Build a Fan Dipole Antenna
A fan dipole uses multiple pairs of wire legs — each resonant on a different band — sharing a single feedpoint and a single coax run. The result is true resonant performance on three, four, or five bands without a tuner, without traps, and without the efficiency penalty of complex matching systems. The fan dipole is arguably the best multi-band wire antenna available: clean SWR on every covered band, no moving parts, and simple construction from basic materials. This guide builds a 3-band fan dipole for 40m, 20m, and 10m — covering the three most active HF contest and DX bands from a single installation.
Parallel Resonant Elements — The Core Principle
A fan dipole works because each pair of wire legs is cut to resonate on its target band. At the resonant frequency of a given leg pair, that pair presents a low impedance at the feedpoint — approximately 73Ω for a half-wave dipole — and draws power from the feedline. The other leg pairs, which are not resonant on this frequency, present high impedance at the feedpoint and take very little power. The antenna effectively selects which leg pair is active based on the operating frequency.
This behavior is why the fan dipole does not need a tuner — at each operating frequency, the resonant leg pair naturally presents a low-SWR feedpoint to the coax. The non-resonant leg pairs are electrically "quiet" on that band.
- At 7.150 MHz: the 40m legs are resonant (~73Ω), 20m and 10m legs present high impedance
- At 14.200 MHz: the 20m legs are resonant (~73Ω), 40m and 10m legs present high impedance
- At 28.500 MHz: the 10m legs are resonant (~73Ω), 40m and 20m legs present high impedance
- Slight interaction between leg pairs exists but is typically small — usually less than 50 kHz shift from the isolated dipole resonance
Leg Pair Interaction — What to Expect
The non-resonant leg pairs in a fan dipole are not completely invisible — they interact slightly with the resonant pair through mutual coupling and capacitive effects. This interaction is the main source of complexity when tuning a fan dipole. Understanding it helps you work through the tuning process efficiently:
- The 40m legs slightly affect 20m resonance: the long 40m legs hanging near the 20m legs add some capacitive loading to the 20m legs, slightly lowering 20m resonance. Cut 20m legs slightly shorter than the standalone formula predicts to compensate.
- The 40m legs slightly affect 10m resonance: similar effect, typically less than 100 kHz shift.
- 20m legs have minimal effect on 40m: the short 20m legs are far off-resonance on 40m — their effect on the longer 40m legs is very small.
- Angular separation reduces interaction: spreading the leg pairs at 15–30° angles from each other reduces mutual coupling and makes each pair more independent.
- Tuning sequence: always tune 40m first, then 20m, then 10m — adjusting a lower-band pair has more effect on higher bands than vice versa.
| Band | Target Freq | Calculated leg (ft) | Cut leg to (ft) | Expected after interaction (ft) | Notes |
|---|---|---|---|---|---|
| 80m | 3.750 MHz | 62.4 ft | 64.3 ft | 62–64 ft | Add 80m legs to cover 80/40/20/10m (4-band fan) |
| 40m | 7.150 MHz | 32.7 ft | 33.7 ft | 32.5–33.5 ft | Tune first — longest legs, most influence on others |
| 30m | 10.125 MHz | 23.1 ft | 23.8 ft | 22.8–23.5 ft | WARC band — add if 30m operation desired |
| 20m | 14.200 MHz | 16.5 ft | 16.8 ft | 16.3–16.7 ft | Slightly shorter than standalone due to 40m coupling |
| 17m | 18.120 MHz | 12.9 ft | 13.3 ft | 12.8–13.2 ft | WARC band — add for 17m coverage |
| 15m | 21.200 MHz | 11.0 ft | 11.3 ft | 10.9–11.2 ft | Often not needed — 40m legs resonate on 15m (3rd harmonic) |
| 10m | 28.500 MHz | 8.2 ft | 8.5 ft | 8.1–8.4 ft | Tune last — most affected by other pairs |
Note on 15m: a 40m dipole naturally resonates near 21 MHz (3rd harmonic). If you already have 40m legs, separate 15m legs may be unnecessary — check SWR on 15m with just the 40m and 20m legs before adding 15m legs.
Complete materials for a 40m/20m/10m fan dipole
Building the 3-Band Fan Dipole (40m, 20m, 10m)
Cut wires, build the feedpoint, spread the legs, raise, and tune one band at a time. Allow 3 hours for the complete build and tuning session.
