Folded Dipole Antenna: Impedance, Bandwidth and Twin-Lead Builds
The ordinary half-wave dipole is the first antenna most amateurs build, and it works well. A folded dipole antenna looks like a small modification: add a second wire parallel to the first and join the two ends. Yet that one change quadruples the feed-point impedance, widens the bandwidth and turns the antenna into something that matches cheap 300 ohm twin lead almost perfectly. This guide explains how the folded dipole works, why the impedance is about 292 ohms, how the number of wires changes the figures, how to build a practical version for an HF band and where the design has limits. The technical description follows the standard reference article on the folded dipole.
What a Folded Dipole Is
The reference defines a folded dipole as a half-wave dipole with an additional parallel wire connecting its two ends. If the extra wire has the same diameter and cross-section as the dipole, two nearly identical radiating currents are produced. The far-field radiation pattern is therefore nearly identical to that of a single-wire dipole, with the same figure-eight shape and the same low gain, but at resonance the feed-point impedance is four times higher.
The reference also gives a useful way to think about the shape. A folded dipole is technically a folded full-wave loop antenna, in which the loop has been bent at opposing ends and squashed into two parallel wires in a flat line. Its broad bandwidth, high feed-point impedance and high efficiency are characteristics more like those of a full loop, even though its radiation pattern looks like an ordinary dipole. Since it is easier to understand a half-wave dipole than a loop, both full loops and folded dipoles are often described as two half-wave dipoles in parallel, joined at the ends.
If you want to remind yourself of the basics first, our guide to your first HF antenna covers the simple dipole and its alternatives.
Why the Impedance Is Four Times Higher
The reason is current. Imagine that the antenna must radiate a fixed amount of power. The total radiating current, which is the sum of the currents in both wires, is twice the current in each wire taken separately, and so it is also twice the current at the feed point. The power radiated depends on that total radiating current and on the radiation resistance of a simple half-wave dipole. The power delivered at the feed point depends on the current in the fed wire, which is only half of the total, and on the feed-point resistance. Setting the two powers equal shows that the feed-point resistance must be four times that of the single dipole.
With the textbook value of about 73 ohms for a resonant half-wave dipole in free space, the folded dipole presents roughly 292 ohms. In real installations the figure changes with height above ground, the surroundings and the wire thickness, so treat 292 ohms as the starting point. Our article on dBi and dBd also uses the half-wave dipole as the reference antenna, which is why its properties matter beyond simple wire antennas.
A Natural Match for 300 Ohm Line
A feed-point impedance of about 292 ohms is very close to the 300 ohm characteristic impedance of flat twin-lead ribbon cable. The reference notes that the folded dipole is therefore well matched to 300 ohm balanced transmission lines. That is why it appears in so many everyday antennas. Half-wave folded dipoles are often used as FM radio antennas, and the versions made of twin lead that hang on a wall are often supplied with FM tuners. They are also widely used as the driven element in rooftop Yagi television antennas, where the high impedance of the driven element is useful because the parasitic elements lower it.
For amateur use, there are two common feeding methods. The first is to connect 300 ohm twin lead directly to an antenna tuner or to a transceiver through a matching network that can handle balanced lines. The second is to use a 4:1 balun at the feed point, which transforms the roughly 300 ohm balanced impedance to about 75 ohms unbalanced, close enough to 50 ohm coax that the SWR is moderate on the design frequency. A balun also isolates the feed line from the antenna and helps to keep common-mode current off the coax. Our guide to baluns and ununs explains the types and when to use each, and it is worth reading before you choose one.
Wider Bandwidth
The second advantage is bandwidth. According to the reference, a folded dipole has a wider bandwidth than a single dipole. The usable range of frequencies, defined by an acceptable SWR, is wider because the antenna behaves partly like a loop and the two conductors share the current. You can see it on the analyzer: the SWR curve of a folded dipole is broader and flatter around resonance than that of a single wire of the same thickness. On the lower HF bands, such as 80 and 40 meters, where a plain dipole may cover only part of the band, a folded dipole can cover a bigger portion without retuning.
The reference adds that the antenna can be used to transform the value of the input impedance over a broad range of step-up ratios by changing the thickness of the conductors on the fed side and the folded side. In other words, a designer can adjust the impedance ratio by using wires of different diameters. For most amateurs building a wire antenna, using two identical wires is easiest, and the four-to-one ratio comes automatically.
More Wires, More Impedance
If two wires multiply the impedance by four, more wires multiply it by more. The reference explains that adding a third parallel wire raises the feed-point impedance to nine times that of a single-wire dipole, which is 658 ohms, a good match for open-wire feed line, and broadens the resonant band further. In general, the radiation resistance is approximately n squared multiplied by 73 ohms, where n is the number of parallel half-wave wires side by side, connected at their ends. The table summarizes the pattern.
| Number of wires (n) | Impedance multiplier | Approximate feed-point impedance | Typical feed line |
|---|---|---|---|
| 1 (plain dipole) | 1 | 73 ohms | Coax, directly or with a 1:1 balun |
| 2 (folded dipole) | 4 | 292 ohms | 300 ohm twin lead or 4:1 balun to coax |
| 3 | 9 | 658 ohms | Open-wire line |
| n | n squared | n squared times 73 ohms | Depends on the value |
The reference also describes a variant in which each parallel wire is slightly too short by the same amount, and a capacitive loading wire, usually dangling, is connected at each end to restore resonance. That design achieves nearly as good performance as the full flattened-loop version in a shorter space.
