Introduction |
The VHF/UHF Yagi Antenna Design page does a good job of calculating the lengths of all the antenna elements, but doesn't really describe the feed arrangement. Originally, this page only described a Folded Dipole as a feed element. While I still describe the Folded Dipole, there are other feed arrangements that may be a better fit for some. However, I am only providing some theory, characteristics, and calculations on the feed arrangements. For actual physical mounting, your on your own. A quick google search will provide you with tons of information on the mechanically connecting the feed to the antenna and feed line.
There are effectively two types of feed arrangements, balanced and unbalanced. Balanced feed line are readily available in various impedances (75/300/450 Ω), but the use of a balanced feedline presents some physical requirements that are not easy to get around. Plus, unless the transceiver is homebrew, the likelyhood of having balanced outputs is small. With most transceivers being equipped with unbalanced coaxial cable outputs (50/75 Ω) the usual preference is a unbalanced coaxial cable feed line.
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Direct Coax Feed |
I have personally run simulations on many Yagi designs, from my Yagi Design page, and found that the feed impedance is very close to 50 Ω at the design frequency. Making a Direct Coax feed very feasable. The diagram on the right give you the idea. The coax center conductor is attached to one side of the driven element. The other side of the driven element is at ground potential and is attached to the boom and the coax shield. That is, assuming you are using a metalic boom.
A drawback to using this arrangement is possible azimuthal squinting (to the left or right when the antenna elements are horizontal) and unbalance of the driven element. However, the effects of this unbalance on the array should be small.
This same arrangement can be done with 75 Ω coaxial cable. Low loss 75 Ω coaxial cable might be a little easier to obtain. Satellite and Cable installers often have excess cable that is just going to be thrown away. However, you might have to adjust the width of the driven element a bit to provide a better match. Even without any adjustment, the SWR should be around 1.5:1 which is perfectly usable. Loss due to SWR should be a minimum.
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Balanced to Balanced |
Knowing ZL and the Center Frequency is usually pretty straight forward. However, having a accurate value for ZA is not always handy. But it is possible to make a educated guess. If you are just trying to match a simple dipole, that is sufficiently away from large metal objects and sufficiently above ground, the feed impedance in the center will be around 72 Ω. But if the dipole is part of a parasitic array (Yagi) the feed impedance of the dipole can be different. It is common for HF Yagis to modify the feed impedance to around 20 Ω, where as VHF Yagis commonly have a 50 Ω input impedance.
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Folded Dipole |
A Standard Dipole, in free space, will have a feed point impedance of around 72Ω. But when it is in the presence of parasitic elements, like Yagi directors/reflectors, the feed impedance can be driven to 20Ω. So the Folded Dipole arrangement can be used to bring the feed point impedance back to a usable level, for 50 or 75 Ω coax, or even higher, for 300/450Ω balanced feed lines.
The Folded Dipole has a couple of advantages, over the Direct Coax Feed described above. For one, the Folded Dipole has a flatter frequency response, enabling it to be used over a wider bandwidth. Further, the existance of parasitic elements (Yagi directors/reflectors) can reduce the feed point of a dipole. The Folded Dipole can then significantly increase the impedance level, enabling the antenna to be matched more easily. Note that the Folded Dipole does not create a specific impedance. It only provides a transformation of the impedance that would normally exist at the feed point of a dipole.
There is also a mechanical advantage, in that, the center of the non-feed element (℄) can be secured directly to a metalic boom, without affecting the impedance transformation ratio or radiation efficiency. This makes it a little easier to mount the element. This is, of course, as long as the feed point does not short to the metalic boom.
The drawing shows the main elements as solid rods, tied at the ends with wire. But the elements can be made from solid wire (insulated or bare) or hollow rods. Common 1/4" copper tubing is often a good choice as long as it's not going to be in a harsh environment. For more strength, and less weight, you might substitute aluminum tubing. For VHF/UHF use it is common to use the elements from an old TV antenna. They are light and readily available these days.
From the equations, you can see that as long as the diameter of the feed elements and the non-feed elements are equal, there will be a 4:1 impedance transformation, regardless of the distance "S" between them. But don't make the distance too big. If the distance "S" starts to approach λ/4, you wind up with a full wave loop, which is not being discussed here.On the right is a somewhat exagerated drawing of the basic Folded Dipole. But, it's only intention is to show you the configuration you are working with.
