The Folded Dipole is commonly used by itself and in many types antennas. A Folded Dipole feed is common in VHF/UHF Yagi antennas and as the main element in many HF antennas. The Folded Dipole is a combination Split Dipole feed element plus a unbroken Impedance Transformer element. The two elements are connected together at each end. This arrangement provides some mechanical and electrical advantages. The actual feed impedance of this combination, is then based on the feed impedance of a Standard Dipole multiplied by the Transformation Ratio.
The folded dipole has several interesting features.
A two wire folded dipole can increase the characteristic feed impedance of a dipole and offer a good match to 300 Ohm balanced feed line.
A three wire folded dipole can increase the characteristic impedance of a dipole and offer a good match to 450 Ohm or 600 Ohm balanced feed line.
Offers a better match over a wider band, which can be important on the lower frequency bands. This is primarily due to effectively making the antenna fatter with more wire.
When fed with a balanced feed line, and an antenna tuner, it can be run on other odd multiples of the fundamental frequency. However, it is not very useful on frequencies that are even multiples.
A Folded Dipole is pretty simple to make. It is often made from lengths of 300 Ω twinlead or 450 Ω ladder line. Because the two conductors in the line are the same size, twinlead and ladder line make a great antennas with about a 300 Ω feed point. Of course, you could make your own spreaders, to keep the wires at a constant separation distance. And then use common #14 AWG electrical wire.
There are two drawings below. The one on the left shows the equations involved in calculating the impedance transformation ratio. On the right is a calculator for determining the wire length and feed impedance for a Folded Dipole. This is a handy calculator for trying to create a Folded Dipole feed section for a multi-element VHF/UHF array, like a Yagi.
The drawing on the right shows the essential elements of a folded dipole. It consists of two parallel elements having a constant spacing S. These elements can be anything from simple wires to copper or aluminum tubing. The bottom element is split in the center and serves as the feedpoint. The upper element has a diameter D2 and the bottom element has a diameter D1. The ends of the elements are connected to form a continuous loop from the feedpoint.
The relationship of those three dimensions, (D, D1, and D2) creates a impedance transformation at the feedpoint that is described by the equation on the right. The Ratio, when multiplied by the standard dipole feed impedance, describes the folded dipole feed impedance.
|HF Folded Dipole|
The Folded Dipole is not limited to the UHF/VHF region. It makes a very good antenna for the HF bands. The drawing below is a example. The feed-point is at the center, on the lower section of the antenna, where a insulator is used. The antenna wire is continuous from one side of the feed-point to the other. The length of which should be one wavelength (1λ), minus a small amount due to the Velocity Factor. At the feed point, a 4:1 Balun is needed to transform the feed-point impedance to a lower value for feeding with coax. As the drawing shows, you can use 50 Ω or 75 Ω coax, without any real issues. When the antenna length is trimmed to resonance, you should still have a low SWR across the band.
The drawing shows three wood/plastic spacers on each side of the feed-point. Use as many as you think are necessary to keep the wire at a consistant spacing. But remember, each spacer adds extra weight, so make them out of a light material.
Below the antenna is a listing of lengths for each amateur band.
In the text areas to the right, enter your initial design information. Enter your expected frequency of operation, the antenna velocity factor, and the nominal feed impedance of a simple dipole.
The Velocity Factor is to adjust for the fact that the propogation of energy in a wire is a little slower than in free space. The value is based on the length to diameter ratio and defaults here to 0.951. Larger diameters may require you to adjust this value slightly higher.
The folded dipole transforms the normal feed impedance of a simple dipole. For a 1/2 wave dipole, in free space, this is approximately 72 Ohms. You may not be dealing with a dipole in free space, but 72 Ohms is close enough to start with. You can adjust it to other impedances in the appropriate text box below.
Then enter the dimensions for your folded dipole antenna. If you make the diameter of both radiator elements, D1 and D2, equal the transformation ratio will be 4. This should transform the 72 Ohm dipole feed impedance to about 288 Ohms. You should note that, when the two diameters are equal, the distance S does not change the transformation ratio. Use the text areas below to enter diameters of each element and the distance between them. You can enter the the data in any dimension you like. You can even mix and match. Output data is presented in both US/Imperial and Metric dimensions.