BALUN ratio is approximate” — BALUN ratios are nominal, not exact. A “4:1” BALUN typically presents 4:1 ± 25% across the HF range, and the variation matters for OCFD design. This is why the OCFD’s SWR varies band-to-band even with a perfect geometry — the BALUN is the variable.
- “Higher impedance feedline reduces loss” — true for the same physical conductor. Switching from RG-8X coax (50 Ω) to 450 Ω ladder line drops loss because the ladder line is air-dielectric and has very low conductor losses; the high-Z is incidental. The lesson: prefer low-loss feedlines, not specifically high-impedance ones.
- “My antenna gets out on all bands so it must be tuned” — antennas radiate (some) regardless of SWR. A 10:1-SWR antenna still radiates some of the power applied to it; the rest reflects to the rig where the protection circuit either folds back or accepts it. “Getting out on band X” doesn’t mean the antenna is matched; it means the antenna radiates a fraction of the transmitter’s power. Verify SWR with a meter — don’t infer it from “I made a contact.”
- “The OCFD’s pattern is the same on every band” — false. On the design band (80 m, fundamental), the pattern is broadside figure-8. On higher bands (where the antenna is electrically multiple wavelengths), the pattern fragments into multiple lobes with off-axis maxima. A 14 MHz OCFD on a 134-ft wire is operating as a 2.5λ antenna with at least 6 main lobes. The pattern is messy but the average gain across azimuth is still ~2 dBd, so most paths work.
- “My BALUN doesn’t get hot, so it’s working fine” — heat is a symptom of saturation, which happens at high power. At 100 W an undersized BALUN won’t visibly heat, but it may still be presenting the wrong ratio and degrading the SWR. The proper check is the SWR profile: if it matches the expected pattern (3.65 / 7.15 / 14.2 / 21.2 / 28.5 MHz minima), the BALUN is doing its job.
16. Resources
- ARRL Antenna Book Ch. 7 (multi-band antennas) — the canonical multi-band reference.
- Louis Varney G5RV original 1958 article (RSGB Bulletin) — the foundational G5RV paper.
- Brian Austin ZS6BKW optimization paper (Radio Communication, 1980) — the ZS6BKW geometry derivation.
- Buckmaster Tech Notes — OCFD design notes from the modern reference vendor.
- Walter Maxwell W2DU, Reflections (3rd ed.) — the standing-wave / impedance / BALUN bible; clarifies a lot of OCFD myths.
- Sevick, Transmission Line Transformers (5th ed.) — the BALUN-design reference; covers 4:1 current BALUNs in detail.
- L. B. Cebik (W4RNL) papers on OCFD modeling — archived at the antenneX library.
- N5DUX OCFD calculator — online tool for OCFD geometry across different design proportions.