The most important dimension is the Length. You want to adhere as closely as possible to the calculated length from the VHF/UHF Yagi Antenna Design page.
The second most important dimension would be the element Diameter. For these designs, the diameter of the element should be limited to between 0.001 and 0.02 wavelengths. As an example, this would mean that for 2 Meters you should keep the diameter between 0.081" (2.053 mm) and 1.617" (41.068 mm). For 70 CM the diameter should be limited to between 0.027" (0.681 mm) and 00.536" (13.627 mm).
The Spacing between the horizontal sections can vary rather widely. This nicely accomodates do-it-yourselfers, but also give you some latitude for varying boom diameters and mounting methods.
The Feed Gap is exactly what it sounds like. This is where you break the loop for attaching the feed. An exact dimension is not necessary, but try to keep the gap as small as practicable. A good rule of thumb would be to keep it no larger than one boom diameter.
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Mounting |
 "Click for Larger Image")
Click for Larger ImageNeither of the choices has any particular advantage over the other, so you can choose the mounting method that best fits your application. The boom running through the folded dipole doesn't seem to have any significient effect of the performance of the overall antenna.
In all of the methods, however, you should be careful not to make an electrical connection between the folded dipole and the antenna boom. The center of the unbroken side of the folded dipole should be at an electrical zero, but there is no guarantee that you have a perfect balance and this could cause unwanted RF currents to flow on the boom.
The most popular method for mounting the folded dipole is to secure a small plastic box above, or below, the boom. The box serves the dual purpose of providing a insulated place for making the feed connections and allows you to easily weather proof the connections. Again, this is probably best described in a picture, like the one below. No actual dimensions are given, because it's unknown to me exactly what you are using. "Click for Larger Image")
Click for Larger ImageThe plastic box can be secured to the boom with two sheet metal screws. If you place solder lugs under these same screws, the coaxial cable shields can be tied here as well, but more on that later. Properly sized rubber grommets in all of the entry holes will help prevent the weather from entering.
Don't drill any feedline entry holes until you decide on the kind of feedline to use. Otherwise you may have some extra holes that you will need to patch up.
The plastic spacer, on the bottom, serves to insulate the folded dipole element from the boom and to secure it in place. For UHF folded dipole feeds, all you should need to do is use a couple of UV stabilized ty-wraps to secure it in place. Larger and heavier elements would be better held in place by a small clamp of sorts. The drawing doesn't show it very well, but, take care to make sure the dipole element, held down by the plastic spacer, does not touch the screws that secure the spacer to the boom. There is nothing that says that the plastic box needs to go on top. If you want it on the bottom, just flip the drawing over.
The drawing shows a square boom but this kind of arrangement can just as easily be done with a round boom. If you are using a non-metalic boom insulating everything is going to be much easier.
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Feeding and Matching |
The next thing to do is figure out how to connect the feed to your receiver or transmitter. As mentioned earlier, the advantage of this kind of feed is the fairly constant impedance over pretty wide frequency range. The usual assumption is that the feed impedance of this kind of feed is about 300 Ohms. The exact impedance is dependent on the relationship between the diameter of the unbroken section to the split section. For more about this relationship see my Folded Dipole page.
The actual impedance is more like 288 Ohms, but 300 Ohms is is a nice round number to work with and is close enough. But 300 Ohms may not be what you are looking for.
If your just connecting this antenna to something like a television, you could simply use 300 Ohm Twin-Lead. This is a parallel conductor transmission line available at most electronic stores. Most older televisions have 300 Ohm antenna connections. The only problem you would have is that this kind of transmission like requires that you space it away from metalic booms and booms.
The usual requirement is for a coaxial transmission line to your receiver. For television reception the common impedance is 75 Ohms. For amateur purposes you might want 50 Ohms. This means that you need some way of transforming the balanced 300 Ohms, at the folded dipole feed point, to the unbalanced 50 or 75 Ohm transmission line.
