## 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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<p>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.</p><p>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).</p>
<p> 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. </p>
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.
<p> 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. </p>
<p> 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?</p>
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.
<p> 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. </p><p> I used to have boxes of those connectors around.. can't find any now.</p>
<p>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. </p>
<p> 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. </p>
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.
<p>WILL LEAD WORK AS ELEMENTS ON A DIY TV ANTENNA ?</p>
I doubt the will. The antenna elements are usually an inch or more wide.
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?<br />
Cables for connecting an antenna or other signal device (DVD&nbsp;player, etc.) to a television have an impedance given in ohms.&nbsp; 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.&nbsp; As the frequency of the current changes in these circuits, so does the resistance to its flow.&nbsp; 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.&nbsp; That is why small coils of wire in a radio circuit are called a choke.&nbsp; 300 ohm twin lead is flat TV signal cable that presents 300 ohms of resistance at television signal frequencies.&nbsp; See this <a href="http://en.wikipedia.org/wiki/Twin-lead" rel="nofollow">article</a>.&nbsp; Coaxial cables more frequently used now for digital signals have an impedance of 75 ohms.&nbsp; A small transformer can convert the signal from 300 ohms to 75 ohms.<br />
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 &quot;characteristic impedance.
thanks, I never knew that and I wire stuff at work all the time, mostly DC though so low ohms readings.<br />
I do not work with these things, but have read bits and pieces about impedance, also known as reactance.&nbsp; In a radio tuning circuit there is an inductance (coil of wire) and a capacitor.&nbsp; One of those is variable by means of a tuning knob.&nbsp; The resistance provided by the inductance reduces or blocks signals.&nbsp; The same is true for the reactance or resistance provided by the capacitor.&nbsp; 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.&nbsp; The result is the frequency you want is allowed to pass relatively freely.&nbsp; <br /> <br /> I am glad to have been of help.<br />
Don't know if relevant but in Finland these &quot;converter boxes&quot; are called &quot;digi-boxes&quot;. Came by when analog transmissions ended here in 2007. <br> Just thought to let you know.<br><br>Digi-digi ^^<br>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.
An old satellite dish should work. There's one on my roof from when I moved in that I don't use. If I pointed it in the direction of the towers and mounted this aerial where the LNB goes, it may be ideal.&nbsp;
Yes, that should work pretty well.&nbsp; A dish focusses to a point where the LNB is mounted.&nbsp; In theory this little antenna is more suited for a parabolic trough as shown in step 6, but I think&nbsp;what you described&nbsp;would work.
g<br />
I do not text, so I am not quite sure what &quot;g&quot; means to you.&nbsp; But, thanks.<br />
Love the simplicity of this! Who needs a \$300 &quot;digital-ready&quot;, amplified&nbsp;ultra-mega-stick?<br /> <br /> Properly &quot;skinned and dressed&quot;, the &quot;pelt&quot; of a standard pop can is about 4&quot; x 8 1/4&quot;, so you can readily make at least two 1 1/8&quot;&nbsp;x 6 3/4&quot; strips from one can. (To get a strip over 8&quot;, try a spiral cut - probably oughta carve a&nbsp;paper template&nbsp;first, then tape it&nbsp;around a &quot;raw&quot; can and trace with a Sharpie,&nbsp;to get the edges to line up nicely.)<br /> <br /> For even more fun, you could select&nbsp;the can(s)&nbsp;and arrange the cut(s) so as to showcase the logo of your favourite product!<br /> <br /> Hey! What if you just mounted two&nbsp;whole, uncut&nbsp;cans to the furring strip?!? That would be so classically &quot;ET-phone-home&quot;! Or would the 4 3/4&quot; cans&nbsp;be too short for DTV wavelengths? Maybe &quot;tall-boys&quot;?<br /> <br /> Aluminum venetian blind strips may also make for easy antenna elements - pre-curved, so if you glued,&nbsp;screwed or stapled two strips back-to-back,&nbsp;it wouldn't need the furring strip, just&nbsp;a good central mounting/connection block....
I have to believe your soda can version would work, as would your Venetian blind version (as long as the blinds are not actually plastic).&nbsp; Give it a try and let us know.&nbsp; Thanks for your comment.<br />
I cobbled this together after my old antenna stopped working. This works really well and we get all the UHF stations in our area (Minneapolis). Thanks a lot. I cut the aluminium strips out of an old discarded gutter. The strips have some ridges on them but it doesn't seem to make a difference.
Thank you for trying this. I am glad it worked for you. Thank you for your comment.
But not all digital TV will be on UHF, in some markets it will be on the VHF channels the associated analog signal has used for years. Here the CBS affiliate will remain on 7 NBC affiliate will remain on 2, PBS on 9. I understand PBS will have a HDTV broadcast, but I don't expect HDTV from the commercial networks in this rural market. www.antennaweb.org should help you find out if you will need both an UHF and a VHF antenna.
Just recently I heard that TV broadcast repeater systems in the mountains of Central Idaho (relatively little population, no cities) will broadcast digital TV on VHF frequencies. I am not surprised other places, like your area of Kansas will do the same. In that case, your old VHF antenna will serve you well. These aluminum tabs do a good job with UHF frequencies, and if the signals are weak, the parabolic reflector behind the aluminum tabs makes a super passive amplifier.
