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Homemade HDTV Antenna
This page describes building a small UHF antenna suitable for use in the greater Seattle area for
reception of over-the-air (free) High Definition Telivision (HDTV) broadcasts. The antenna described
is being used indoors in Lynnwood, which is about 12 miles from our broadcast towers. It provides
good reception but probably not as good as commercially available larger outdoor antennas. I have
been experimenting with different designs and I'll update this page based on the results.
Disclaimer: The author of this page makes no warranty about the contents of this page, or the
suitability or safety of the described equipment when used with any electronic or electrical
equipment. This page does not describe safety practice for electrical construction or antenna installation.
Beware that outside antennas represent a significant hazard for lightning and electrical shock.
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Goals and Local Information
My main goal was the ability to recieve our local PBS station, which is KCTS, since I knew they
have high quality HDTV broadcasts regularly. I also checked on the frequencies used by the local
stations, since this will have a large effect on the antenna design. And here they are:
- UHF #18: 494-500MHz = KCPQ (FOX)
- UHF #25: 536-542MHz = KTWB (WBS)
- UHF #38: 614-620MHz = KOMO (ABC)
- UHF #39: 620-626MHz = KIRO (CBS)
- UHF #41: 632-638MHz = KCTS (PBS)
- UHF #48: 674-680MHz = KING (NBC)
FOX and WBS have very little high definition content, even though they do broadcast digitally (DTV).
So the frequencies of interest all lie between 614 and 680MHz, which means that, in our area,
we can use a relatively narrowband antenna design. A commercially available antenna, in comparison, needs
to provide good gain from 470 to 890MHz, which is a much more difficult problem. Our narrowband requirement
lets us get away with an antenna design which is significantly more compact than a broadband antenna
of the same gain.
Another advantage that I have in my location in Lynnwood is that the transmitters are all in approximately
the same location as seen from my house. This means that I can use a relatively high gain (directional)
antenna without needing an antenna rotator.
I wanted an antenna that could be either unobtrusively mounted outside or, if inside, would be small
and inoffensive. I forced my family to suffer for years with rabbit ears with little foil clumps on the
end, and we had had enough of that!
Antenna Options
My first inclination was to buy a four element "bow-tie" UHF array antenna. My understanding of what a bow
tie antenna is, is that it is a folded dipole in which each half has a taper, thus forming the
characteristic bow-tie shape. The taper de-tunes the dipole so that it has similar sensitivity over a broad
frequency range. There is a wide variety of geometrically scalable broadband antennas like this, but I
couldn't find any good references on this approach, so I temporarily
set it aside. Several of the good commercial antennas are of this type, so I intend to revisit it
later.
Another option which I may consider is a helical antenna. Although stations transmit vertically (linearly)
polarized signals, I think that the helical will still do a good job, although it is designed for
circular polarization. It may need to be a little longer
however, perhaps 2 wavelengths (about 40 inches), which is starting to be not so compact.
The design I settled on is the classical Yagi-Uda design (invented in Japan by Uda and described in
English translation by Yagi). This design comprises a set of parasitic director elements placed in front
of a dipole element which in turn is in front of a reflector element. The nominal element spacing is
just under 1/4 wavelength. A key advantage is the availability of amateur radio support and free
software tools for design analysis. The main disadvantage to the Yagi design is that it has a cutoff
frequency beyond which the gain plummets. I also found that in my designs, the antenna impedance was
relatively low, but since I put an amplifier at the antenna terminals, matching to the transmission
line will not be an issue.
First Design
This is the design for my first antenna, which is fabricated from 3/16" dia. hollow brass tubing which is
shoved into a wooden dowel. A Radio Shack coaxially powered 10dB broadband cable amplifier is placed
at the active element, but it's gain is not included in the predicted gain shown below:
| ELEMENT |
Length |
Dist from reflector |
| Reflector | 8.42" | 0 |
| Dipole | 8.42" | 4.21" |
| guide 1 | 7.6" | 7.60" |
| guide 2 | 7.6" | 10.95" |
| guide 3 | 7.6" | 14.29" |
| guide 4 | 7.6" | 17.63" |
| guide 5 | 7.6" | 20.97" |
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| Freq (MHZ) |
Gain (dBi) |
Front/back (dB) |
Z (ohms) |
| 500 | 3.47 | 0.6 | 25-135j |
550 | 4.29 | 0.2 | 26-88j |
600 | 7.85 | 2.0 | 25-34j |
625 | 10.57 | 6.2 | 32-8j |
650 | 11.60 | 16.1 | 33+7j |
675 | 11.01 | 7.1 | 31+69j |
700 | 2.72 | 1.3 | 9+57j |
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The antenna front/back ratio is the ratio of sensitivity looking forwards to looking immediately backwards.
As you can see, off of the design frequency the antenna has no rejection of rearward signals. For me,
this isn't important since there are no stations in the other direction or significant sources of reflection.
The column lablelled Z is the complex impedance. Don't worry if this is Greek to you. To the engineer types,
the impedance matching issues will be discussed below.
If you intend to use this design, note that the diameter of the elements and their spacing is very
important. Using smaller diameter tubing will make each element more narrowband which will make the
entire design more freqency selective, which is not really a good thing since this design is already
too weak at the low frequencies.
