VHF/UHF Yagi Antenna Feed Design
by Martin E. Meserve
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Antennas | VHF/UHF Yagi Feed
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Feeding and Matching
300 Ohm Feed
75 Ohm Feed
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.
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.
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.
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.
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
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.
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
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
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.
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.
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.
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
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
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