Homemade DTV Antenna
In June of 2009 the government-mandated switch to digital television (DTV) went into effect in the United States. With this change comes more channels along with DVD-quality picture and sound. To enjoy these benefits of DTV, all you need is a converter box (or a TV with a built-in ATSC tuner) and a decent antenna. This page explains how you can build a great little antenna for not a lot of money and start enjoying over-the-air digital.
My Digital Conversion Experience
I live about 30 miles from the city of Chicago and I've always gotten marginal-quality analog TV reception with my indoor antenna. Because of this I've been paying the cable company a monthly fee for a very basic subscription of about 20 analog channels. I don't really watch a lot of TV, but when I do I at least want a picture without static.
When I first found out about over-the-air digital TV I was pretty excited. I had heard that DTV broadcasts were in the UHF band and UHF channels had always been among the best analog reception for me. So I went out and bought a converter box and hooked it up to a cheap UHF loop antenna. Amazingly enough I got many channels with good clarity. I had a few had dropouts here and there, but overall the picture quality was better than cable. Then I noticed a couple channels were missing. After visiting antennaweb.org, I found out why.
It turns out that not all digital TV is broadcast in the UHF band. In fact, in Chicago there are two stations (CBS2 and ABC7) that still broadcast over VHF. Since a VHF antenna was required, I resorted to adding a pair of rabbit ears to my UHF loop. So now I've got free over-the-air digital TV, but at the expense of a really hokey-looking antenna atop my entertainment center.
I started searching the Internet for a better solution. There are several space-age looking "HDTV-ready" antennas out there, but surprisingly many of them were only designed for the UHF band. I managed to find a few that advertised VHF and UHF coverage, but upon closer inspection the VHF band was pulled in by a set of rabbit ears. $50 for an antenna that still uses rabbit ears? No thanks.
Building My Own
I have a pretty solid background in electronics, so I decided to try my hand at building an antenna from scratch. I figured that with a little research, I could make one that's better performing and less obtrusive than what's commercially available.
I started by researching some basic designs and finally settled on the half-wave folded dipole. I chose this antenna design for several reasons.
- Easy to understand, well documented antenna design.
- Can be built with readily-available 14 gauge copper wire. (The same stuff used deliver electricity to household outlets.)
- Good bandwidth covering a full octave.
- 300 Ohm impedance connects to commonly available matching transformers.
Digital TV is broadcast in two bands, VHF High (RF channels 7-13) and a portion of UHF (RF channels 14 - 51). According to information at cgsnetwork.com, these channels correspond to frequencies 174 - 216 MHz and 470 - 698 MHz respectively.
Looking at the DTV broadcast frequencies, I determined that it's just not possible to build a single folded dipole that performs well over the entire range. This did not surprise me however, since analog TV always required using two different antennas, one for VHF and another for UHF. So I decided to build two antennas and use an old-fashioned VHF/UHF combiner to bring the signals together.
The UHF Antenna
I started with the UHF antenna. I figured that being smaller it would be easier to construct and I wouldn't waste much wire if it took me a couple tries to get it right.
The first thing to do is calculate the measurements of the folded dipole. I chose the center of the UHF band for my target frequency of 584 MHz. The formula to calculate the length of a folded dipole is taken from the ARRL Handbook For Radio Communications that I checked out from my local library.
l = 468 / f where l is length in feet and f is frequency in Hertz or l = 5616 / f where l is length in inches and f is frequency in Hertz
Using the formula, I found that the length of antenna required for my UHF target frequency is 5616 / 584 or 9.616 inches. Since measuring tapes have fractions and not decimals, I rounded to 9 5/8 inches.
The distance separating the wires in a folded dipole design (s), although not extremely critical, should be small compared to it's length. I decided to wrap the wire around a screwdriver handle to form the curves, so s is about 3/4 inch.
So now the trick is to take all of the measurements and build an actual antenna. I start by cutting a 20 1/4 inch (2l plus an extra inch) length of 14 gauge wire and marking the center of it. The extra inch is so I can make curves on the end and still end up with the correct width. From the center marking I measure out 5 1/16 inches in either direction, a quarter of the length of wire, and mark these distances as well. These last two marks show me roughly where the ends of my dipole will be.
The VHF Antenna
Designing the VHF antenna is the same process as the UHF antenna except that the center frequency is 195 MHz. This translates to a length (l) of about 24 3/4 inches. I'll be using the same screwdriver handle to form curves on the ends of the dipole so again I added an extra inch to the length of wire to compensate. That makes the total length of wire 25 1/4 inches with a center mark at 12 7/8 inches and quarter-length marks at 6 7/16 inches from the ends.
Putting It All Together
To form the folded dipole elements, I take my straight piece of wire and place one of the quarter-length marks on top of my screwdriver handle. I then gently bend the wire around the screwdriver handle, lining up the end of the wire with the center marking. I repeat the process with the other side making sure the two ends have a gap of about 1/8 inch between them. After a little adjustment, I've got two nice-looking folded dipoles ready for mounting.
Since I have two antenna elements, I need to combine the signals. I also need to convert the 300 Ohm antenna elements to the 75 Ohm input of my DTV converter box. I can accomplish both of these tasks with a relatively inexpensive UHF/VHF 300 Ohm combiner from Radio Shack. I can simply cram the ends of my folded dipoles under the screw terminals of the combiner and tighten them down so that the UHF element is stacked on top of the VHF element. If I want to get a little fancier, I could solder on some fork terminals first.
The only thing left to tackle is hooking it up to the converter. It's not very practical to attach the 75 Ohm output of the UHF/VHF combiner directly onto the converter box. That leaves me a couple options. I can get a cable to attach it, but it will need a male F-connector on one end and a female F connector on the other end. That's not a real common item as most cables are male on both ends. My other option is to rig something up with a gender changer. I can plug one end into the UHF/VHF combiner and attach any standard coax cable to the other end.
Enjoying Over-The-Air Digital
Now that the antenna is done, I simply place it on top of my entertainment center, cable it to my converter box and point it toward the broadcast towers in my area. Most of the channels come in very well, but I do live 30 miles from the towers and occasionally there is some break-up in the picture of weaker stations. Fortunately, I was able to boost my signal strength with an inexpensive 12dB RF amplifier from a local home improvement store.
All-in-all I am very happy with my antenna design. It has several advantages:
- Good performance
- Fairly inexpensive
- Relatively easy to build
- Much less obtrusive than rabbit ears
Best of all, I learned a lot while building it, plus I get bragging rights for doing it myself.