Introduction: Good Digital Antenna Cheap!
In the USA we have all heard that analog TV signals will disappear on February 17, 2009. Everyone receiving a signal through the air on an older television will need a converter box for Digital TV.
We bought a box, but it was a while later that we learned we needed a special antenna to go with it. We bought the one shown in the photo. It also comes with an active (powered by a voltage adapter) signal amplifier. This one is $37 at Amazon.com (plus shipping and handling). I must be slow mentally, because it took me a while to realize the Digital TV signal utilizes a basic UHF antenna.
We live 30 miles (48 km) from the local broadcast towers. An antenna like this should give us super fine reception. But, pointing it in the exact direction of the tower is critical. It does not function well without the signal amplifier, and the picture pixelates or goes to "No Signal" when someone walks across the signal path in another room. Mounting it in the attic might help, but one of our stations is located about 45 degrees clockwise from most of our stations.
For pennies I made a better antenna.
Step 1: Start Making the Antenna
Cut two strips of thin sheet aluminum 1 1/8 inch (3 cm) by 6 3/4 inches (17 cm). I considered cutting the aluminum from old soft drink cans. To get the needed length I would have needed to cut down the side and across the bottom. Do that if you have no other sheet aluminum.
Step 2: Mount the Aluminum Strips
I drilled a small hole in each piece of aluminum and used carpet tacks to mount the outer end of each to a piece of furring strip commonly known as 1 x 2. The distance between the two pieces at the center is 7/8 inch (2.2 cm). That means you need about 15 inches (38 cm) of furring strip.
Step 3: Attach Twin Lead
Strip and attach a piece of 300 ohm twin lead (TV antenna wire) with two sheet metal screws. Attach a 300 to 75 ohm signal or impedance matching transformer to the other end of the twin lead.
Step 4: Provide a Strain Relief
I used to make "T"-shaped VHF antennae from twin lead. The connections always twisted and pulled until the thin wires broke off. Add a strain relief to keep that from happening. I used some electrical tape to keep the twin lead from twisting.
Step 5: Add a Support to the Furring Strip
I wanted the furring strip to rest so the face of the aluminum strips, not their edge, would receive the signal. Twin lead is stiff and stubborn stuff. I drilled into the back of the furring strip and added a dowel rod to prop the furring strip up.
Connect the 300 to 75 ohm transformer to the input on your converter box. Connect the converter box to the TV. Run a scan so the converter finds available stations. Experiment with the placement of the antenna for your best reception and enjoy. Also try placing your antenna so it is vertical rather than horizontal.
When I was finished, I found this cheap antenna performs better than the commercial antenna in the first panel of this Instructable. It is not as dependent on directional alignment and once properly aligned, no channels pixelate or fail with "No Signal." It did not lose a picture when someone walked across the signal path.
Step 6: Need Amplification?
You can add passive (no electrical power needed) amplification to your signal by using a parabolic reflector made of sheet aluminum. The graphic shows the aluminum strips (here in dark red) set off from a flat parabolic reflector also known as a parabolic trough. You would devise your own scheme for mounting the reflector and the aluminum strips.
Note: I have always wanted to try making the reflector from a wire screen. It should work, but it would need more support to hold its proper shape.
Step 7: What Is a Parabola and How Do I Make One?
The definition of a parabolic curve is "a line equidistant between a point and a straight line." Go to a large table top and make a large sheet of paper from small pieces of paper. Newsprint works if you are using a felt tip marker. Make it a color like red or green so it stands out from the printing on the newspaper. Draw a straight line across the bottom. At its center make a line perpendicular to the straight base line. At about 14 inches (35.5 cm) mark the focal point. The distance from the base line to the focal point is completely your choice, but it affects the size of the reflector you make. Mark a point midway between the base line and the focal point. This is the first point you have plotted on your parabolic curve.
The "B" lines are always perpendicular to the base line. The "A" lines change their angle in order to connect with the "B" lines. Use two yardsticks and a square. Plot points where the yardsticks cross over one another while the measurement shown on each yardstick is the same. The more points you plot, the more accurate your parabolic curve will be. If you are careful, you can plot only the left half of the curve and then flip it over to copy it onto the right half of the paper.
The end result is that any signal (TV, radio, sound, light) coming to the parabolic curve in parallel rays will be gathered in strength at the focal point and amplified because they are concentrated.
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