Cut and Label All Six Wire Legs
Cut wire for all three band pairs before assembly — it is much easier to keep track of six pieces of wire when they are laid out together than when some are already installed. From a single spool of #14 CCS wire:
- 40m legs: cut 2 pieces at 33.7 ft each — mark both ends with red tape
- 20m legs: cut 2 pieces at 16.8 ft each — mark both ends with blue tape
- 10m legs: cut 2 pieces at 8.5 ft each — mark both ends with green tape
Lay all six pieces out on a flat surface and verify lengths with a steel tape before proceeding. A measurement error discovered now costs nothing to fix — a measurement error discovered after installation costs several trips up and down a ladder.
Wind the Current Choke
Wind 8 turns of RG-8X coax through the FT-240-31 toroid. A current choke is particularly important on a fan dipole because the multiple wire legs create more opportunity for unbalanced currents to develop — the non-resonant legs interact with the feedpoint in complex ways that can drive common-mode current onto the coax shield. A well-wound choke makes the fan dipole behave as designed and makes SWR measurements reliable across all three bands.
Build the Multi-Wire Feedpoint
The feedpoint center must connect three wire legs to each side of the coax — center conductor and braid. Use a commercial multi-dipole center if available, or fabricate from a 3"×4" piece of 1/4" polycarbonate with three stainless screw terminals per side. Space the terminals to allow clean wire connections without crowding.
Strip 1.5 inches from one end of each wire leg. Form a loop with round-nose pliers. Assign each wire to a terminal — keep the same color on the same side (all red and all blue and all green on the coax-center-conductor side; the other three on the braid side). The key rule: all legs on the same side connect to the same electrical node. Tighten each connection firmly with lock washers and nuts.
Install the Feedpoint Spreaders
The spreaders hold the leg pairs separated at the feedpoint — preventing them from lying against each other and coupling too strongly. Use 18-inch sections of fiberglass kite rod or thin fiberglass tubing. Attach the spreaders to the feedpoint center at 15–20° angles from the main wire axis on each side. The 40m legs run most nearly horizontal; the 20m legs spread 15–20° below; the 10m legs spread 15–20° below the 20m legs.
Tape each leg pair to its spreader with a small tie wrap 12 inches from the feedpoint — this maintains the angular separation near the feedpoint where coupling is strongest. Further from the feedpoint, the wires can drift freely since they are far enough apart that coupling is reduced.
Install End Insulators on All Six Legs
Thread the far end of each wire leg through its egg insulator and secure with the standard wrap-and-solder method. Attach Dacron rope to each insulator. For a 3-band fan dipole, you have six end ropes — three per side. Plan how these will be anchored before raising the antenna:
- The 40m leg ends (longest) anchor at the maximum horizontal distance — approximately 32–33 feet per side of center
- The 20m leg ends anchor at approximately 16–17 feet per side of center — often on a lower branch or lower fence post than the 40m legs
- The 10m leg ends anchor at approximately 8 feet per side — can be very low (2–3 feet) or simply secured to the 40m or 20m support rope with a small tie wrap
Raise the Antenna
Raise the feedpoint to the target apex height — for an inverted-V configuration, 30+ feet for the feedpoint. Secure the center support. Then pull each leg pair to its end anchors:
- Pull the 40m legs out to their end anchors first — these define the maximum span and carry the most mechanical tension
- Route the 20m legs to their anchor points — they should droop slightly below the 40m legs due to their shorter length and the angular spread
- Secure the 10m legs — they may simply be tied off to the 20m support ropes or to low stakes near the center
Check that the leg pairs have visible angular separation when viewed from below the feedpoint. The three pairs should form a fan shape — narrow near the feedpoint and spreading outward. If two pairs appear to be running parallel or crossing, adjust the spreaders and anchor points before proceeding to tuning.
Tune 40m First
Connect the NanoVNA at the radio end of the coax. Sweep 6.8 to 7.5 MHz. Find the SWR minimum — this is the 40m leg pair resonance. With legs cut to 33.7 ft, expect resonance at 7.0–7.10 MHz (slightly low due to long legs). Trim both 40m legs equally in 2-inch increments until resonance reaches 7.150 MHz.
The 40m tuning has the most effect on the other bands, so complete it fully before moving on. After each 40m trim, note whether the 20m and 10m resonances shift — if they do, the leg pairs are coupling more than expected. This is normal; just complete 40m tuning first.
Tune 20m Second
Sweep 13.5 to 15.0 MHz. Find the 20m SWR minimum. Due to coupling from the 40m legs, 20m resonance is typically 50–200 kHz lower than the standalone dipole formula predicts. With 20m legs at 16.8 ft, expect resonance around 14.0–14.1 MHz — already close to the target or possibly needing only a small trim.
Trim both 20m legs equally in 1-inch increments until resonance reaches 14.200 MHz. On 20m, each 1-inch trim per leg raises resonance approximately 10–15 kHz. After final 20m trim, verify 40m resonance has not moved — it typically does not change from 20m trimming.