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Feed Setup for use with 300 Ohm Twin Lead or External Balun |
The picture on the right shows a possible entry method for feeding your folded dipole with 300 Ohm Twin Lead or if you are going to use an external balun. The mounting posts are simply machine screws with two nuts and a solder lug. The solder lug should be mounted inside the box and is for wiring the post to the feed. The exact method for attaching the wire to the feed depends on what you are using for the feed. If you are using aluminum tubing a good method would be to flatten the ends and attach the wire with a small nut and bolt. If your just using heavy gauge wire, the simplest method would be solder. For soldering, you may want to make these connections before you put the feed lines in the plastic box, for obvious reasons.
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Feed Setup for use with 75 Ohm Coax |
If you are looking to use a 75 Ohm coaxial cable as a transmission line, you will need a impedance matching transformer. If you are only using the antenna for receiving, an external matching transformer, purchased from most any electronic parts store, can simply be attached to the feed arrangement pictured above and then secured to the boom with Ty-Wraps to provide stress relief. But, you can make your own matching transformer out of a short length of coaxial cable and connect it right inside the plastic box.
The drawing on the right is intended to give you an idea of the configuration, for a matching transformer made out of coaxial cable. The two important aspects of this arrangement are the length of the 1/2 Wavelength Phasing Section and the connections to the folded dipole feed point.
The length of the 1/2 Wavelength Phasing Section is dependent of the frequency of operation and the velocity factor for the coaxial cable you are using. A half wavelength is calculated in the usual way using 491.8/F(Mhz) to obtain the length in Feet. The velocity factor is then used to adjust for the reduction in the speed, as the wave travels through the coax.To help you in calculating he proper length for your needs, below is a little calculator. Just enter the frequency, in MHz, that your are working with and then select the type of coax you are using. If you don't find the exact coax that you are using in the table, select one that has a similar velocity factor. The required length (L) will be listed in US/Imperial and Metric dimensions.
Be careful with the type of coax you select. For VHF/UHF needs, some coaxial cables may not have the bend radius capability that you need. For example, a balun for 900 MHz would be a little less than 5 inches. A RG-11 type of coax may not make the bend, without the cable crimping. A better choice might be the more flexible RG-59 cable.
| Frequency (MHz) |
Type | Velocity Factor |
Impedance |
This diagram above is included to illustrate that the dimension calculated is only for the shielded section of the coax loop. Make sure you add some extra length for connecting to the feed points. |
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| RG-59 | .66 | 75 Ohm | ||
| RG-59/U Foam | .79 | 75 Ohm | ||
| RG-11/U | .66 | 75 Ohm | ||
| RG-11/U Foam | .80 | 75 Ohm | ||
| RG-6 | .75 | 75 Ohm | ||
| RG-58 | .66 | 50 Ohm | ||
| RG-8 | .66 | 50 Ohm | ||
| RG-8x | .78 | 50 Ohm | ||
| RG-8/U Foam | .80 | 50 Ohm | ||
| RG-213 | .66 | 50 Ohm |
The diagram on the right illustrates the assembly of the 1/2 Wavelength Phasing Section and Folded Dipole Feed. The feedline connects to one side of the Folded Dipole Feed section along with one side of the 1/2 Wavelength Phasing Section. The 1/2 Wavelength Phasing Section exits the plastic box through a rubber grommet and comes back in through another. This end is then connected to the other side of the Folded Dipole Feed. The shields for all of the coax ends are tied together and soldered to a lug inside the box. If the boom is metal, and the screw holding the ground lug creates a physical connection, that's OK. If the boom is not metal, that's OK too.
You have a lot of space to get creative here. The drawings are only a suggested method. For example, if you had a box big enough, you could probably keep everything but the feedline in the box. But that is usually not the case, except for higher frequency antennas. Or, you could use coaxial connectors at the places where the coax enters the box.
In one of the drawings the 1/2 Wavelength Phasing Section is shown as a simple U-Shape, whereas, in the drawing above, it is shown looped at the bottom. Either way is acceptable. For lower frequencies, the 1/2 Wavelength Phasing Section may be quite long. Just roll it up into a 6 or 8 inch loop, secure it with electrical tape, and then lash it to the boom. Higher frequency antennas will have shorter 1/2 Wavelength Phasing Sections and may not need to be spooled up.
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