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Outdoor antenna design

  1. Apr 26, 2017 #1
    What i need are the ff:
    • basic guidelines on how to create an outdoor antenna (you can send links :))
    • the most suitable type of antenna that can receive 70 MHz tv station frequency. The antenna will be mounted less that 3 m from the ground.
    • computations that i need to know (you can also send links)
    i know i could just search the net but i basically have a little knowledge about antennas so i don't quite understand what i searched if its right or what :( you know that feeling haha
     
  2. jcsd
  3. Apr 26, 2017 #2

    Drakkith

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    The other option is to buy an actual textbook on antenna theory. You're almost guaranteed to get good information if you purchase a book.
    You can probably find an older edition of this book for very cheap: https://www.amazon.com/Antenna-Theo...qid=1493257072&sr=8-1&keywords=antenna+theory
     
    Last edited by a moderator: May 8, 2017
  4. Apr 26, 2017 #3

    davenn

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    you have given no indication of gain required, how far you are from the TV transmitter etc

    a basic folded dipole and balun ( buy the BALUN) I have annotated a pic below
    upload_2017-4-27_12-51-0.png

    this is a vertical polarised dipole, you will need to see what is used locally, look at your neighbours antennas

    That's very inefficient ... yes, it will work but not well .... height is everything !


    if you don't think you could engineer an antenna, just buy one, they are not overly expensive



    Dave
     
    Last edited: Apr 26, 2017
  5. Apr 26, 2017 #4

    jim hardy

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  6. Apr 28, 2017 #5
    thank you for responding! Is there a way where i can combine the folded dipole antenna with another kind of antenna (except yagi-uda) so that i can make the antenna more efficient. (And if there's a way where i can make the antenna efficient with that height :( )
     
  7. Apr 28, 2017 #6

    jim hardy

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    Seems to me first thing you need is an estimate of signal strength where you are.
    Oh No ---
    FCC has moved their map page from what i linked in my Yagi thread

    to here
    https://www.fcc.gov/media/engineering/dtvmaps
    Whew i thought it was gone !

    here's what it shows for my next door neighbor's house. He gets far better signal than i do..

    clicking on a station tells you estimated signal strength . That'll tell you how exotic an antenna you need. And it gives compass heading to the transmitter to help you aim it .

    If you're only interested in 70 mhz tune an antenna for it. That's what i did with my Yagis, i needed them being so far down in a valley in a fringe area anyway.

    upload_2017-4-28_7-41-54.png

    you can zoom in right to the spot in your yard where you plan to put the antenna.

    old jim
     
  8. Apr 28, 2017 #7

    berkeman

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    What do you mean by "more efficient"? There are optimum antenna designs for various situations. You can improve gain for more directional antennas, for example.

    Also, you really should try again to learn about antenna design in general. You will get a lot out of spending some time reading. Start with the Wikipedia article, and then branch out into it's references and related reading.

    And what do you have against Yagi antennas?
     
  9. Apr 28, 2017 #8

    sophiecentaur

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    Your question is a "how long is a piece of string?" kind of question. There are many possible answers until you actually specify what you need this antenna to do. I could recommend going out into the street and looking up at all the antennae in the neighbourhood. See what types they are and, if they are directional, where they are pointing. If you are in a town and you want your local station(s) then you may be asking the question because you have a set top antenna which doesn't give a good signal. If you just stick it up on the roof with a long coax lead, that might solve your problem almost for free.
    If you are straining to watch programmes from a very distant transmitter then you will need to try harder. If you are prepared to show a bit of interest in the topic then you could contact a local radio ham group and you may find them very helpful and enthusiastic about your problem.
     
  10. Apr 30, 2017 #9
    I researched about antennas. I read that the radius of the dipole rod affects the bandwidth but i don't seem to find the computations for the radius. The only thing the i found is an online calculator but the site didnt give the formula. :(
     
  11. Apr 30, 2017 #10
    I'm sorry. I was cramming since i dont have enough time to create the project but i started to read now. :)
     
  12. Apr 30, 2017 #11

    sophiecentaur

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    This is another level of sophistication and of no consequence to you - particularly for a receiving antenna. A piece of wet string will work if you can get it high enough and many TVs work ok with a wire coat hanger stuck in the back antenna socket.

    Define your precise problem, for a start.
    You really have to make up your mind about what you want from this thread - and from the whole exercise. There are Gigabytes of stuff, written about antennae on the www and a lot of it is rubbish. A lot of the rest is far too hard for you, unless you are in this for the long haul and want to do an EE course in antenna design.

    A good text book on em wave and antenna theory would have all you need to know but that would be overkill and probably just hurt your brain (it hurt mine for more than twenty years!!).

    If all you want is better TV reception then a very simple half wave dipole (even without a balun and fancy matching network) stuck up on a roof or even in an attic, could solve a simple reception problem. You will get more value from a bit of experimentation and reading radio ham websites than digging deep into the theory.

    P.S. Even 3m is better than 1m! But the trouble with low sited antennae can be multipath reception rather than just signal strength. 70MHz (4m ish) is a very long wavelength to be building a 'proper' directional antenna indoors
     
    Last edited: May 6, 2017
  13. Apr 30, 2017 #12
  14. May 5, 2017 #13

    vk6kro

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    One problem with yagi antennas for TV is that they have to work over a wide bandwidth.

    So, an element that might work as a reflector at 77 MHz could start behaving like a director at 70 MHz.

    This would cause the frequency response to vary widely over the band required and it may cause the antenna to accept noise or other signals from the rear, which you may expect it to reject.

    I would look at the antennas of my neighbours and go to buy something similar. Design of TV antennas is not easy and requires plenty of expensive test equipment or lots of luck.
     
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