Good Digital Antenna Cheap!





Introduction: Good Digital Antenna Cheap!

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

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 (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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    51 Discussions

    How this antenna can be 300 ohm? It looks like a simple dipole with ~75 ohm impedance, not a folded dipole with 300 ohm impedance.

    And if you need 300 ohm cable and you don't have 300 ohm twin lead, you can try using an Ethernet cable connecting 3 out of 4 pairs in series (each pair has 100 ohm of impedance).

    Built this and it works great. I have built a few smaller antennas and this one has worked the best so far. I want a small antenna that I can just put behind the TV out of sight. All the other antenna I tried had drop outs. This one does not so far but maybe weather has something to do with it? Did not bother with the little converter. Just stripped the wire put the copper wire on one side and the twisted mesh on the other. Maybe it would be better if I used the converter? Don't know. The tower is across the lake from me and I will never get more than the one channel so as there is no other tower within 200 miles of me with other stations so nothing fancy was required.

    2 replies

    I am glad it is working for you. Thank you for the report. If the broadcast tower is that near to you, you might be able to receive a useful signal on the proverbial gold tooth in your mouth. Seriously, proximity does cause a signal to flood a device and expands reception possibilities.

    if i can find one of those little dodads i want to try it to see if it makes any difference although since digital is an either or proposition it will be hard to tell without some type of meter.

    another question.. why can I not just cut the coax and connect the copper wire to one side, twist the braid and connect it to the other? Do I need the flat cable and converter? Why not just skip that part altogether?

    3 replies

    Two words: impedance matching. Impedance is a type of resistance that factors in changes due to the frequency of the current in the signal. TVs with a coaxial cable input require an antenna matched to 75 ohms impedance. An antenna such as I described has an impedance of 300 ohms. Coaxial cable is matched to 75 ohms. Flat twin lead is matched to 300 ohms of impedance. And, you need a 300 to 75 ohm impedance matching transformer. Without matching the impedance in all of the ways I described will result in no signal or a very distorted signal, probably no signal.

    I understand that part but why would the aluminum strips have an impedance of 300 ohms? It is just 2 metal strips. As long as I make a solid connect to the strips why does it matter? The antenna I use now is just copper wire bent into shape with a connect to my TV using marettes. I get dropouts. The exact same drop outs I got when I tried using a commercial rabbit ears type of setup with coax cable connections.

    I used to have boxes of those connectors around.. can't find any now.

    TV signal reception is full of anomalies, whether analog or digital. I remember that we got one particular local channel just fine, but someone half of a block away could not get it at all. Impedance is strange. A small coil of wire may have almost no resistance to a DC current, but feed a radio frequency current into it and the effective resistance soars. As for why a simple antenna is 300 Ohms, I remember simple UHF bow tie antennae for older analog TVs. They were always treated as 300 Ohm antennae and used 300 Ohm twin lead. The antenna I made essentially duplicates a old UHF bow tie antenna. I expect a TV engineer from the early days of TV could explain the answer.

    I have an old satellite dish that I am going to reaim towards the tower to see if I can get a good signal. if I do that do I have the aluminum face the dish or face the tower? I am assuming it should face the dish.

    1 reply

    The dish is a parabolic curve and will focus the incoming signal in a way that greatly amplifies the signal strength. But, you will need to have the dish accurately pointed at the signal source. And, you will need the blades of the antenna located as near as possible to the focal point of the dish. I hope I have understood your question. I am not completely sure what you mean by the aluminum. I assume you mean the two blades or dog ears of the antenna. The signal will go to the dish and be reflected to a focal point near the dog ears.

    what do you mean by 300 ohm wire, that the resistance of the wire is 300 ohms? or is it a wire rating of some kind?

    4 replies

    Cables for connecting an antenna or other signal device (DVD player, etc.) to a television have an impedance given in ohms.  Impedance is a form of resistance to the flow of current and is different from simple resistance in that it involves circuits in which coils of wire (inductances) or capacitors interact with currents at various frequencies.  As the frequency of the current changes in these circuits, so does the resistance to its flow.  A coil of wire may present only a couple of ohms of resistance to a straight DC current, but apply a current at radio frequency and that coil presents several hundred ohms of resistance to the high-frequency current.  That is why small coils of wire in a radio circuit are called a choke.  300 ohm twin lead is flat TV signal cable that presents 300 ohms of resistance at television signal frequencies.  See this article.  Coaxial cables more frequently used now for digital signals have an impedance of 75 ohms.  A small transformer can convert the signal from 300 ohms to 75 ohms.

    300 oms means that twin lead has a characteristic impedance of 300 ohms. Most UHF TV antennas use a folded dipole as the driven element, a folded dipole has a characteristic impedance of 300 ohms that's why twin lead has been used over the years, so there's no impedance mismatched. LOL just dawned on me you are feeding a plain dipole that has a characteristic impedance of 75 ohms with the twin lead. No matter rabbit ears use twin lead, and all works sometimes. that why the term is "characteristic impedance.

    thanks, I never knew that and I wire stuff at work all the time, mostly DC though so low ohms readings.

    I do not work with these things, but have read bits and pieces about impedance, also known as reactance.  In a radio tuning circuit there is an inductance (coil of wire) and a capacitor.  One of those is variable by means of a tuning knob.  The resistance provided by the inductance reduces or blocks signals.  The same is true for the reactance or resistance provided by the capacitor.  But, at the selected or desired frequency of the station you want to hear, the reactance from the capacitor has the effect of countering the reactance or inductance from the coil of wire.  The result is the frequency you want is allowed to pass relatively freely. 

    I am glad to have been of help.

    Don't know if relevant but in Finland these "converter boxes" are called "digi-boxes". Came by when analog transmissions ended here in 2007.
    Just thought to let you know.

    Digi-digi ^^
    Over and out.

    Try perforated aluminum or perforated stainless steel. Either one will work great and no worries about structural strength. Even heavy mesh metal screen (like a good strainer) can be used with just a little bit of aluminum or steel rod for support.