Check 15m Before Adding 10m Legs
Before tuning the 10m legs, sweep 20.5 to 22.5 MHz to check whether the 40m legs provide adequate 15m coverage. A 40m dipole resonates near 21 MHz (3rd harmonic of 7 MHz). If the SWR on 15m is already below 2:1 across the 15m band without any dedicated 15m legs, you may not need dedicated 15m elements. This is a significant simplification — eliminating one leg pair reduces feedpoint complexity and coupling interactions.
Tune 10m Last
Sweep 27.5 to 30.0 MHz. Find the 10m SWR minimum. With 10m legs at 8.5 ft, expect resonance around 28.0–28.4 MHz. Trim both 10m legs in 0.5-inch increments until resonance reaches 28.500 MHz. On 10m, each 0.5 inch trimmed from both legs raises resonance approximately 30–40 kHz.
After completing 10m tuning, do a final verification sweep of all three bands in sequence — 40m, then 20m, then 10m. Document the resonant frequency and minimum SWR on each band. If all three show SWR below 1.5:1 at the target frequencies, the fan dipole is complete.
Weatherproof the Feedpoint
With all three bands tuned, weatherproof the feedpoint. The fan dipole feedpoint has more connections than a single-band dipole — more entry points for moisture. Apply self-amalgamating tape starting from below the coax entry, working upward with 50% overlap until all connections, the coax SO-239, and 2 inches of coax jacket are fully covered. Apply a PVC tape outer layer for UV protection. Leave the spreader rods exposed — they do not need weatherproofing.
Inspect the feedpoint weatherproofing annually. The multiple wire connections in a fan dipole feedpoint create more potential moisture pathways than a simple dipole. Rewrap if any tape shows cracking, separation, or moisture intrusion.
Record, Photograph, and Verify
Record the final trimmed leg lengths for all six wires, the resonant frequency and minimum SWR on each band, the apex height, and the installation date. Photograph the feedpoint from multiple angles showing the spreader arrangement and leg separation. Photograph the full antenna from below showing the fan shape.
Use the Reverse Beacon Network to verify on-air performance. Transmit CW on each band (40m, 20m, 10m) briefly and confirm spots appear from the expected regions. Compare signal strength reports across bands — a well-built fan dipole should show comparable signal quality on all three covered bands, within 2–3 dB of each other at similar distances.
Adding 80m to the Fan Dipole
Expanding the 40m/20m/10m fan dipole to include 80m is the most common upgrade. The 80m leg pair, at 62–64 feet per leg, dominates the feedpoint mechanically and requires dedicated end anchor points well beyond the 40m leg endpoints. Key considerations:
- The 80m legs need anchor points approximately 60+ feet from the center — significantly further than the 40m legs at 33 feet
- The 80m legs may need to run in different directions than the other legs if space is constrained
- The 80m legs coupling effect on 40m is significant — after adding 80m legs, re-tune the 40m legs (they will have shifted lower in frequency)
- On 40m, the large 80m leg pair acts as a loading element that slightly lowers 40m resonance — trim both 40m legs slightly after adding the 80m pair
- The feedpoint must accommodate four connections per side — add terminals to the feedpoint center
- A 4-band fan dipole (80/40/20/10m) from one installation is an excellent all-HF antenna for most stations
Adding WARC Bands (17m, 12m, 30m)
The WARC bands (30m, 17m, 12m) are highly active but require specific leg pairs since they have no harmonic relationship with the standard 40m, 20m, or 10m legs. Adding all three WARC band pairs creates a 6-band fan dipole (40/30/20/17/15/12/10m — or a subset) from one feedpoint.
- 30m legs: 23.1 ft each — run between the 40m and 20m legs angularly
- 17m legs: 12.9 ft each — run between the 20m and 10m legs
- 12m legs: 9.4 ft each — run near the 10m legs with small angular separation
- Each additional pair requires its own terminals at the feedpoint and its own end anchors
- A 5 or 6-band fan dipole becomes mechanically complex at the feedpoint — consider a homebrew polycarbonate plate with 12 terminals total (6 per side)
- Tune in order from lowest frequency to highest: 40m → 30m → 20m → 17m → 12m → 10m
- Interaction between adjacent WARC legs and the existing legs requires patience — plan for 2 hours of tuning for a full 6-band fan
Fan Dipole vs OCFD for Multi-Band Coverage
Both the fan dipole and the OCFD cover multiple HF bands from a single feedpoint and single coax run. The comparison:
- Fan dipole: true resonant performance on each covered band; no tuner needed; each band independently tunable; more complex construction; more wire and supports needed
- OCFD (Windom): single 135-foot wire; simpler to install; covers 5 bands (80/40/20/15/10m) but not at true resonant efficiency on all; requires 4:1 balun; sometimes needs a tuner on 15m and 10m
- Winner for SWR quality: fan dipole — each band is genuinely resonant
- Winner for ease of installation: OCFD — one wire, simpler feedpoint
- Winner for space efficiency: OCFD — 135 feet of wire covers 5 bands vs 140 feet total covering 3 bands for a 40m/20m/10m fan
- Winner for adding WARC bands: fan dipole — add a leg pair; OCFD does not naturally cover 30m, 17m, or 12m
Inverted-V vs Flat Configuration
A fan dipole works well in either flat or inverted-V configuration. The choice depends on available supports:
- Flat fan dipole: requires three support points per side — center support plus end anchors for each leg pair at different heights and distances. All leg pairs run nearly horizontal. Best RF performance but most complex support arrangement.