Building a Twin-Lead Folded Dipole
A folded dipole made from 300 ohm twin lead is cheap and quick to build. The steps below are a general guide; adjust the details for your band and your installation.
- Choose the band and calculate the length. The starting length of a half-wave dipole in feet is about 468 divided by the frequency in megahertz, but twin lead has a velocity factor that shortens the electrical length, and the exact figure depends on the cable. Cut a little long, and plan to trim. Our article on velocity factor explains why cable and wire lengths do not match the free-space numbers.
- Join the two conductors at both ends. The two wires of the twin lead are shorted together at each end, which forms the loop. Solder the connections and seal them against moisture.
- Cut one conductor at the center. The feed point is a gap in one of the two wires at the center of the antenna. The two cut ends connect to the feed line.
- Attach the feed line. Connect either a 300 ohm twin lead feed line or a 4:1 balun at the center. Use a strain relief so that the weight of the line does not pull on the joint.
- Hang it up. Support the ends with insulators and keep the antenna clear of metal objects. Keep it as high and as straight as practical.
- Measure and trim. Use an antenna analyzer to find the resonant frequency and shorten the antenna evenly at both ends if it is too low in frequency. If you do not have an analyzer, see our guide to the NanoVNA antenna analyzer, which shows how to take a sweep.
Twin lead is light and flexible, but it has weaknesses. Rain and ice change its characteristics, and the plastic can deteriorate in sunlight over the years, so inspect it regularly and replace it when it cracks. If you need a more durable version, you can make the antenna from two separate wires held apart by spacers, or from ladder line, which is less affected by weather. The article on ladder line versus coax compares the feed line types.
Where Folded Dipoles Are Used
The reference lists several of the best-known uses. The folded dipole is used as an FM broadcast receiving antenna, as the driven element of rooftop television Yagis and in many other broadband antennas. It is also the basis of at least one amateur design. The T2FD antenna is a folded dipole with a resistor added on the second wire, opposite the feed point, which produces a very wideband antenna at the cost of some efficiency. We cover it in our guide to the T2FD antenna.
In Yagi design, the folded dipole is a popular driven element because the parasitic elements around it reduce its impedance. When the impedance falls by a factor, the folded design brings it back toward a convenient value. That is why many commercial VHF and UHF Yagis use a folded dipole in the driven position.
Limits to Keep in Mind
The folded dipole is not a miracle antenna. It does not add gain over a regular dipole at the same height, because the radiation pattern is nearly the same as that of the single-wire version. The extra wire adds weight, wind load and complexity, and the high impedance means that you need a matching device or a balun when you use coax. It is also normally resonant on one band; on other bands the feed-point impedance changes in a complicated way, and while a tuner and balanced line may allow multiband use, that is a different approach from the single-band design discussed here.
Height and surroundings matter as they do for any horizontal wire. A folded dipole mounted low over poor ground will have a different impedance and pattern than one mounted at a half wavelength. If you want to understand how height changes things, read our article on antenna height and takeoff angle. Common-mode current on the feed line can also distort the pattern and bring noise into the receiver, so a good balun or choke at the feed point is worth installing.
The Bottom Line
A folded dipole is a half-wave dipole with a second parallel wire joined at both ends. The pattern stays the same as that of a simple dipole, but the feed-point impedance rises to about 292 ohms, the bandwidth increases, and 300 ohm twin lead or a 4:1 balun can feed it without trouble. Add a third wire and the impedance rises to about 658 ohms, and in general it grows with the square of the number of wires. It is an inexpensive project for a single band, a useful driven element for a Yagi and the starting point for broadband designs such as the T2FD.
Frequently Asked Questions
What is the impedance of a folded dipole?
About 292 ohms at resonance, which is four times the roughly 73 ohms of a simple half-wave dipole. The exact value changes with height, wire thickness and surroundings.
Does a folded dipole have more gain?
No. The radiation pattern is nearly the same as that of an ordinary dipole, so the gain is similar. The advantages are the higher impedance and the wider bandwidth.
How do I feed a folded dipole with coax?
Use a 4:1 balun at the feed point. It transforms the balanced impedance of about 300 ohms to about 75 ohms unbalanced, which is close to 50 ohm coax.
Can I make one from 300 ohm twin lead?
Yes. Short the two conductors together at both ends, cut a gap in one conductor at the center for the feed point, and trim the length with an analyzer. Allow for the velocity factor of the cable.
What happens with three wires?
The impedance rises to nine times that of a single dipole, about 658 ohms, which suits open-wire feed line, and the resonant bandwidth broadens further.
Is a folded dipole good for multiband use?
Not by itself. It is designed for one band, although a tuner and balanced feed line can make it usable on others with some loss.