The overall layout looks like this:
Where L is the element length and D is the distance from the reflector which is the leftmost element. The
dipole element is also referred to as the driven element (which comes from use of the antenna for
transmitters).
In this design I intended to overcome the issue of matching the cable impedance to the 75 ohm coax
line that my SIR-T151 receiver uses by placing a Radio Shack 10dB in-line amplifier right at the
dipole. All that I needed to do is provide a balanced (dipole end) to unbalanced (amplifier) end converter.
This is sometimes called a "balun". A convenient method for doing this is to use a half-wave coax converter
which will be shown in a figure which will be added. In this method, the left half dipole connects to the
coax center lead. The right side connects to a half-wave length piece of coax which is folded
over on itself. The end of this half
wavelength piece is also connected to the center coax lead. The sheilds are all soldered to each other.
What I didn't realize was that this converter makes the
impedance looking into the antenna end one fourth the antenna impedance. Since at my design frequency the
antenna impedance is already half the amplifier input impedance, this is not so great.
It turned out that this antenna worked pretty well anyway. When placed on a short pole on my front porch,
all local broadcasters can be received, but I have to rotate the antenna slightly to pick up all of them. Then
I mounted the antenna on the side of my house, but in this location there's some obstruction from my
neighbor's house, so it only reliably recieves PBS and ABC (617 and 635MHz center frequencies). This is
using 25 feet of RG6U coaxial cable which is supposed to have a loss at this frequency of 1dB/10 feet so that's
2.5dB of additional loss. Also the noise figure of the RAdio Shack amplifier is probably not so great.
It's main function is really just to drive the coax with a matched source. Using the signal strength
meter in the SIR-T151 shows about 8-9 'bars' which is halfway between "no signal" and "strong". I noticed however
that this reciever never shows anything less than about 8 'bars'.
Second Design
My second design de-tuned the director elements towards slightly higher frequencies to lower the drop-off
at the top end, and also I took out one element to make it more compact. I also used a different construction method. Later I found
out that most people tune the dipole/reflector up in frequency and the director elements down.
| ELEMENT |
Length |
Dist from reflector |
| Reflector | 8.42" | 0 |
| Dipole | 8.42" | 4.3" |
| guide 1 | 7.6" | 7.6" |
| guide 2 | 7.5" | 10.8" |
| guide 3 | 7.4" | 14.1" |
| guide 4 | 7.3" | 17.4" |
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| Freq (MHZ) |
Gain (dBi) |
Front/back (dB) |
Z (ohms) |
| 500 | 2.9 | -0.2 | -- |
550 | 3.8 | -0.1 | -- |
600 | 7.4 | 4.0 | 25-37j |
625 | 9.8 | 8.9 | 26-7j |
650 | 11.1 | 10.9 | 32+28j |
675 | 11.1 | 70.0 | 60+47j |
700 | 9.2 | 13.0 | 17+91j |
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The second antenna seems to do a better job with NBC as this was close to the cutoff on the first antenna.
I did some experimenting with locations and I found out that this antenna actually gets its best performance
inside our house, on the bannister facing the south-facing picture window. The entire antenna is packaged
within a 12" x 20" x 2" envelope, so it's compact enough to be semi-unobtruse as an indoor device,
although its location in the room and orientation is critical.
The construction method for the second antenna is to take two sheets of one inch thick polystyrene home
insulating foam and cut each to the approximate overall size of the antenna. One piece is then
marked out for the location of each element and a groove cut for each element using a sharp knife.
The elements are then embedded in the foam, the dipole (active) element assembled and placed in a
cavity cut out of the foam, and then the top sheet of foam is glued over the bottom. This produces a
monolithic assembly in which the antenna elements are embedded within the polystyrene foam. This foam has
a small effect on the antenna which is that it raises the dielectric media content, which raises the
element impedances and probably also raises the resonant frequency associated with the design. However, this
is a small effect because the foam is mostly air.
The other big change I made was in the matching design. I tried out something called a gamma-match,
which is a parallel rod over one dipole element with a capacitor to the coax going to the rod and a
movable shorting bar from the rod to the dipole element. In this case, the coax shield is soldered to the
center of the active element which is a single piece of tubing, and the coax center lead is soldered to
a small (appx 10 pF) leaded capacitor which in turn is soldered to the parallel rod. The leads must be
extremely short since they have enough inductance to make mincemeat of our careful design. I also got
best reception by connecting the rod to the end of the dipole anyway so it degenerated into a sort of folded
dipole design. I've seen the same design used in HAM antennas. The advantage of this approach over the other
halfwave coax balun is that the impedance is nominally the same as the antenna design impedance. A
quarter wavelength of 50 ohm coax could also be used to reflect the 35 ohm (nominal) impedance of this
design up to 70 ohms which better matches the amplifier 75 ohm input impedance.
After gluing the foam top and bottom pieces together, the pieces are then filed and sanded to a smooth
and attractive shape. I will then cover it with fiberglass and West system epoxy. I may add a thin
wood veneer cover.
The following photos provide more detail of the antennas.
