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VHF/UHF Yagi Antenna Feed Design
by Martin E. Meserve

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Introduction

Mounting

Feeding and Matching

300 Ohm Feed

75 Ohm Feed

Introduction

The VHF/UHF Yagi Antenna Design page does a good job of calculating the lengths of all the antenna elements but it kind of leaves you flat for actually feeding the antenna. Although it's not really mentioned, the long boom yagis, designed from that page, are intended to use a Folded Dipole antenna feed. That is what this page is all about.

The Folded Dipole has several useful characteristics. It's easy to build and adjust, its bandwidth is good for over an octave. For example, 50 Mhz to 100 Mhz or 120 Mhz to 240 Mhz. And, its characteristic impedance is a more or less a constant 300 ohms. This doesn't mean that the whole yagi will have that kind of bandwidth. The directors and reflectors respond best for the design frequency, giving you an overall bandwidth of about 7% of the design frequency. So, a antenna designed for 145 MHz should give you an effective bandwidth of about 10 MHz.

Below 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.

Yagi Folded Dipole

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Introduction

Mounting

Feeding and Matching

300 Ohm Feed

75 Ohm Feed

Mounting

Here again, you have several choices. Some of this depends on your choices for the antenna boom. Initially you have to decide on the orientation of the folded dipole feed in relation to the parasitic elements. The picture on the right shows three different orientations. On top, the folded dipole feed point is parallel to the parasitic elements. In the middle, the unbroken section of the folded dipole is parallel to the parasitic elements. An then on the bottom, the parasitic elements are parallel to the aperature between the feed point section and the unbroken section.

Neither 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.

Yagi Folded Dipole

Yagi Feed Mounting

The 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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Introduction

Mounting

Feeding and Matching

300 Ohm Feed

75 Ohm Feed

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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Introduction

Mounting

Feeding and Matching

300 Ohm Feed

75 Ohm Feed

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.

Yagi Folded Dipole

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Introduction

Mounting

Feeding and Matching

300 Ohm Feed

75 Ohm Feed

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 left 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 the 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.

Yagi Feed Coaxial

The little diagram below 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.

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


Yagi Folded Dipole

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.

Yagi Folded Dipole