- Inverted-V fan dipole: all leg pairs slope downward from a single apex support. Only the apex needs to be tall — end anchors for all leg pairs can be at low height (3–8 feet). Significantly simpler support arrangement. The most practical configuration for most installations.
- In an inverted-V, the longer (lower-band) legs slope more gently and the shorter (higher-band) legs droop more steeply — this naturally creates the angular separation needed between leg pairs
- An inverted-V fan dipole at 40 feet apex with three band pairs is one of the most effective and most practical multi-band HF antennas available to residential operators
Why do my fan dipole legs need to be spread at an angle?
Angular separation between leg pairs reduces mutual coupling — the electromagnetic interaction between adjacent wires. Without separation, parallel wires couple strongly and the resonant frequency of each pair is significantly affected by the presence of the other pairs, making tuning very difficult. With 15–30° of angular spread, coupling is reduced enough that each pair can be tuned relatively independently. The angular spread also gives each leg pair its own physical space, reducing the chance of wires tangling in wind. A fan dipole with no angular spread — all legs parallel — is very difficult to tune and performs poorly.
My 20m SWR is still high after tuning — what is wrong?
Several common causes: the 40m legs may be coupling too strongly to the 20m legs, shifting the 20m resonance and broadening the SWR dip. Try increasing the angle between the 40m and 20m leg pairs. Also verify the current choke is providing adequate common-mode rejection — if the choke is inadequate, the coax acts as a seventh wire connected to the feedpoint, confusing the impedance measurements. A third possibility: the 20m legs have been trimmed too short. If resonance is above 14.350 MHz, splice additional wire onto both legs and retune.
Can I add a 5th band to an existing 3-band fan dipole?
Yes — adding a band is simply a matter of adding a new leg pair to the existing feedpoint and tuning it. You will need to modify the feedpoint to accommodate additional wire connections and add new end anchors for the new legs. After adding the new pair, re-tune all bands from lowest to highest frequency — the new wires will have shifted the resonances of the existing bands slightly. This shifting effect is typically small (25–75 kHz) and is corrected in the re-tuning pass. Plan for 30–45 minutes of re-tuning after any band addition.
Does a fan dipole perform better than a trap dipole?
Yes, noticeably. A fan dipole provides true resonant performance on each covered band — the active leg pair behaves identically to a dedicated single-band dipole. A trap dipole uses resonant traps that introduce a small but real resistive loss (typically 0.5–1 dB per trap) and slightly narrow the bandwidth on each band. For casual operation the difference is modest — perhaps 1 dB average. For weak-signal DX work or digital modes where every dB matters, the fan dipole's advantage is meaningful. The fan dipole's additional complexity (more wires, more end supports, more tuning effort) is the price of that performance improvement.
How much does each leg pair affect the others?
With good angular separation (20–30°), the interaction is modest — typically 50–150 kHz shift in resonant frequency. The longest legs (40m) have the most effect on shorter legs because the long wires are closer to the shorter wires' resonant frequency harmonics. The shortest legs (10m) have minimal effect on longer legs because short wires are far off-resonance at the longer bands' frequencies. This is why the tuning sequence (40m first, 10m last) is important — adjusting longer legs first, then shorter ones, minimizes the re-tuning iterations needed.
Can I build a fan dipole with different wire gauges for different bands?
Technically yes — thinner wire for the shorter (higher-band) legs saves some weight. However, mixing wire gauges from different sources introduces small velocity factor differences that complicate tuning. The practical recommendation is to use the same wire gauge and type for all legs from the same spool. The weight savings from using thinner wire on 10m legs — perhaps 20 grams — is not worth the additional tuning complexity. Keep it simple: identical wire throughout, and color-code the leg pairs with electrical tape for identification.