I live in Idaho! im in the valley, though
Thanks for the comment. You are over on the eastern end of the state.
How'd you know?
I think your profile says you are from Ririe, Idaho. I checked Google Maps for its location.
LOL. A good example as to why to provide limited personal information in profile sections. In creating or discussing an instructable or anything that matter, you can end up revealing to the extent of your possessions, to those who would seek to relieve you of them. Mild paranoia perhaps, but who can predict what could take place. In cases where on can't avoid using their ham call sign, enough info is provide so one get get driving instructions to the ham's home.
If you want to learn a boatload about what I own, anyone with my full name can do a Google search and you get a bunch of hits on things I have reviewed at Amazon. None of them are very rare or fabulously expensive, though. Still, you would learn a lot about my interests.
With luck you'll be in range of a repeater, that Phil spoke of. Kansas isn't all flat. A nearby small town in a hole, had better and more TV than we living on the flat. The had a tall tower with good receiving antennas that repeated the signals into the hole. I was never aware if they where passive or active repeaters.
With luck you'll be in range of a repeater, that Phil spoke of. Kansas isn't all flat. A nearby small town in a hole, had better and more TV than we living on the flat. The had a tall tower with good receiving antennas that repeated the signals into the hole. I was never aware if they where passive or active repeaters.
I suppose the rural population density, is to low for broadcasters to be interested. Except for the smallest of villages most towns have CATV. I have been toying with the idea of putting up a passive or flea powered UHF repeater, to improve my reception of UHF translator stations. I have to be careful not to mess up the local CATV reception of the translators.
We have satalite TV also, but we cant get 10.2, 10.3, 8.2 etc....
EDIT: through the satalite: we still have to use the convertor. we get 10.1, 6.1, and 8.1
It said on the DTV website that you will not need a digital antenna. You can use your old antenna. But this is good for people that want better quality.
The old antenna will likely work if it has a UHF section. We just added another converter box for another TV. It was connected to our old high gain VHF-UHF antenna located in the attic. That antenna is not adjustable as it is installed. It works pretty well if the weather is just right. But, if a front is coming some of the stations pixelate. Some of our broadcast towers are also in a much different location. We switched over to one of the homemade antennae described in this Instructable. It rests on the floor next to the TV cabinet. When a station begins to pixelate, we can change the direction it points just a little and reception is good again. We cannot do that with our old antenna. Earlier today I saw an advertisement for a local firm that will install a digital antenna on your roof for \$172.50 and there is no extra charge for the antenna! A homemade antenna can be an advantage.
I made one using the measurements described out of aluminum foil glued to cardboard and it works great. I also took a piece of 12x18 inch cardboard, covered it with aluminum foil, bent it into a rough parabola and mounted it behind the cardboard antenna using a coat hanger. That more then tripled the signal strength of just the cardboard antenna and took all of 20 minutes. Looks kind of funny but works great!
Thank you for describing your adaptation. One of the great things about Instructables is that users can adapt what someone has posted so it better fits their needs or the materials they have available. I did something like that with my Instructable on a "Pretty Good Postal Scale from Old CDs." To anyone who jokes about the way your antenna looks I would say, "Do you want to receive a good signal or pay big money to look good?" Thanks again. Your comment has brightened my day.
Great Instructable and just in the nick of time. I'm going to try this later after I dig up my old TV wires and stuff. Wanted to also say that you can actually get the needed 3cm x 17cm from an old soft drink can. Simply cut diagonally across the body of the can; one can will yield enough for both strips (you might need to cut the can's bottom off first, to get a good cutting angle). Although, one edge of the strip might be slightly beveled, and I'm not sure this would do to signal reception.
Thanks for the tip on cutting cans on the diagonal. A slight wedge shape on one edge of the strips probably will have no discernible effect. When it comes to the dimensions I gave, the truth is I just eye-balled what I thought looked good from what worked when I made up a couple of these for UHF channels back some years. I think you will be OK. And, you will not spend a bunch of money on an antenna, like we did.
Last night we had a little front come through. One channel in particular becomes difficult to receive when that happens whether by analog or digital. I had to change the orientation of the little antenna described here and we got the station, although it did want to pixelate now and then, especially if someone walked near the signal path. This morning I moved the antenna back to its previous position and we are getting all channels just fine without moving the antenna. Also, back in the 1970s I made an antenna like this with a parabolic reflector. Suddenly we were getting UHF channels from more than 50 miles away.
While I can't speak to whether or not this antenna will do what you need (I don't have the technical chops for understanding antennas), I do feel the need to point out that <a rel="nofollow" href="http://www.pbs.org/digitaltv/faq.php#difference">HDTV is NOT the same as DTV</a>. <a rel="nofollow" href="http://www.youtube.com/watch?v=PXPf5Eldz3M">DTV broadcasts</a> will be capable of transmitting HDTV content, but that doesn't mean your television will show a better picture. You'll need an HDTV-capable display for that.<br/>