Fan Dipole Antenna: Multiband HF Wire Without Traps
A fan dipole is the simplest way to get several resonant HF bands from a single feedline. Instead of traps, loading coils or a tuner, it uses one half-wave dipole per band, all connected in parallel at the same feedpoint and spread apart like the ribs of a fan. On each band the dipole cut for that frequency takes the power and the others mostly ignore it. The result is a direct 50 ohm match on every band you built it for, with the full efficiency of a plain wire dipole. This guide explains how the idea works, which band combinations are easy and which are awkward, how to size and trim the wires in the right order, how to feed it, and what goes wrong when the spacing or the trimming sequence is ignored.
How a Fan Dipole Works
A half-wave dipole presents a low impedance, in the region of 50 to 75 ohms, at its resonant frequency, and a much higher impedance well away from it. When you connect two or three dipoles of different lengths to the same feedpoint, the one that is resonant on the band in use looks like a low impedance and the others look like high impedances in parallel with it. Most of the current flows into the resonant element, so the antenna behaves almost like a single dipole for that band. The word “almost” matters: the non-resonant wires are still coupled to the active one, and that coupling shifts the resonance of the active element downward and slightly changes its pattern. That is why a fan dipole is always trimmed as an assembly, never as a set of individually cut wires.
The standard reference description of a dipole antenna gives the starting point for each element: the full length in metres is about 143 divided by the frequency in MHz, or about 468 divided by the frequency in MHz for the length in feet. In a fan, cut each wire a few percent longer than that, because the interaction with the neighbouring wires lowers the resonant frequency and you will trim from there.
Fan Dipole Versus the Other Multiband Wires
There are several ways to cover more than one HF band with a wire antenna, and each trades something different. The table below compares the common options for a station that wants three or four bands from one feedline.
| Antenna | How it covers bands | Needs a tuner? | Efficiency | Main drawback |
|---|---|---|---|---|
| Fan dipole | One resonant dipole per band in parallel | No | Full dipole efficiency on each band | Interaction makes trimming slow; many wires in the air |
| Trap dipole | LC traps isolate sections of one wire | No | Slight loss in traps | Traps add weight, cost and narrow bandwidth |
| Doublet with ladder line | Non-resonant wire, matched by a balanced tuner | Yes | High if the tuner and line are good | Needs a tuner and care routing the open line |
| Off-center-fed dipole | Feedpoint placed where several bands share a similar impedance | Usually no | Good, with a 4:1 or 6:1 balun | Common-mode current, some bands missing |
| End-fed half-wave | One wire, 49:1 transformer, harmonic bands | Rarely | Good, small transformer loss | Needs a counterpoise plan and a hot coax shield |
Our separate guides cover the trap dipole and the doublet fed with ladder line in detail. The fan dipole wins when you want the simplest electrical design and have the patience to trim it properly.
Which Bands to Combine
Not every combination is equally easy. The rule of thumb is that the elements should differ in length by a reasonable margin, so that their resonances are well separated and the interaction is modest. The classic three-band fan is 40, 20 and 15 meters, or 80, 40 and 20 meters. Here are the main considerations:
- Harmonic pairs. A 40 meter dipole is already roughly resonant on 15 meters as a three-half-wave wire. Adding a separate 15 meter element to a fan that already has a 40 meter wire creates two resonances close together, which can be hard to tame. Many builders leave 15 meters to the 40 meter wire and trim that wire for a compromise. If you want a dedicated 15 meter element, be prepared to adjust both.
- Adjacent bands. 20 and 17 meters, or 17 and 15, are close enough that the elements pull each other strongly. It can be done, but the trimming takes longer and the bandwidth on each may be narrower than on a lone dipole.
- Widely spaced bands. 80 and 20, or 40 and 10, interact only mildly. These are the easiest to build and trim.
- How many wires. Three bands is comfortable. Four is common. Beyond that the feedpoint becomes a heavy bundle, the wind load grows, and the mutual coupling becomes difficult to predict. For five or more bands a doublet with a tuner or a trap design is usually a better plan.
Choose the bands you will actually operate. A fan for 40, 20 and 15 meters covers daytime DX, night-time DX and the most popular contest bands with three wires, and it is a good first multiband antenna after the single-band dipole described in our first HF antenna guide.
Mechanical Layout and Spacing
The longest element is the backbone of the antenna. It carries the mechanical load and is supported at its ends in the usual way, as a flat top or an inverted V. The shorter elements hang below it, or fan out to the sides, with their far ends tied off to the longest wire with insulating cord so that everything stays in a fixed relationship. The spacing between elements at the far ends matters: if the wires lie almost parallel and only a few centimetres apart, the coupling is strong and the resonances shift unpredictably; if they are spread by a metre or so at the ends, the interaction is much smaller and trimming is easier. A common arrangement for an inverted V is to hang the shorter elements at a slightly different angle so that they separate naturally as they leave the feedpoint.
