diy uhf TV antenna

1 beerismy: There is most likely NO RF out on the back of the TV.... it is probably an antenna or RF INPUT. If it is a "F" type coaxiall jack the nominal impedance will be 75 ohms, if you have two screws for the antenna or RF input, the nominal impedance will likely be 300 ohms. Note that the impedance is NOT the same as DC resistance like you would measure with your multimeter. A simple UHF antenna could be a stiff wire loop of about 7 inches in diameter. This will have a DC resistance of near ZERO ohms but an impedance of about 300 ohms at UHF frequencies so if you are connecting it to a 75 ohm input jack you will need a 300 / 75 ohm balun matching transformer for best performance. electricitym . . .

Andy replies:

Get a copy of the ARRL handbook or, even better, the ARRL Antenna Handbook...

You have a lot of getting up to speed to do before you will actually accomplish anything. Once you understand the basics, you can do almost anything with coat hangers. Otherwise, you will only create bent up wires that may or may not work at some frequencies and not at others...

It ain't black magic --- antenna design has very definite rules and they are not difficult. It's a lot like hanging sheet rock. If you understand what you are doing, it is easy. If you don't , you have a mess.... good luck...

We all started just the way you are now. You may find that it is a hell of a lot of fun. Or you may decide, "the hell with it". But there's no reason for "trial and error" when the actual discipline is very methodical and reasonable....... about the complexity as changing the oil in your new truck for the first time....

Andy W4OAH

Sometimes its an open loop, at least DC resistance. Sometimes its a DC closed loop. When its shorted, there will be near zero ohms DC resistance. In the world of RF and antennas, impedance has no bearing on the DC resistance. An antenna has a specific impedance over a certain range of frequencies, and the transmission line feeding the antenna must match the antenna impedance, as well as match the input impedance of a TV or other device. I started out with an ARRL antenna book and a ARRL handbook back in about 1962. Got my Novice class then.

greg

Years ago I made a Yagi antenna with an aluminium square tube with bars screwed on with stanless steel screws. It had about 20 elements on it. But now I want to make another one and I don't have a plan any more. Is there any good plans available on the web? I must admit I have not even looked yet.

i have read a bit more on this and have successfully created a simple dipole which gets VHF & some UHF too.my favorite channels come in with no snow, but with some ghosting.

i read that raising the dipole in the sky would cure it but would require building a tower.

would adding a reflector help with this? i read that a dipole's reflector is a piece of wire a bit longer than the diapole. would it make any difference if i get more drastic and use a rectangle of foil or chicken mesh to block out everything coming from behind the diapole?

thx

Rotating the dipole will help remove ghosting. I used to buy corner reflectors, hardly worth making your own. If they don't sell them, you will have to try doing what you said. If you found formulas for making yagis, the distance between and the length of  ?ÿ ratio over a driven and reflector rod combination.

greg

Repaired that post.

Rotating the dipole will help remove ghosting. I used to buy corner reflectors, hardly worth making your own. If they don't sell them, you will have to try doing what you said. If you found formulas for making yagis, the distance between and the length of elements can be adjusted. You can increase the size of the back reflector as you mentioned, and will increase the front to back ratio further than a plain reflector eolement.

greg