Spreaders made of fibreglass rod, plastic tube or even dry wood dowel can hold the elements apart at fixed intervals along their length. They are not essential, but they keep the spacing stable in wind, and a stable antenna is one that stays tuned. At the feedpoint, bring all the wires to one centre insulator and connect the left-hand legs together and the right-hand legs together. A proper centre insulator with strain relief is worth having here, because the combined weight of several wires pulls hard on the connection.
Height
A fan dipole follows the same height rules as any other dipole: the pattern and the feedpoint impedance depend on height in wavelengths, so the 80 meter element at 10 metres is low and radiates mostly upward, while the 20 meter element at the same height is a decent DX antenna. If one band matters most, choose the height for that band and accept what the others do.
Trimming in the Right Order
This is the part that separates a fan dipole that works from one that frustrates its builder. Because the elements interact, trimming one changes the others. The sequence that works is:
- Cut all elements long. Use the standard formula, then add two to three percent to each wire.
- Hang the antenna at its final height with all elements in place. Trimming on the ground, or with wires missing, gives results that will not survive the move.
- Measure the lowest band first. The longest element is affected least by the shorter ones. Sweep it with an analyser and trim both legs equally until the resonance sits where you want it in the band.
- Move up one band at a time. Trim the next shortest element, then the next. Each will have been pulled downward in frequency by its neighbours, so you will usually find them resonant below the band and will trim upward.
- Go back and recheck the lower bands. Trimming the higher elements shifts the lower ones a little. One more pass is normally enough.
Fold the excess wire back on itself rather than cutting it off until you are satisfied, because the next adjustment may need it back. An antenna analyser or a NanoVNA sweep across the whole HF range makes this far quicker than an SWR meter and a transmitter, and it shows you where each resonance has landed instead of making you hunt for it.
Feeding the Fan Dipole
A fan dipole is a balanced antenna fed with unbalanced coax, so it needs a balun or a common-mode choke at the feedpoint exactly as a single dipole does. A 1:1 current balun, which in practice is a number of turns of coax or a few beads on the line, keeps RF current off the outside of the coax shield. Without it the feedline becomes part of the antenna, the SWR changes when you move the coax, and noise picked up along the cable arrives at the receiver. Our article on baluns and ununs explains the choices and how to wind one. The balun reference page covers the underlying theory.
The feedpoint impedance of each element is close to that of a lone dipole, modified slightly by the interaction and by height. Expect an SWR below about 1.5:1 at resonance on each band once the trimming is done, with the usual dipole bandwidth: wide on 10 and 15 meters, moderate on 20 and 40, and narrow on 80 meters, where a single element may cover only part of the band at a low SWR. On 80 meters many operators cut the element for the part of the band they use most and accept a tuner, or a second element, for the rest.
Common Problems and How to Fix Them
Resonances will not settle. The usual cause is elements that are too close together or that swing relative to each other in wind. Increase the end spacing, add spreaders, and tie the ends off so they cannot move.
One band is fine, the next is far off. Check that the element for that band was cut long enough to start with. If you trimmed it short, the only remedy is to splice in extra wire. This is why you fold rather than cut during trimming.
High SWR on a harmonic band. If you rely on the 40 meter wire for 15 meters, its third-harmonic resonance is typically a little above the band. A short extension stub near the feedpoint, or accepting a modest SWR with a short coax run, is the usual answer.
SWR changes when you touch the coax. Common-mode current is on the feedline. Add or improve the choke at the feedpoint.
Everything shifted after a rain shower. Wet insulation and wet cord change the resonance slightly on every wire antenna; on a fan with tight spacing the effect is bigger. Spreading the elements reduces it.
The Bottom Line
A fan dipole gives you several HF bands at full dipole efficiency with nothing but wire, insulators, a choke and one coax run. The price is patience: the elements interact, so you must trim them in order, at final height, with everything in place, and you must keep them apart and stable. Choose bands with well separated wavelengths, start with three wires, use an analyser, and you will have a multiband antenna that needs no tuner and has no lossy parts anywhere in the path from radio to sky.
Frequently Asked Questions
How far apart should fan dipole elements be?
Close together at the feedpoint by necessity, but spread by half a metre to a metre at the far ends. The wider the spacing, the weaker the interaction and the easier the trimming.
Does a fan dipole need a balun?
Yes. It is a balanced antenna fed with coax, so a 1:1 current balun or common-mode choke at the feedpoint keeps RF off the feedline and stabilises the SWR.
How many bands can a fan dipole cover?
Three is easy and four is common. Beyond that the wire bundle becomes heavy and the interactions hard to manage; a doublet with a tuner or a trap design is usually better for five or more bands.
Can I add a 15 meter element to a 40 meter fan dipole?
You can, but the 40 meter wire is already near resonance on 15 meters, so the two will interact. Many builders let the 40 meter element serve 15 meters and trim it for a compromise instead.
Which element do I trim first?
The longest one, for the lowest band, because it is affected least by the others. Then work upward in frequency and finally recheck the lower bands once more.
Why is my fan dipole resonant below the band on every wire?
Because the elements couple to each other and pull the resonances down. That is normal and is why each wire is cut a few percent long and then trimmed as part of the assembly.